Quantitative and sensory evaluation of odor retention on polyester/wool blends

Abstract

The odor sorption and emission characteristics of fabrics affect their functional performance, particularly for sportswear. This study systematically evaluated the impact of wool in polyester/wool blends on body odor retention properties. The odor sorption and emission of polyester/wool blend fabrics were quantitatively evaluated by an infrared-based spectrometer and by sensory testing. It was found that wool had the highest odor sorption capacity among all the test fabrics. Both odor sorption and emission are greatly affected by the presence of wool in the blends. Fabrics containing 20% wool had substantially reduced odor intensity compared to 100% polyester. The body odor retention properties of the 20/80 wool/polyester blend and the 100% polyester fabrics were also compared by wear trial, which further confirmed that the wool/polyester blend effectively reduced body odor intensity compared to 100% polyester.

At low levels of body odor, fabric blends with at least 20% wool were shown to perform well, as they may not have reached their sorption capacity. Under higher and more extreme levels of body odor, where sorption plays a more important role, 100% wool showed higher sorption capacity than the blends.

The data and methods presented in this paper provide a basis for optimizing the fiber composition of active wear garments with respect to odor.

Keywords

chemistry fabrication materials odor polyester properties sorption testing wool

It has been long accepted that sweat is initially odorless but becomes odorous after being degraded by certain bacteria on the surface of human skin.¹ It was reported that body odor intensity increased with increasing numbers of aerobic bacteria, particularly corynebacteria.² Therefore, various strategies have been employed by the textile industries to control odor development within textiles. The most common method is to apply biocides to impart antimicrobial properties to the fibers and fabrics.^3–5 Different types of antimicrobial materials have been coated on the surface of the fibers and fabrics, including nano titanium dioxide/nano silver/water-borne polyurethane,⁶ silver chloride,⁴ quaternary ammonium compounds,^7,8 polyhexamethylene biguanides (PHMBs),⁹ triclosan^10–12 and chitosan.¹³ Currently, silver, PHMB, quaternary ammonium compounds and triclosan are used in commercial antimicrobial textiles consisting of synthetic and natural fibers, while chitosan is still in the development stage. Depending on fiber type, these biocides can be used either as finishing agents or incorporated into the fibers during extrusion.⁵ The imparting of antimicrobial properties on fiber or fabric could inhibit the growth of odor-causing bacteria, which could prevent odor build-up within the textile. Other methods for controlling body odor include finishing the textile materials with fragrance oils to mask the odor,^14,15 and also finishing the textile material with cyclodextrins and active carbons to absorb odor.^16,17

However, recent research has found that the bacterial number per square meter is not necessarily a good predictor of the odor intensity of worn textiles. Odor intensity is affected by the fiber type from which the garment is made.¹⁸ McQueen et al.¹⁸ found that bacteria survived better on wool than both cotton and polyester fiber, yet wool was assessed by a panel of assessors to have superior odor resistance compared to the other fiber types. Therefore, understanding the relationship between fibers, fabrics and body odor is important, especially with respect to the risk to public health from emerging antibiotic-resistant bacterial strains linked to the increased use of antibacterial and antibiotic products.^19,20 More especially, there are risks of non-pathogenic bacteria developing resistance through the use of common household antibacterial products, including deodorants, and such resistance is able to be transmitted to pathogenic bacteria.²⁰ Control of body odor through methods that do not alter a person’s natural microflora is also desirable.

Herein, because of the excellent natural odor resistance of wool, this study aimed to evaluate how wool percentage in polyester/wool blends affects the odor intensity of a garment after wear. Fabric samples were knitted on the same machine from yarns with different blend ratios of polyester and wool. The fabric processing variables were controlled to yield almost identical fabric specifications. The sorption/emission performance of four model body odor compounds was quantitatively evaluated by both an infrared (IR)-based spectrometer and by sensory testing. The body odor retention on the optimized wool blend and polyester fabric was also compared in a wear trial. It is expected that wool blend fabric would be an effective and natural way to minimize body odor in active wear.

Experimental details

Materials

All the yarns were custom made with the required blend ratio of wool and polyester (Merino Ultimate, China). The average diameters of wool and polyester fibers were 18.7 and 18.0 µm, respectively. Fabrics were knitted on the same machine with fabric processing variables controlled so as to obtain similar fabric specifications, as shown in Table 1. These fabrics are typical of those used in base layer clothing.

Table 1.

