A review of garment ventilation strategies for structural firefighter protective clothing

Abstract

The purpose of this review article is to evaluate ventilation, within protective clothing, for its benefit towards heat loss. Literature from ventilation studies in the sports apparel, outdoor clothing, military, chemical, and firefighter protection industries will be examined for future research opportunities. Challenges to ventilation such as garment placement, protection, wearability, and durability will be discussed in the context of turnout suits. Ventilation designs will be considered for further evaluation in structural firefighter turnout garments. This article serves as the first comprehensive review of ventilation literature for structural firefighter turnout ensembles. Researchers, technologists, and functional apparel designers may all benefit from such a review. The value of ventilation and its potential contribution to current firefighter turnout research will be discussed.

Keywords

ventilation heat loss protective clothing firefighter

Introduction

Structural firefighter turnout suits are designed to provide protection against fire and thermal hazards. They must also protect the firefighter from liquids from the fire ground and blood borne pathogens. The protective aspect of clothing in general, also creates an air layer between the skin and the clothing, which is denoted the “microclimate.” As modern turnouts have become increasingly encapsulating, the microclimate becomes close to the temperature and humidity at the human skin, there has been corresponding interest in developing designs that provide enhanced heat loss from the microclimate to reduce the heat strain. Approaches can include more air permeable turnout materials and open garment designs. One heat strain reducing garment strategy that can be considered to improve heat loss is the incorporation of ventilation into the turnout. The functional theory of garment ventilation is based on the assumption that vents enhance body heat loss to the environment primarily by enabling convective and evaporative heat transfer through the clothing or by venting microclimate air via openings (“vents”) in the suit that allow direct air exchange between the microclimate and the environment. The incorporation of vents into turnout suits is particularly challenging since any heat strain reduction that is realized must be accomplished without compromising the fundamental need to provide protection against thermal, liquid and toxic environmental hazards.

While the concept of using vents in functional clothing to increase air exchange and heat loss is well known, there are relatively few studies specifically addressing ventilation approaches and effects in structural firefighter gear. Additional literature sources can be found that apply ventilation concepts to other types of functional clothing, particularly clothing designed for outdoor and sports applications. This paper, therefore, will provide a review of ventilation in structural firefighter turnout clothing. It will discuss findings from studies of ventilation in other types of protective and active wear clothing. This review is designed to be a useful source of information of value to researchers seeking to design and incorporate ventilation features into firefighter turnout suit designs.

Heat strain in protective clothing

Humans are homoeothermic, which means they require a balance between the amount of heat produced by the body and the amount of heat the body loses to the environment.^1–3 Protective clothing hinders heat loss as it often consists of multiple fabric layers for protection against puncture, chemicals, heat and flame, steam, and gases.² When multiple layers of clothing are worn the air and moisture exchange from the body to the environment is reduced. For workers who operate in warm environments or who perform heavy physical activity, heat strain may occur as a result of the personal protective clothing being worn.⁴ For firefighters, thermal protection is of utmost importance; however, a contradiction occurs between the necessary thermal protection required and the need for heat loss.¹

Heat loss through protective clothing

The body's primary mechanism for removing excess heat is through sweat evaporation, which is reduced by high humidity and lack of air movement when clothing is worn.^4,5 When the body performs physical activity it produces heat which must escape from the body in order to maintain a balanced core temperature. If this heat cannot escape, heat strain, fatigue, stroke, and even death may occur for the wearer. The addition of clothing, especially multiple layers, creates air gaps which further hinder the ability of sweat to evaporate to the outward environment. Humid conditions also play a role in reducing sweat evaporation as well, along with the type of fabric and material in the clothing.

Convection is the transfer of mass and heat by a gas or liquid, or in this application, air exchange.³ In order to increase heat loss through convection, the ambient air temperature must be less than the microclimate air temperature within the suit. The greater the difference between the two gases, or air temperatures, the greater the heat exchange.³ There are two types of convection: natural convection, which occurs due to existing temperature gradients, and forced convection which occurs with air movement, due to wind or “pumping,” due to human activity. As a baseline, garments are often tested in a dry environment, with no wind and no movement. This eliminates the effects of pumping when the body is moving, such as walking, and forced convection effects when wind is present.⁶

Pumping or bellows ventilation is defined as a mechanism of air exchange which results from rhythmical movements of the limbs and body during physical activity.⁷ This effect occurs as movement forces air around clothing and brings air in or pushes it out through openings.⁸ These openings can be of traditional design such as the collar, cuffs, and bottom hems or they can be purposely designed as vents. Pumping is simulated in manikin testing by making the manikin perform a walking motion.

The second type of heat loss which occurs within the clothing system is evaporation of sweat, which is an “extremely effective” tool for heat transfer.³ This type of heat loss is a last resort for the body when heat cannot be properly removed through convection.^3,9 Even in low temperature environments, with the addition of protective clothing, the body may rely heavily on sweat evaporation in order to prevent overheating.⁹ Only at very low work rates would the wearer not sweat as the amount of heat loss by sweat evaporation increases as the metabolic rate rises.¹⁰ This concept is illustrated in Figure 1.

Figure 1.

