Lung function and functional exercise capacity in underground semi-precious stone mineworkers

Abstract

BACKGROUND:

Semi-precious stone mining may cause occupational lung disease. The impact of inhaling silica on workers’ exercise capacity has only been partially studied.

OBJECTIVES:

To study lung function, exercise capacity, and identify factors associated with functional impairment.

METHODS:

In a cross-sectional study of 193 current miners from Ametista do Sul, Rio Grande do Sul, Brazil, medical and occupational data were collected. The diagnosis of silicosis was established by the history of dust exposure and chest radiographic findings. All workers performed a spirometry and a 6-minute walk test (6MWT).

RESULTS:

Of the sample 51 (26.4%) had silicosis. Time working in mine was 14.7±8.7 years. Spirometry showed a normal, restrictive or obstructive ventilatory pattern in 75.1 %, 13 % and 9.3 % of the workers, respectively. The diagnosis of silicosis and length of time working in mining negatively affected lung function, although exercise capacity was preserved. In the multivariate analysis, time working in mining, diagnosis of silicosis and education remained significant for forced expiratory volume in one second (FEV₁; r = 0.60; r² = 0.36; p < 0.001) and age and height for distance in 6MWT (r = 0.66; r² = 0.43; p < 0.001).

CONCLUSIONS:

Our results show impaired lung function and preserved exercise capacity in current mineworkers exposed to silica. Length of time working in mining, presence of silicosis and lower education were factors associated with reduced lung function.

Keywords

Pneumoconiosis silicosis exercise 6-minute walk test

1 Introduction

Silicosis, resulting from inhalation of silica particles, is the most common pneumoconiosis in Brazil. Exposure to silica occurs in different activities such as construction, mining, ceramic industry, metalworking, quarrying and sand blasting [1]. The country is also well known for the extraction of semi-precious stones, which are used in the manufacture of jewelry, fashion accessories and home ornaments. It is the main economic activity in the Mid-Upper Uruguay micro-region, located in northwestern of the State of Rio Grande do Sul – Brazil.

Previous studies have shown impaired lung function in patients with silicosis [2 –8] and also in workers without pneumoconiosis that have been exposed to silica [9, 10]. It is also common knowledge that impaired lung function due to obstructive or restrictive lung diseases may compromise functional exercise capacity [11 –13]. The easiest way to evaluate functional exercise capacity is the 6-minute walk test (6MWT) [14], which is a simple and inexpensive test that requires no technology or advanced training for technicians. Although several studies have evaluated lung function in pneumoconiosis, especially in silicosis, to the best of our knowledge, data on exercise capacity in this patient group are scarce [15 –17]. Furthermore, the relationship between lung function and functional exercise capacity has not been studied in current semi-precious stone mineworkers.

Therefore, we carried out a study to evaluate lung function and functional exercise capacity and to analyse the factors associated with functional impairment in workers engaged in mining semi-precious stones in northwestern Rio Grande do Sul State in Brazil.

2 Materials and methods

We conducted a cross-sectional study among mineworkers in the city of Ametista do Sul, Rio Grande do Sul, in southern Brazil. A total of 420 men attended a routine periodic health examination in the Occupational Health Center between the months of November 2013 and March 2014. Of these, 348 (83%) composed a convenience sample in a study designed to assess the prevalence of silicosis and health related quality of life [18]. From the initial sample, 193 consented to underwent a 6MWT and were included in the current study, which evaluated the relationship between spirometric variables and functional capacity. However, none of the 42 miners (12.1%) of the original sample who were temporarily absent due to illness or retired because of silicosis agreed to take part on the current survey. This study was conducted in accordance with the amended Declaration of Helsinki, and was approved by the Institutional Review Board of the Hospital de Clínicas de Porto Alegre, number 331.531 and all participants signed the informed consent prior to enrolment.

Data were obtained through an interview performed by a registered nurse, and from medical records review. Data on education level, smoking habits, respiratory symptoms and lifetime occupational history were collected using a standardized questionnaire. All workers included in the study had a routine medical consultation and underwent chest X-ray, spirometry. Silicosis was diagnosed based on exposure history and findings of the chest X-ray, which was interpreted by two certified B readers, a radiologist and a pulmonologist, according to ILO International Classification of Radiographs of Pneumoconioses [19]. The agreement between readers was excellent as indicated by kappa = 0.91 (95% confidence interval). Silicosis was diagnosed in those patients whose chest X-ray showed rounded opacities with profusion equal to or greater than 1/0. Spirometry was performed with a portable MicroLab MK8^® spirometer, according to the Brazilian guidelines [20, 21] and normal standards for Brazilian population [22]. Pulmonary function tests were categorized into four groups on the basis of the predicted percentages of forced vital capacity (FVC), forced expiratory volume in one second (FEV₁), as well as the FEV₁/FVC ratio: normal (CVF > 80% predicted, FEV₁ _> 80% predicted, FEV₁/FVC≥70%), restrictive (CVF < 80% predicted, FEV₁ _< 80% predicted, FEV₁/FVC≥70%), obstructive (CVF > 80% predicted, FEV₁ _< 80% predicted, FEV₁/FVC<70%) or mixed pattern (CVF < 80% predicted, FEV₁ _< 80% predicted, FEV₁/FVC<70%).