Description of fabrics

Fabric code	Designed value	Tested value	Wales/ cm^a	Courses/ cm^a	Mass g/m^2b	Thickness/ mm^c	Yarn count/tex
Polyester	100% Polyester	100% Polyester	15.2	21.6	189	0.53	23
10% Wool	90Polyester/10wool	87.3Polyester/12.7wool	17.2	20.0	198	0.52	23
20% Wool	80Polyester/20wool	78.1Polyester/21.9wool	17.2	20.8	200	0.56	23
40% Wool	60Polyester/40wool	57.8Polyester/42.2wool	18.0	20.4	205	0.59	23
50% Wool	50Polyester/50wool	49.6Polyester/50.4wool	18.4	20.8	205	0.61	22
Wool	100% Wool	100% Wool	18.4	20.0	200	0.53	21

AS 2001.2.6—2001.

ASTM D3776-96(2002).

ISO 5084:1996.

Regain measurement

The moisture regain of each fabric was measured by oven drying according to ASTM D2495. In summary, the fabric samples were oven dried at 105℃ for 2 hours. They were weighed for the first time, which was named as the oven dry mass. The fabric samples were then conditioned (20 ± 2℃ with humidity of 65 ± 2%) for 48 hours and reweighed, which was named the conditioned weight. The difference between the oven dry mass and the conditioned weight was calculated as moisture regain. This measurement for each fabric was carried out in triplicate to get an average result.

Spectrometer testing of odor sorption and emission

The odor detection system used in this study was composed of three main parts, namely a gas pump, a sample chamber and an IR-based spectrometer, as shown in Scheme 1. Since the light wavelength absorption of a compound is dependent upon the number and types of its functional groups, there could be several wavelengths that one compound absorbs strongly. Therefore, the wavelength that gave the best resolution, that is, the greatest absorbance compared to background, was chosen for quantitative analysis of odor compounds.

Scheme 1.

Schematic diagram of the odor detection system.

The odor sorption property measurement was carried out as follows. Prior to testing, the fabric was laundered three times to remove any finishes and laid flat to dry, which followed domestic washing and drying procedures for textile testing ISO 6330:2012 (Procedure No. 3N). A total of 2 g of fabric was placed into the sample chamber and the sample chamber was then sealed. A set amount of odor compound was injected into the pump and quickly gasified through the pump to the sample chamber and then circulated in the sealed odor detection system. The real-time odor sorption property of each fabric was then monitored by the IR-based spectrometer.

To monitor the odor emission process, 2 g of fabric sample was placed in a sealed container together with a set volume of odor, which was then kept for 8 hours at 20℃ to reach its odor sorption equilibrium. The fabric sample was then placed into the sample chamber and sealed. The odor emission process was monitored by the IR-based spectrometer.

Wear trial

To further confirm the reduced odor intensity of wool blend, a wear trial was performed of a matched pair of the polyester and the wool blend fabric, which reduces the odor intensity at the minimum wool percentage. Approval to undertake this research project was given by the Human Ethics Advisory Group (HEAG), Deakin University.

Fabric samples were cut into squares, 220 mm × 220 mm, and their edges were overlocked. They were folded in half along the course direction and shallow arc stitched to facilitate fitting the fabric specimen into the underarm area of a T-shirt, with one fabric stitched into the left-hand underarm region and the matched pair stitched into the right-hand side. An in vivo wear trial experiment was conducted with nine healthy participants. All participants were in good health and had not received antibiotics for at least two months. The participants were conditioned for three days before wearing any test fabric, as follows:

no consumption of strong/spicy food for the duration of the study;

showered with soap provided;

no deodorant, perfume or antiperspirant was worn on the exercise day.

The fabrics were laundered three times to remove any finishes and laid flat to dry for fabric relaxation prior to cutting the specimens.

All participants attended an intense spin class for 1 hour wearing the T-shirts with the fabric samples mounted. The fabric pairs were collected after the exercise session, sealed in a sample bag and stored at room temperature in the dark to simulate the home conditions, and to allow the microbial community to grow on the clothing. The fabric samples were subjected to a sensory test 24 hours after the wear trial.

Sensory test

Olfactometry testing was performed according to the Australia Standard “Determination of Odor Concentration by Dynamic Olfactometry” AS/NZS 4323:3:2001, accredited for compliance with ISO/IEC 17025 – Testing. This test was carried out using five trained panelists with a four-port olfactometer (Odormat Series V02) device by The Odor Unit Pty Ltd (Sydney, Australia). The odor bags were transferred to the olfactometer device. The determination of odor intensity with dynamic olfactometry was declared as odor units (ou). The assessment was performed in an air- and odor-conditioned room. The room temperature was maintained between 22℃ and 25℃.