Sensible and evaporative heat losses for comfort related to metabolic rate.¹⁰

In hot and humid environments the need for sweat evaporation is even greater. The body's available cooling power through sweat evaporation is not only dependent upon the environmental conditions, but the fitness level of the individual, the state of acclimation of the body, the type of clothing being worn, as well as the evaporative efficiency of the clothing materials.⁹ One way to impart change and increase sweat evaporation in the clothing system, beyond improving materials, is to increase the rate of air exchange between the clothing ensemble and the outer environment.¹¹ Air exchange between a specific location within a garment and the environment involves three parts which include: (i) air exchange between local body parts' microclimates, (ii) air exchange through the fabric layers to the environment, and (iii) air exchange through garment apertures with the environment.¹² Of these interactions, the most effective for heat loss is air exchange between the microclimate and the environment through garment apertures.¹²

Ventilation in clothing to increase body heat loss

In order to alleviate the buildup of heat within the clothing system, ventilation may be employed. Ventilation is defined as, “the amount of ambient air that flows under the garment after passing through the fabric and/or through (designed) openings.”⁴ The presence of ventilation features within a clothing system has the potential to remove excess body heat and reduce the discomfort caused by excess sweat.⁶ These ventilation features allow for sweat and heat to escape more easily from the microclimate and away from the wearer's skin. For protective clothing, garment design improvements and fabrication changes are necessary to increase the amount of heat loss while maintaining protection.¹³ These design improvements have shown measurable differences in skin temperature during physical activity on human subjects.¹³ For prototype structural firefighter turnouts, garments which incorporated both ventilation openings and fabrication changes had the lowest recorded skin temperature and greatest ventilation, as measured by temperature differences compared to standard control turnouts. As Watkins describes in Clothing The Portable Environment, design openings in the arms, waistline, and cuffs, along with a spacer placed between the skin and first layer of clothing, may be added to increase the ventilation in protective clothing.³

To determine if ventilation is a viable approach for increasing heat loss, an understanding of heat transfer, convection, and evaporation is necessary. Dai and Havenith evaluated this relationship by measuring the local ventilation of the clothing microenvironment, thermal insulation, and evaporative resistance in various air movement conditions and by making the manikin stand and walk.⁸ This study involved the use of a tracer gas dilution method to measure localized ventilation in two jackets while the manikin was standing still and while walking in wind speeds of 0.4 and 2 m/s.⁸ During natural convection, when the manikin is stationary and there is no wind, the ventilation was the lowest among all of the tests. The only air exchange which occurs in this condition is the permeation through the fabric of the jackets. With the addition of walking, pumping effects can be seen as the ventilation increased by over 100% for both jackets evaluated.⁸ Forced convection, through increased wind speed, was found to contribute more to ventilation than pumping. Jacket A ventilation increased by over 800% with forced convection alone.⁸ Heat transfer increased as both the wind speed and body walking were introduced. The conclusion of this study was that clothing ventilation affects more on evaporative resistance than thermal insulation.⁸ The heat and moisture transfer within a clothing ensemble is affected not only by the specific materials used but also by the design of the garment, including the style, size, fit, and added accessories.¹⁴

The addition of a ventilation spacer placed between the skin and the first layer of clothing have been suggested by Watkins and evaluated by Reischl and Stransky.^2,3 However, this evaluation was done in conjunction with various other types of ventilation within the same firefighter turnout suit. Future research should evaluate ventilation designs separately, including microclimate spacers placed within the turnout suit. If cold and/or dry air moves into the microclimate of a clothing system it will act as a “heat sink” and mix with the warmer air that is present in the microclimate¹ leading to heat loss when exchanged with the environment. However, if hot air moves into these layers, it causes an increase in temperature of the microclimate air, leading to an increase in skin temperature.¹ This may cause discomfort for the wearer and prolonged exposure to this condition may lead to additional thermal strain due to the imbalance of heat transfer from the body.¹ Very little experimental data is available on this issue, but the general trend warrants concern, so any microclimate ventilation system should address this issue and incorporate design features to allow for a reduction of ventilation under very hot conditions.

Clothing ventilation measurement methods

To measure whether or not ventilation has a benefit towards heat loss, methods using a sweating manikin or trace gas dilution technique should be used. A sweating manikin measures the thermal insulation and evaporative resistance of garments. Thermal insulation can be defined as “the resistance to dry heat transfer by way of conduction, convection, and radiation.”¹⁵ Evaporative resistance is “the resistance to evaporative heat transfer from the body to the environment.”¹⁶ From these measurements, a predicted total heat loss (THL) value can be calculated for the overall garment. The trace gas dilution method, can be used to measure the ventilation index of the garment. The ventilation index is calculated from the measure of the micro-environment volume and the tracer gas rate of air exchange measure.¹⁰ This index describes the air exchange characteristics of a garment and can be used to rate the performance of protective clothing ensembles.¹⁰ This method is more complicated than the sweating manikin but can be done in a shorter amount of time. To the authors’ knowledge no clear direct comparison between results from the sweating thermal manikin THL method and the direct ventilation method have been made yet. Finally, thermo-regulation models may be used to evaluate the effect of ventilation in various environmental conditions and real-life scenarios that firefighters would encounter. Each of these methods will be discussed in detail below.

Total heat loss (THL) using a sweating thermal manikin

Evaluating ventilation on the manikin level allows for the entire clothing ensemble to be assessed, including pockets, accessories, boots, gloves, hood, and helmet. This method takes into account the amount of body surface area covered by different materials and various numbers of layers, the fit of the garment, and the increased surface area for heat loss.^17,18 The sweating manikin includes separately controlled heated sections with built in pores for sweating.¹⁹ A typical sweating manikin that is often used contains a fluid pre-heater inside the manikin to ensure the water coming through the pores is maintained at the proper temperature, according to the standard test methods. The manikin is connected to a computer software program which monitors the heaters, fluid temperature, and flow set points for each section.¹⁸ This allows for an evaluation of dry air exchange (thermal resistance) and wet moist vapor exchange (evaporative resistance).