Results

Odor compound selection

The compounds of body odor were reported by McQueen et al.² Three compounds were chosen as representative of body odor, which were acetone, acetic acid and butyric acid. An alkaline odor, ammonia, which commonly exists in human body sweat, was also chosen as a representative odor in this study.²¹ These four compounds were used as model body odors to evaluate the odor sorption and emission properties of the fabric blends.

Test odor sorption and emission using the odor detection system

Odor ammonia retention

Figure 1(a) shows the change in ammonia concentration within the sample chamber over time with the presence of different polyester/wool fabrics. It can be observed that the wool fabric reached its sorption equilibrium after about 80 minutes. The 100% wool fabric sorbed the highest amount of ammonia of all the fabric blends, which was 92%. The 100% polyester sorbed the least, which was 49%. For the polyester/wool blends, the higher the wool ratio was, the more ammonia was sorbed, as shown in Figure 1(b).

Figure 1.

(a) Time-dependent ammonia concentration change for different fabric blends. (b) Odor ammonia sorption after 80 minutes by the polyester/wool blend fabrics.

Results for the ammonia emission after 8 hours storage are shown in Figure 2. The 100% polyester showed the highest ammonia emission among all the fabric samples with an ammonia intensity of 7.3 au after 125 minutes. This is followed by the 10% wool blend fabric with an intensity of 4.8 au, and 2.1 au for the 20% wool blend. Further increasing the wool percentage in the blends resulted in a similar ammonia emission amount.

Figure 2.

(a) Time-dependent ammonia concentration during the emission process for the polyester/wool fabrics. (b) Ammonia emission intensity for different fabric blends.

To calculate how much of the sorbed ammonia was released, the emission percentage was calculated according to the following formula

\begin{matrix} Emission % = \frac{Emitted ammonia}{Sorbed ammonia} * 100 % \end{matrix}

where sorbed ammonia refers to the ammonia intensity reduction at 80 minutes and emitted ammonia refers to the ammonia intensity after reaching its equilibrium at 125 minutes. The calculation result is shown in Figure 3. It can be seen that 25% of the total sorbed ammonia was emitted from the 100% polyester fabric. Introducing wool into the blends substantially reduced the emission percentage. The emission rate reduced to 13% for the 10% wool blend and to 6% for the 20% wool blend. Further increasing the wool blending percentage resulted in only a minor reduction of the emission rate. This result indicates that 20% wool blend is the minimum blending ratio with a greatly reduced ammonia emission rate than 100% polyester and similar emission rate as 100% wool.

Figure 3.

Emission percentage of the sorbed ammonia.

Odor acetic acid retention

Figure 4(a) shows the residual concentration of acetic acid after being sorbed by different fabrics. It can be observed that wool sorbed the highest amount of acetic acid, which was 93%, and reached its sorption equilibrium after about 120 minutes. Polyester sorbed the least acetic acid, which was 54%. For the blends, the higher the wool percentage was, the higher acetic acid sorption was observed, as shown in Figure 4(b).

Figure 4.

(a) Time-dependent acetic acid concentration change for different fabric blends. (b) The relationship between the odor sorption by fabric blends and different wool blending percentages.

The acetic acid emission from the fabric blends was monitored by the odor detection system. As shown in Figure 5, polyester showed the highest acetic acid emission among all the fabric samples, and the acetic acid emission intensity reached 4.0 au after 80 minutes. This is followed by the 10% wool blend, which reached 2.9 au after 80 minutes. Further increasing the wool percentage in the blends resulted in a similar acetic acid emission amount, which was around 1.2 au.

Figure 5.

(a) Time-dependent acetic acid emission for different fabrics. (b) Odor intensity emitted from different fabric blends.

To calculate the percentage of acetic acid emitted from the fabric samples, the emission percentage was calculated according to the following formula

Emission % = \frac{Emitted acetic acid}{Sorbed acetic acid} * 100 %

The calculation result is shown in Figure 6. It can be seen that polyester emitted the highest percentage of acetic acid, which was 2.6% of total sorbed acetic acid. Introducing wool into the blends reduced the acetic acid emission rate. The emission rate reduced to 1.7% for the 10% wool blend, and 0.6% for the 20% wool blend. Further increasing the wool percentage resulted in a minor reduction of emission. This result suggests that 20% is the minimum wool blending ratio to achieve an acetic acid emission rate substantially less than polyester and similar to wool.

Figure 6.

Emission percentage of the sorbed acetic acid.