Thermal resistance is tested in accordance with ASTM F1291-10 Standard Test Method for Measuring the Thermal Insulation of Clothing Using a Heated Manikin,¹⁵ which determines the insulation value of a clothing ensemble. ASTM F1291-10 provides the measurement of resistance to dry heat transfer from a heated manikin to a relatively calm, cool environment.¹⁵ Dry heat loss measurements are computed into thermal resistance values for the ensemble. Measurements should be taken on the ensemble prior to adding vents and once ventilation is in place to compare the effect of ventilation on heat loss.

To measure evaporative heat loss, ASTM F2370-10 Standard Test Method for Measuring the Evaporative Resistance of Clothing Using a Sweating Manikin is used.¹⁶ The manikin temperature is the same as ASTM F1291 (35℃) to ensure there is no dry heat transfer occurring as well.¹⁶ Evaporative resistance measurements can be combined with thermal resistance measurements to determine the overall predicted THL for the suit when ventilation is added.

THL of the full clothing ensemble can be determined based upon the thermal and evaporative resistance measurements. It is an effective measure for estimating the overall ventilation effect on heat loss throughout the garment. The measurements taken from the sweating manikin will provide the most comprehensive data for evaluating the benefit of ventilation to reduce heat strain in clothing. The type of heat loss (dry or wet) can be compared, along with the effects of forced convection and pumping, by increasing the wind speed and making the manikin walk. With this method, the researcher is able to isolate which condition contributes the most to an increase in heat loss. Furthermore, the distribution of the sweating thermal manikin in multiple zones should allow some evaluation of local effects of ventilation. Depending on vents size and distribution, compared to the manikin zones, differences may be found in heat loss change between the zones, indicating localized ventilation effects.

Thermoregulation modeling

Time and expenses do not often allow for sweating manikin testing in every environmental condition possible. Instead, thermoregulation or physiological modeling can be used to determine the human response while wearing a particular garment, but in various temperatures and humid environments. There are many different thermoregulation models including those developed by Gordon et al.,²⁰ Konz et al.,²¹ Wissler,²² Fiala et al.,²³ and Huizenga et al.²⁴ Thermoregulation models can be beneficial for structural turnout ventilation research. Data taken from sweating manikin testing can be input into the RadTherm® software, developed by Thermoanalytics (Calumet, MI, USA) to simulate what would occur in various environmental conditions for each type of protective clothing, or type of ventilation design. This would provide a more comprehensive database on the effects of ventilation in various ensembles and conditions.

Trace gas dilution method and ventilation index

A different procedure for estimating the volume of air flowing through the micro-environment of clothing, under various working conditions, is the use of a trace gas dilution method to calculate a ventilation index. The original method, developed by Crockford et al.,²⁵ requires two techniques; both a measurement of the microenvironment volume and a tracer gas to measure the rate of air exchange.^10,25,26 Calculations between these two techniques results in the air exchange properties of a garment which can be described in terms of a ventilation index.¹⁰ Calculation of the ventilation index is illustrated in:

Q = R \times V

(1)

where Q = ventilation index (liters/minute), R = air exchange rate (min⁻¹), and V = micro-environment volume (liters).¹⁰

The ventilation index, Q, can be used as a quantitative rating scale, as shown in Figure 2, which enables comparisons between different garments. The index shows the required liters/minute needed for effective ventilation in various types of clothing with specified activity levels and wind speeds. This would be beneficial research to determine how different ventilation designs perform compared to one another, as well as to compare other types of protective clothing. The ventilation index may also be used as an index of volume air exchange.

Figure 2.

Ventilation index values of selected garments on a linear scale reproduced from Birnbaum and Crockford.¹⁰

Before the introduction of the trace gas dilution method for clothing ventilation measurements, air exchange rates were measured in a similar way for the ventilation of rooms and buildings.^10,25 Tracer gas methods for clothing involve the use of tubing systems to distribute and sample a tracer gas in the microclimate to determine the ventilation to the environment. These tubing systems are designed to take equivalent samples at different body parts and can be placed on a manikin or human for wear testing. The distribution system for supplying the tracer gas is placed between the undergarments and the clothing system being worn. These tubes are perforated with small holes, proportionate to the surface area of the body which it covers, to distribute the tracer gas.¹⁰ The sampling tubing system is placed on the surface of the skin where it continuously samples the microclimate air through a set of equal length tubes, which are only open at their distal ends.¹⁰ The trace gas method leaves the head, hands and feet free of tubes; areas where clothing is not usually worn.²⁶

Dukes Dobos et al. adapted Crockford et al.’s method in order to consider regional changes in garment designs as separate sections apart from total ventilation.^4,25 This method allows for local modifications to be made to the garment. They created the second technique for measuring clothing ventilation. This technique is simple and fast, measuring the air exchange rate and air speed.²⁷

Lotens and Havenith further developed the Crockford et al. tracer gas technique using a continuous flow of argon (Ar) instead of nitrogen (N₂).^28,29 Lotens and Havenith developed a steady state method, whereas the Crockford et al. method was an unsteady state method.¹² A continuous flow of tracer gas is used to look at the steady state dilution of the gas in the microclimate. Because the microenvironment volume does not need to be calculated with this method, and due to its simplicity, it is easier to use in research.

A comparison of the two tracer gas dilution methods, which was conducted by Havenith et al.,²⁹ showed good reproducibility, validity, and usability of both methods. A major problem with the Crockford et al. method, however, is the technique used to measure the microenvironment volume. This technique is both unreliable and cumbersome. Further limitations of the Crockford et al. method include the limited time constant of the measurement apparatus which causes an upper limit to the ventilation that can be reliably measured.²⁹ The Lotens and Havenith method also has disadvantages. The distribution of the tracer gas over the entire skin surface becomes a problem factor at very high ventilations (above 1000 L/min).

Comparison of measurement methods

Compared to the sweating manikin technique alone, the trace gas method for measuring ventilation has some advantages. Its most notable advantage is speed of testing. A measurement can be taken in three to six minutes, instead of one to two hours with a sweating manikin.^25,26 The tracer gas method can also be used to measure ventilation at localized body segments, allowing for determination of effective vents placed in specific areas of the garment,^12,29 and it can be used to measure directly on humans wearing the clothing. A sweating manikin, however, will provide thermal and evaporative resistance measurements at various locations of the body, broken down into the zones of the manikin. An advantage of the sweating manikin technique is the calculation of a predicted THL value which relates directly to overall total heat loss and can be compared back to sweating hot plate THL on the fabric level. The sweating manikin also produces both dry and wet heat loss, allowing for the separation of convection and evaporation differences. Both methods have their advantages and disadvantages and both are beneficial. A need exists for future research to compare both methods and validate them against one another.

Examples of ventilation in protective clothing

It is important to understand previous research conducted on ventilation in order to know what strategies and measurement techniques have proven effective for various types of protective clothing. Previous findings from other areas of protective clothing, such as industrial, military, outdoor and sports apparel, can be used to determine what types of ventilation designs could be incorporated into structural firefighter turnout gear.

Ventilation in industrial protective clothing

Thermal protection can be important for the clothing of industrial employees such as miners, soldiers, and pilots. These types of thermal protective clothing may also lead to heat stress of the wearer, depending on exact requirements. New materials and design improvements have been introduced in some occupations to help eliminate the accumulation of excess heat and moisture within the microclimate of worker's clothing.^30,31

In order to determine how ventilation affects the microclimate humidity during light exercise, the ventilation index was measured in three body locations while wearing five different work shirts.³² The study found the chest had the greatest ventilation and the upper arm had the lowest.³² This finding shows that microclimate ventilation differs among the regions of the body due to varying air currents caused by body movements, respiration, fabric drape, and thickness of air layers.³² For these reasons, it is important to measure clothing ventilation at various body regions, not just air permeability of the fabric, as to attain a realistic evaluation of the clothing system.³²

A direct ventilation measurement technique was used to evaluate four helicopter pilot suits for their thermal comfort properties. Four suits were evaluated including two single layer suits, a double layer suit, and a membrane based garment. Results showed that both single layer suits had the fastest rates of air exchange compared to the double layer and membrane based ensembles.³¹ The two air permeable suits had an air exchange rate of 0.058 R min⁻¹ and 0.014 R min⁻¹, respectively.³¹ Results showed the suit with the loosest fit and larger neck, ankle, and wrist openings provided greater area for air to exchange, hence why it performed so well.³¹ However, a single layer suit does not provide enough layers and air gaps for adequate thermal protection. The loose fit also impedes the ergonomic function of the garment. Therefore, it is best to increase air exchange by changing fabric permeability and adding garment openings.³¹

Ventilation in sports and outdoor apparel

Ventilation features are widely used in the sports apparel industry to release body heat and moisture.³³ The first example is vertical ventilation of the side seams of a t-shirt. In this study, mesh fabrics were studied in sports t-shirts and evaluated on a thermal manikin for their thermal insulation properties. T-shirts that incorporated mesh fabric in various locations, including the chest, back, and vertical side seams, and at different widths, were evaluated.³³ As the area of the mesh fabric increased, the dry thermal insulation of the t-shirt decreased, showing an increase in heat loss, as would be expected. The t-shirts with mesh designs placed in the vertical side seams were considered to be the most ideal for releasing body heat.³³

Garments in the outdoor clothing arena may be looked at also. Common ventilation designs in this area can be found on outdoor jackets in the form of “pit zips” and eyelet openings in the underarm region. Ruckman et al. conducted a ventilation study on outdoor jackets using three different designs of venting: “pit zips,” “venting pockets,” and “venting back.”⁶ The “pit zips” were implemented in the underarms as this region demonstrated the greatest perspiration in preliminary research of the study.⁶ Figure 3 illustrates the design of the “pit zips” in outdoor jackets.

Figure 3.

Illustration of “pit zips” design in outdoor jackets.⁶

These vents would induce forced convection which would draw in more air through the open vents and allow for greater air and moisture exchange to the outer environment. This design also has the potential to maintain a sufficient amount of protection, even when active, as liquid should not penetrate the area during normal active motions.⁶ This study on outdoor jackets found the chest area had the highest skin temperature readings. The authors concluded, therefore, that the chest area should be vented further to increase heat loss in this region.⁶

Ventilation in structural firefighter turnouts

Research on the ventilation of firefighter turnout gear includes active ventilation strategies, the effect of different moisture barrier materials, and the opening of garment cuffs to reduce heat strain. An original study by Reischl and Stransky evaluated a new structural firefighter prototype which contained a larger neck opening, ventilation spacers, bellows in the sleeves, and vertical ventilation spacers in the pants.² The vented prototype had lower heat buildup when compared to the closed clothing system; this difference was attributed to the zippered openings located in the seams of the pants.² Design changes were implemented by Reischl et al. in a second study to create an advanced prototype with increased heat dissipation characteristics.¹³ This study included shortening the coat, opening the collar, adding ventilation openings and spacers throughout the coat and pants, and increasing the collar height.¹³ Conclusions showed improved ventilation in the arm and back regions, but no significant improvement in the chest or leg regions of the prototypes tested.¹³ The authors suggested increasing the number of ventilation openings in the coverall legs to increase heat loss and consider the addition of vertical ventilation openings in the trousers of firefighter turnouts.¹³

A study which evaluated the opening and closing of the sleeve and pant cuffs, opening of the collar, and the replacement of a belt with suspenders, also found positive results for heat loss.⁴ A trace gas dilution technique was used to measure localized ventilation around various regions of the body. This study found the greatest improvement in heat loss was due to ventilation in the chest area when the coat was opened at the collar and in the front.⁴ Opening of the sleeve cuffs did not contribute a great amount to ventilation but the pant cuff opening lead to an increase in ventilation in the leg and crotch areas.⁴ The design of firefighter turnout gear has continuously changed over time; therefore, ventilation research on current designs is necessary.