Odor butyric acid retention

Figure 7(a) shows the time-dependent butyric acid concentration change with the presence of different fabric blends. It can be seen that wool fabric sorbed 75% butyric acid after 200 minutes, which was the highest sorption among all the fabric samples. Polyester sorbed the lowest amount of butyric acid of 41%. It was also found that the fabric blends have an increasing butyric sorption property as the wool percentage in the blends increased (Figure 7(b)).

Figure 7.

(a) Time-dependent butyric acid concentration change for different fabric blends. (b) The relationship between the odor sorption and wool blending percentage.

Figure 8.

(a) Time-dependent butyric acid emission from different fabric blends. (b) Odor intensity emitted from different fabric blends.

The butyric acid emission from fabric blends was monitored by the odor detection system. As shown in Figures 8(a) and (b), the polyester showed a low emission intensity of 1.3 au. The blends showed a slightly increased butyric acid emission intensity from 0.8 to 2.1 for 10% and 50% wool blends, respectively. The 100% wool showed the highest emission intensity of 2.3 au at 80 minutes.

To calculate how much butyric acid was emitted from the fabrics, the emission percentage was calculated according to the following formula

Emission % = \frac{Emitted butyric acid}{Sorbed butyric acid} * 100 %

As shown in Figure 9, all the fabric samples showed similar emission rates for butyric acid. The emission rate for the polyester was 2.5% and was maintained between 1.2% and 2.5% for all the blends.

Figure 9.

Emission percentage of the sorbed odor butyric acid from different fabric blends.

Odor acetone sorption

Figure 10 shows the residual concentrations of acetone after being sorbed over time by different fabric blends. The residual concentration of acetone has been normalized to make it comparable for different fabrics. It can be seen from Figure 10 that the acetone concentration slightly decreased with the presence of wool fabric and a negligible concentration change was found for all the other fabric blends. This result indicates that almost no acetone was sorbed by the fabric blends. Because of this very small level of sorption, acetone emission is not investigated in this study.

Figure 10.

Time-dependent acetone concentration change for different fabric blends.

Sensory evaluation test

Sensory measurement is an important tool in odor analysis and is frequently used in product development and quality evaluation in the food industry, where customer satisfaction is critical. A sensory panel can assess odor in its entirety and compare odor varying in quantity. To evaluate the body odor sorption and emission performance of different fabric blends, the fabric samples were firstly pre-conditioned and then a set volume of body odor compounds mixture of ammonia, acetic acid, butyric acid and acetone was applied onto each fabric sample. These fabric samples were then incubated in gas-tight containers for 8 hours to simulate body odor build-up in a permanent wear situation. Then each fabric was placed in a separate sealed air bag for the sensory test.

Overall odor intensity ratings for the fabric samples are shown in Figure 11. These fabrics showed great differences in odor intensity under the same conditions. The 100% polyester fabric showed the strongest odor intensity with a concentration of 122,300 ou. This is followed by the 10% wool blend with an odor concentration of 31,600 ou. All the other fabric blends showed similar odor concentration of less than 900 ou. This result indicates 20% is the minimum wool blend percentage for achieving greatly reduced odor emission compared to 100% polyester. This result is consistent with what was observed from the ammonia and acid sorption and emission tests using the odor detection system.

Figure 11.

Fabric odor concentration in odor units, “ou”.

Wear trials

Nine male participants were invited to take part in this wear trial and detectable odor intensity was obtained from the worn fabric pairs of five participants. Table 2 lists general information about the five participants in this study.

Table 2.

Participant general information in this wear trial

Participant	Gender	Age (yr)
1	M	27
2	M	29
3	M	29
4	M	32
5	M	34

The odor intensity of the worn fabric pairs from the five participants was measured by olfactometry as shown in Figure 12. In the olfactometry test, an odor intensity below 16 ou indicates almost no detection of odor. Substantial differences were found between polyester and the 20% wool blend fabrics for each of the five worn pairs. For participant 1, the 100% polyester fabric showed an odor concentration of 332 ou. This is compared with a greatly reduced odor concentration of 76 ou for the 20% wool blend. A similar trend was found for the other four pairs. The odor concentration of the remaining four 100% polyester fabrics ranged from 49 to 97 ou, while the odor concentration of 20% wool blends was below 16 ou, which means almost no odor was detected for their matched 20% wool blend fabrics. This wear trial result further confirms that a 20% wool blend can substantially reduce odor intensity compared to 100% polyester under the same wear conditions.

Figure 12.

Fabric odor concentration after the wear trial.