Firefighter user needs

Firefighter working conditions

Firefighting activities account for only 10 to 20% of all duties firefighters perform.³⁴ Up to 99% of a firefighter’s time may be spent performing other firefighter working tasks where the threat of heat and flame is nonexistent.³⁵ These other firefighter working tasks include responding to goodwill calls, emergency medical services, motor vehicle accidents, performing vehicle extrications, and conducting search and rescue operations.³⁶ Under these conditions, extreme heat and flame exposure does not pose a threat and the heavyweight self-contained breathing apparatus (SCBA) is not worn.

Overhaul is a working condition performed by firefighters once a fire is under control. This activity involves opening walls, tearing down ceilings, and extinguishing any remaining hot spots.³⁷ In this condition, thermal strain increases greatly as heavy physical activity is performed in an extreme hot and humid environment. Vents with lower exposure level risks may be deployed for improved heat loss in this condition.

The potential for ventilation exists in all firefighter working conditions but is most limited during firefighting operations, within a live fire. Clothing ventilation is most readily applicable during normal working conditions which firefighters perform the majority of their working time. During activities such as vehicular extrication or search and rescue, vents would be beneficial for heat loss and pose no threat for heat and flame exposure.

Firefighter turnout ensemble

A traditional, United States, firefighter’s turnout ensemble consists of the coat, pants, helmet, hood, SCBA, gloves, and boots. Coat and pant garments consist of three component layers: a durable, protective outer shell that serves as the first line of defense; a thin inner layer known as the moisture barrier which prevents water and some chemicals from penetrating through; and the thermal liner which is directly in contact with the firefighter’s base layers. The thermal liner provides the majority of thermal protection within the three-layer composite system.

Activities performed by firefighters often involve carrying, pushing, pulling, holding, turning, wielding, throwing, or lifting; all of which can lead to overexertion.³⁸ The thermal insulation provided by the turnout suit far exceeds what is necessary for the majority of tasks the firefighter performs that are not fire-related, therefore creating a negative impact on the wearer’s comfort between 80–90% of their working time.³⁴

For firefighters, the consideration of additional accessories and reinforcement of materials is vital for improving heat loss. The SCBA is required for respiratory protection. It provides critical respiratory support and protection but can also be very cumbersome and adds a substantial amount of weight to the physical work performed by the firefighter.^39–42 Not only does the SCBA increase the amount of heat produced by the body, it sits in the region of the body where the sweat rate is highest.⁹ The weight of the SCBA has been identified as the factor which negatively affects physiological strain the most, along with its impermeability and fit, by increasing the heart rate and oxygen consumption rate.^39,40,42

The SCBA should be worn during firefighting and hazardous materials incidents including overhaul. When firefighters perform other tasks such as vehicle extrication, rescue operations, and even fire investigations, the SCBA is not part of the protective personal equipment (PPE) protocol.⁴³ Therefore, vents placed in the back and other regions where the SCBA would eliminate their benefit, is a limitation during firefighting conditions only, which are between 5–20% of a firefighter’s working time.

Application of clothing ventilation in structural firefighter turnouts

Based upon ventilation designs in other types of protective clothing and performance apparel, implementation into structural firefighter turnout suits depends upon the working condition and required level of protection. For example, an open vertical vent or “pit zip” in the underarm region would not be practical during live firefighting conditions. This vent would be potentially beneficial during all other normal working conditions. There are some types of ventilation, however, which may be directly applicable for structural firefighting operations such as rivet vents in outdoor jackets or mesh vents on the inside layers. Future research is needed to evaluate the heat loss potential of these vents in structural firefighter turnout suits before being tested on the human wear level.

Other considerations when designing vents into structural firefighter turnout suits include environmental conditions and additional equipment worn on top of the suit. Extreme hot or cold environmental conditions could eliminate the benefit of ventilation openings. The ability to open or close vents may also be effected by other components of the turnout ensemble such as the SCBA, additional tools, or gloves. It should be noted the SCBA closing off vents is actually a beneficial design feature that enhances safety during firefighting operating conditions. Ventilation would only be activated when the SCBA was not required for protection. The tactics and training procedures used by fire departments should consider these potential issues with new designs and adjust them accordingly.

Durability and maintenance considerations

Currently, there is no research on the effect of ventilation designs on the durability, cleaning, and retirement of structural turnout gear. The addition of ventilation creates the need for a re-evaluation of the durability and maintenance of the newly designed garment. NFPA 1851, Standard on Selection, Care, and Maintenance of Structural Fire Fighting Protective Ensembles, covers the selection, care, cleaning, storage, and retirement of structural turnout gear. Ventilation adds new openings, interfaces, and seams into the turnout suit creating more opportunities for degradation, abrasion, tears, and potential problems for normal cleaning routines. Accessories, such as zippers and Velcro closures may need to be added to ensure protection is not compromised during firefighting activities. Such accessories could create issues when cleaning the garment. To ensure durability is maintained with ventilation, additional accessories would need to be included within the NFPA standard for evaluation during both routine and advanced visual inspections. These inspections include the evaluation of all parts of the full ensemble for rips, tears, cuts, missing hardware, thermal damage, shrinkage, and closure system functionality.⁴⁴

Functional design, wearability, and ergonomic effects

When placing vents within protective clothing, consideration must be given to the overall functional design of the garment, including the end use wearability and range of motion. Functional clothing includes all garment assemblies which are engineered specifically for a pre-defined performance, over and above the garment's normal functions.^45,46 Structural firefighter gear falls into the protective category of functional clothing where the garment's properties determine life and death for the user.⁴⁵ Essential requirements for the functionality of turnout gear include sufficient low weight insulation, reduced bulk, improved ergonomic designs, and proper mobility.⁴⁵

The goal of any new development for the functional design of protective clothing must consider improving the ergonomics and overall comfort of the clothing system, while maintaining the necessary protection.³⁴ For firefighters, it is imperative their gear be ergonomically designed in order to crawl, crouch, climb, and manipulate their way through various physical obstacles.⁴⁶ The ability to perform these activities should not be reduced by the addition of ventilation into the clothing system. Therefore, some form of ergonomic assessment should be performed to evaluate the new ventilation designs in structural turnout gear.

Research strategies for venting firefighter turnouts for heat strain reduction

There are various strategies for designing ventilation systems into protective clothing garments. If the protective clothing involves multiple layers, as is the case for firefighter turnout gear, ventilation of all or only one layer should be explored to determine which strategy is most effective. Placement of the vents within the garment may be dependent upon the amount of heat buildup and sweat which occurs on the specific areas of the body. Further research is necessary to determine this relationship. Extra layers and accessories added to the garment for other purposes, such as radio pockets, suspenders, hook closures, and zippers on firefighter suits specifically, should be considered for their effect on the ventilation system.

Active versus passive ventilation

A ventilation design may be active or passive depending upon the type of activity being performed and the necessary protection required. An active vent is one which can be opened or closed, depending upon the work scenario and its protective requirements. A passive vent is always in place, working to release heat at all times and does not have an open or close feature. In Reischl and Stransky’s study on ventilation of firefighter turnouts,² active vents were placed along the inseams of the trousers that could be “opened” or “closed” by using zippers and Velcro closures. These vents would be active during normal working operations, where protection from heat is not necessary, and closed during firefighting activities where direct exposure to flame is a threat. During operations leading up to fighting a fire, such as placing a ladder, unwinding a water hose, and connecting to a fire hydrant, vents can be active, or open. When the firefighter is ready to enter the burning building or fight a fire, the active vents may need to be closed depending upon the type of protection required. Further testing to determine the thermal, liquid, and chemical protection of vented turnout suits is needed.

Examples of passive vents are grommets, or eyelets, which form small, open circular holes for air to flow through. This type of ventilation feature is often found in sports apparel jackets and pants. These tiny, open holes created by the grommets are most often found in the underarm and crotch areas. While these holes may directly expose the base layer of the wearer, they could be implemented into a structural firefighter suit. The placement in the underarms and crotch area should help maintain protection, as those areas are normally covered during standing and walking positions. The ability of this strategy to maintain both thermal and chemical protection would need to be evaluated.

In structural firefighter turnout gear, one such example of a passive vent is a vented moisture barrier.⁴⁷ The middle layer of the turnout gear, the moisture barrier, may be vented around the middle of the torso. A mesh liner is put into place to allow for air and moisture vapor to transfer through. The moisture barrier is often vented in this manner as to maintain liquid protection but increase its ability to allow sweat evaporation to occur.⁴⁷ As the patent for a protective garment with vent features by Curtis shows,⁴⁷ these passive vents may be placed in almost any area of the coat or pants including the upper back, sleeves, neck, knees, crotch, and seat of the pants. The Curtis patent involves placing a pair of vertical vents in the back of the moisture barrier which are formed by gaps, cuts, or openings formed through the fabric.⁴⁷ To provide at least some protection in all positions, the design may include some features, such as loops, which limit the expansion of the vent.⁴⁷ These passive vents may be placed in the moisture barrier, outer shell, or both layers.

Full versus partial layer ventilation

As described in the Curtis patent for protective garment ventilation, structural turnout gear may be vented in one layer or through all three.⁴⁷ The moisture barrier layer poses the greatest hindrance for heat loss, as Cui et al. showed in their study on the water vapor transmission rate of firefighter clothing.⁴⁸ The thermal liner, however, has the highest thermal and evaporative resistance of the three layers in the base composite. Ventilation of both the outer shell and moisture barrier may be explored as well. To determine the layer which is most effective for ventilation, research should be conducted in which all three layers are examined individually and together, in various combinations.

It should be noted that the focus of this review article is on traditional, three-layer, structural firefighter turnout suits. There are other types of suits worn by firefighters for specific work operations or in other geographic regions which involve various differences in fabrication and construction. Both the number of layers and the type of material being vented should be considered.

Garment placement of ventilation

Ventilation should be placed in a garment with considerable thought and attention. Areas of the body where sweat occurs more rapidly and at higher volumes should be studied and chosen for ventilation placement. The effect of pumping and forced convection on the placement of vents should not be ignored either. Finally, each garment includes individualized accessories and reinforcements that add layers and different material types to the base composite. These additional layers can impact the effect of venting.