Discussion

The sorption/emission results of ammonia, acetic acid and butyric acid indicated that the wool percentage played a key role in odor retention on the polyester/wool blends. We believe that both sorption and emission are important factors in understanding how the blending ratio reduces body odor, and these two factors prioritize differently under high and low levels of body odor. Under high levels of body odor, sorption would play a more important role than emission. Since 100% wool shows the highest sorption capacity among all the tested fabrics, it would provide the lowest body odor intensity under the same conditions. However, fabric blends would not reach their sorption capacity under low levels of body odor conditions. Hence the emission percentage, which reflects how much of the sorbed odor was released, would be more important to odor retention performance.

It has been reported that the concentration of generated body odor after 1 day wear is within the parts per billion (ppb) concentration range,² which is much lower than the testing odor parts per million (ppm) concentration range. This indicates that the 20% wool blend would not reach its sorption saturation on the first few wear cycles. Odor emission from the fabric was systematically tested in this study. The odor emission results from both the odor detection system and the sensory test show that the 20% wool blend greatly reduced the odor emission compared to 100% polyester and it was comparable to the odor emission of the 100% wool fabric under the same test conditions. This indicates that 20% wool is the minimum blending ratio that is required to reduce the odor intensity of a garment to a level equivalent to 100% wool under the test conditions.

We believe that a combination of sorption and emission of odor compounds, body soil sorption and bacteria biodegradation are responsible for the odor retention on fabrics. One explanation for the effective body odor reduction of the 20% wool blend compared to 100% polyester concerns the odor sorption of the wool fiber. Wool contains carboxyl groups and ammonia groups, which present as peptides. These functional groups provide sorption sites for odor ammonia and acids. The many reactive sites resulting from the basic amino acid groups (i.e. arginine, lysine and histidine) are capable of forming bonds with ammonia and acid molecules, which may contribute to the better odor resistance of wool. Pucher²² also found that wool sorbed acetic acid into the fiber and the sorption rate increased as the relative humidity increased from 20% to 80%. This could be attributed to the fact that moisture has a plasticizing effect on wool, which lowers its glass transition temperature (T_g ∼ 35℃) to a temperature encountered during wear. In contrast, polyester has a much higher T_g (>69℃) and it is not influenced by moisture.²³ Therefore, polyester remains in a glass state and the sorption of body odor acid and ammonia into the fiber is likely to be slow, which may lead to the higher odor emission of polyester compared to wool. We believe that a combination of this high sorption but low emission may be one of key reasons for the better odor resistance of wool blends than 100% polyester in this study.

Another reason for the effective body odor reduction of the 20% wool blend may be attributed to fabric performance on body soil absorption and its subsequent bacteria biodegradation. It was reported that the absorption of oily soils from human sweat is inversely related to the textile fiber/fabric’s affinity to absorb moisture.²⁴ Compared to wool, polyester is highly hydrophobic, which enables it to readily attract oily soils. These soils provide the odorous precursors for microbial degradation and are readily available for transferring into odor.²⁵ Conversely, the hygroscopic wool fiber may absorb fewer non-polar oily soils on its surface.

A possible link between the ability of a fiber or fabric to absorb moisture and its ability to absorb/emit odor was also reported.² In this study, the moisture regain of a series of fabric blends was measured, as shown in Figure 13. It can be seen that the 100% polyester has a very low moisture regain of 0.13%, and the regain of the fabric blends increased linearly with increasing the blending ratio of wool. The 20% wool blend had a regain of 2.5%, and 40% wool blend had a regain of 4.8%. It was reported that above 4% regain, the difference in soiling characteristics was less apparent.² This could be another reason for the obvious odor intensity difference between the 100% polyester and 20% wool blend, and the small difference when the wool percentage is further increased from 20% to 100%, as shown in Figure 11.

Figure 13.

Moisture regain of fabric blends with different wool percentages. Note standard deviations are included for each point on this plot, which ranges from 0.006 to 0.220.

Conclusion

In comparison with the 100% polyester, wool polyester blends effectively increase sorption of three odorous compounds (ammonia, acetic acid and butyric acid) and reduce their emissions as body odor. Among all tested ratios, the 20% wool blend shows substantial reduction of odor intensity compared to the 100% polyester and similar performance as the 100% wool. The substantial odor retention difference was further confirmed through a wear trial. This study opens the door toward a natural way for reducing body odor intensity, which may have promising applications in fields such as sport, business and even space-related activities.

Footnotes

Acknowledgements

The authors acknowledge Julie Zhang for fabric testing of this research and Dr Trevor Mahar for constructive comments on a draft version of this paper.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by Australian Wool Innovation Limited.

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