Body sweat mapping studies can be a useful tool for determining where the greatest concentration of perspiration occurs. Smith and Havenith conducted a body sweat mapping study to investigate regional sweat rates at two exercise intensities (55% and 75% VO_2max) in moderately warm conditions (25℃, 50% RH, 2 m s⁻¹ air velocity) on nine male runners.⁹ For ventilation specifically, this study indicates where the greatest amount of sweating occurs. As exercise intensity increased, results from the body sweat mapping study show the sweat rate of all areas of the body, except for the feet and ankles, increase significantly.⁹ The regions of the body with the greatest sweat rates were the upper, middle, and lower back areas, followed by the forehead which had the highest sweat rate for the head region.⁹ Figure 4 shows the absolute regional sweat rates from the Smith and Havenith study at the 75% VO_2max exercise rate.

Figure 4.

Absolute regional median sweat rates of male athletes at 75% VO_2max exercise intensity.⁹

These high sweat regions align with other studies which found the torso and lumbar regions to have the greatest sweat rates of the whole body.^49–51 Based upon these results, ventilation systems should be placed in these areas to increase the cooling power of the body.

When placing vents within the garment design, their effect on heat loss through pumping, or movement, must be considered. If placed in the wrong area the pumping effect could be hindered or all together removed from the design of the garment. Ventilation should be added to increase the effect of pumping and convective heat loss during movement, such as walking, bending, and crawling. In the firefighter turnout gear ventilation study, conducted by Reischl and Stransky,² the lower portion of the back of the coat incorporated a “bellows” design as can be seen in Figure 5. This bellows design should move counter to body motion and create air movement in the location it was placed.² This is an example of pumping effects in a location where sweat rates are at their highest on the body.

Figure 5.

Bellows ventilation design of structural turnout gear.²

Another study, which assessed bellows ventilation of clothing, was performed using a human wear trial on one subject to measure skin and microclimate temperature, along with humidity and heat flow.⁷ The effectiveness of bellows ventilation was demonstrated as the results showed a significant increase in temperature and humidity when motion was suddenly stopped. When walking was resumed, the temperature and humidity dropped and the heat loss increased.⁷ Literature shows the ability of pumping or bellows ventilation to significantly increase heat loss through sweat evaporation in protective clothing.

Challenges associated with ventilation in structural firefighter turnout gear

While many of the ventilation strategies described in previous research have proven effective for the relief of heat stress, the most important aspect of firefighter turnout gear, or any protective garment, is its protection. Protective clothing provides a wide range of protection against liquids, steam, heat, flame, chemicals, and toxic gases. A balance must be struck between protection and comfort. Therefore, ventilation designs should not compromise the integrity of a garment by reducing its protection. Three types of protection: liquid, thermal, and gas and vapor will be discussed below along with the corresponding test for evaluation.

Liquid protection

Firefighters and emergency first responders face the hazard of liquids in the form of chemicals, blood-borne pathogens, and hot water. A recent resurgence in added protection for firefighters against chemical, biological and radiological hazards occurred following the terrorist attacks of the last decade.⁵² Therefore, venting applications should maintain a sufficient level of liquid protection of the protective clothing. Some ventilation applications directly expose the wearer's skin in the case of open, zipper vents on the outside layers of a structural firefighter suit. These zipper vents would remain open during, “regular work and support operations,” but must be closed during firefighting activities.¹³

Not only is it important to protect the wearer from toxic chemicals and blood-borne pathogens, but water is a hazard as well. In the act of firefighting, water poses a threat in the form of burns. The majority of burns firefighters suffer from are steam and hot liquid burns.⁵³ These occurred when misuse of protective equipment led to gaps in the sleeve or face mask, causing burns to the hands and wrist, as well as where the mask meets the face and ears. It is important that as ventilation designs are added, such as open eyelets or open zipper vents, that training procedures be put into place to ensure proper protection is maintained during firefighting activities/exercises. These designs should also be evaluated for their protection before being implemented into protective clothing.

Water not only threatens the firefighter in the form of burns, but can also impede the benefit of ventilation by allowing the suit to take on extra weight in the form of liquid. This added weight from water in the suit would lead to a higher metabolic production rate by the body, increased heat storage, and lower heat loss. Therefore, the implementation of ventilation into protective clothing must consider how the specific design will impact liquid protection.

Liquid-tight integrity test

To demonstrate adequate liquid protection of ventilation designs, a liquid-tight shower test should be performed. NFPA 1992, Standard on Liquid Splash-Protective Ensembles and Clothing for Hazardous Materials Emergencies, specifies testing procedures for liquid-splash protection of protective garments during HAZMAT incidents.^54,55 For structural firefighter gear specifically, this standard specifies a “shower” test to demonstrate liquid-tight integrity. Proper liquid protection of protective clothing should prevent the penetration of such liquids as hot water, chemicals, blood, and body fluids from contacting the wearer’s skin.⁵⁶ All clothing by design possesses some form of openings, particularly in the interfaces between layers, in order to don and doff.⁵⁶ When adding ventilation designs into protective clothing, it is vital to determine if these openings, whether a mesh fabric, zipper, or eyelet, have an effect on the liquid integrity of the protective garment. If any evidence of liquid on the garment is present at the conclusion of the test, be it visual, tactile, or absorbent toweling, the garment should be considered a failure for liquid-tight integrity.⁵⁴

Thermal protection

In the case of firefighter turnout gear, thermal protection from heat and flame is the utmost priority for the design and use of the garment. Thermal protection must not be sacrificed for the benefit of heat loss. Ventilation strategies, such as a mesh fabric layer in the moisture barrier, can improve water vapor transport to the outer environment without compromising protection. The firefighter must be protected from both convective heat, which is transferred through air exchange, as well as direct flame during a flashover condition. Open vents, which directly expose the wearer's skin, would pose a danger for both convective heat and flame to burn the wearer if actively “open” during firefighting operations. Therefore, it is important to consider the type of vent and placement on the suit for firefighter protective clothing, not only for liquid protection, but for thermal protection as well.

Flash fire manikin evaluation system

To ensure thermal protection of the firefighter turnout has not been compromised by the addition of venting systems, a thermal protective performance test on the ensemble level should be performed. A Flash fire manikin can be used to evaluate the full ensemble, including the coat and trousers, as well as the boots, gloves, hood, and SCBA mask (worn without SCBA cylinder). Testing should be completed according to NFPA 1971, Standard on Protective Ensembles for Structural Fire Fighting and Proximity Fire Fighting.^57,58

Vapor and aerosol protection

Ventilation designs, especially those which are actively open to the environment, cause concern surrounding toxic industrial chemicals and airborne pathogens. Firefighters are routinely exposed to gases and smoke particulates (aerosols) that contaminate the skin if absorbed through the clothing layers. Recent studies have linked the effects of smoke exposure in firefighters to higher risks of cancer and cardiovascular disease for firefighters.^59–62 Previous studies have found the airborne exposure levels of firefighters, even during overhaul activities, to be greater than the occupational regulatory limits established by Occupational Safety and Health Administration (OSHA).^59,63,64 The firefighter turnout suit should not allow hazardous levels of contaminants to be absorbed by the skin, nor should the skin be directly exposed to hazardous levels of toxins during firefighting activities. Therefore, turnout suits with ventilation designs should be tested for their chemical protection, taking into account short term and preferably also long term toxicity effects of the vapors. Those designs which provide benefit towards heat loss and the greatest protection should be implemented into final prototypes.

Positive versus negative pressure ventilation

Clothing ensembles are most often negative pressure systems, except those for the highest levels of chemical and biological protection, such as NFPA 1991 impermeable protective garments, which have additional ventilation from exhaust air or air fans and are designed to be closed, fully encapsulated, positive pressure garments. Structural firefighter turnouts are negative pressure systems in which air from the environment is drawn into the suit naturally, through the collar, cuffs, hems, and pant leg openings, because of the continuous motion and pumping effects. When ventilation openings are implemented, more air can be drawn in from the outer environment and more air from within the clothing system may be transferred outward. On the other hand, when ventilation is added into the suit, whether it be a mesh vent, larger neck opening, or fully vented zipper openings, the pressure difference between the clothing system and the environment decreases. These openings in the fabric and garment design for air exchange decrease the flow resistance between the outside environment and inside of the clothing.

For firefighters, this leads to both thermal and chemical protection concerns, especially as the turnout is often considered as a low class chemical protective suit. In a negative pressure system, the wearer could be more easily exposed to chemicals, flame, liquid, and toxic vapors. Therefore, the specific strategies used for venting a structural firefighter turnout suit must be carefully evaluated to ensure protection is not compromised. Some research however, shows the addition of air flow into an impermeable system actually increases the protection factor of protective clothing,⁶⁵ probably because of the reduction in negative pressure in the clothing system.

A study on CBRN protective clothing evaluated the effects of microperforations in an impermeable liner system.⁶⁵ When evaluated through the man-in-simulant-test (MIST), the microperforations did not seem to have any effect on protection.⁶⁵ In fact, results showed the air flows underneath the suit were low and the pressure distributions were significant for ventilation.⁶⁵

MIST test

The MIST serves to evaluate full protective clothing ensembles for their ability to provide adequate chemical protective performance.⁶⁶ MIST testing involves human test subjects wearing full protective ensembles in a specifically designed MIST chamber where they are exposed to a low toxicity simulant for chemical warfare.⁶⁶ When considering structural firefighter turnout gear, which is often worn as a low-class chemical protective ensemble, ventilation designs should be evaluated for their impact on chemical protection performance. This testing is ideal for evaluating if the ventilation strategies, zippers, and Velcro added for vent coverings are sufficient for maintaining the garment's original chemical protection, the results should be compared to the protection that currently available garments provide.

Conclusions

Previous research studies show that ventilation of structural firefighter turnout gear, as well as other types of protective clothing, can be an effective tool for improving human thermal comfort. By reducing the amount of heat buildup within the clothing microclimate, through increasing air exchange and sweat evaporation, heat strain can be reduced. For firefighting in particular, it is essential the thermal protection be maintained, while balancing the amount of heat buildup within the turnout suit, in order to reduce the amount of heat strain experienced by the wearer. Various ventilation strategies have been implemented in other areas of protective clothing which achieve this benefit towards heat loss and show promise for its implementation in structural firefighter turnout ensembles.

Future research is necessary in order to determine which types of ventilation designs are most effective for structural firefighter gear, and protective clothing in general. Active and passive ventilation strategies should be evaluated, along with full and partial layer venting. Research is needed to determine how the addition of ventilation impacts thermal, liquid, and chemical protection to the wearer. Various measurement systems including the sweating manikin and trace gas dilution methods should be compared and validated.

Research to determine ways of reducing heat strain in firefighter turnout gear is important for decreasing injuries and fatalities caused directly, or indirectly, by fatigue, decrease in cognitive performance, heat exhaustion or stroke. The buildup of heat inside clothing could be detrimental to the health of the wearer to the same extent, or more, than the hazards the protective clothing was supposed to protect against. Garment ventilation design is one avenue for heat loss that should be explored. This review of ventilation literature and its potential inclusion in structural firefighter gear serves as a prelude to further ventilation research.

Footnotes

Funding

This work was supported by the Department of Homeland Security, Federal Emergency Management Agency, Fiscal Year 2012, Assistance to Firefighters Grant program, fire prevention and safety (grant number EMW-2012-FP-01185).

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