Functional exercise capacity was assessed using the 6MWT. The test was performed according to the guidelines of the American Thoracic Society (14). Subjects were instructed to walk as fast as possible for six minutes in a 30-metre corridor, and standardized verbal stimuli were provided each minute. The distance in meters (6MWD) was recorded at end of the 6MWT and the percentage of the predicted value was calculated according to previously described reference equation [22]. Functional capacity was considered reduced when the 6MWD was less than 80% of predicted. SpO2% (pulse oximetry) was measured using a pulse oximeter, dyspnea and lower-limb discomfort were assessed using the Borg scale [23] before and immediately following the test. The degree of functional impairment was determined prior to the 6MWT, based on the Modified Medical Research Council Dyspnea Scale (MMRC) [24], which was validated for use in Brazil [25].

Statistical analysis was performed using the Statistical Package for Social Sciences^® (SPSS) version 18.0. The data are presented as frequencies and percentages for categorical variables and as mean±SD or median and interquartile range (IQR) for numeric variables. To study the effect of length of time in mining work on functional parameters, the time was stratified as less than 20 years and 20 years or more and also into periods of five years (<5; 5-10; 10-15; 15-20 and > 20 years. To assess difference between variables in different groups of workers, the Student t test, Mann-Whitney test or Chi-Square test was used. Pearson or Spearman correlation test was used to study the relationship between functional capacity and other variables. Stepwise regression was used in the multivariate analysis, using a 0.20 cut-off point for the selection of variables. P values < 0.05 were considered as indicative of statistical significance.

3 Results

A total of 193 men, who were still working in mining, were included in the study. Most of the workers performed various activities in the mine such as drilling, explosive detonating, cutting and debris removal. Of these, 51 (26.4%) had radiographic changes consistent with silicosis. The demographic and clinical characteristics of the workers are described in Table 1. The age ranged between 18 and 64 years, the majority were non-white (74.1%), married, non-smokers and had a low education level. Education ranged from 1 to 13 years. The age at first silica exposure was lower than 20 years in 125 workers (64.8%) and 63 workers began to work in mining before age 15. Working time in the mine ranged from 0.5 to 37 years. Workers with silicosis were older (44.9±8.2 years vs 34.4±10.5 years; p < 0.0001) and had been reported by 12 (23.5%) workers with silicosis and 15 (10.6%) workers without the disease (p = 0.02). Respiratory symptoms were more frequent among workers with silicosis (p < 0.05).

Table 1
Sociodemographic characteristics and occupational history in 193 mineworkers^a

Variable Value

Age, years 37.2±11

Ethnicity

White 50 (25.9)

Non-white 143 (74.1)

Height, m 1.72±0.07

Weight, kg 75.1±12.6

BMI, kg/m² 25.4±3.5

Education, years 5.9±2.7

Education level

Incomplete elementary 106 (54.9)

Complete elementary 48 (24.9)

Incomplete secondary 27 (14.0)

Complete secondary 12 (6.2)

Marital status

Single 49 (25.4)

Married 142 (25.4)

Widowed 2 (1.0)

Smoking status

Nonsmoker 130 (67.4)

Current smoker 32 (16.6)

Former smoker 31 (16.1)

Smoking, pack-years (n = 63) 10 [3 –20]

Occupational history

Age at first exposure, years 19.9±7.2

Time on the job, years 14.7±8.7

Variable	Value
Age, years	37.2±11
Ethnicity
White	50 (25.9)
Non-white	143 (74.1)
Height, m	1.72±0.07
Weight, kg	75.1±12.6
BMI, kg/m²	25.4±3.5
Education, years	5.9±2.7
Education level
Incomplete elementary	106 (54.9)
Complete elementary	48 (24.9)
Incomplete secondary	27 (14.0)
Complete secondary	12 (6.2)
Marital status
Single	49 (25.4)
Married	142 (25.4)
Widowed	2 (1.0)
Smoking status
Nonsmoker	130 (67.4)
Current smoker	32 (16.6)
Former smoker	31 (16.1)
Smoking, pack-years (n = 63)	10 [3 –20]
Occupational history
Age at first exposure, years	19.9±7.2
Time on the job, years	14.7±8.7

^aData are presented as mean±SD, number (%); median [IQR]. Abbreviation: BMI: Body mass index.

The lung function data showed normal spirometry in 145 workers (75.1%), a pattern suggestive of restrictive ventilatory defect in 25 (13%), obstructive ventilatory defect in 18 (9.3%) and mixed pattern in three (2.6%) workers. Among the 51 workers with silicosis, spirometry was normal in 22, it showed a pattern suggestive of restrictive ventilatory defect in 18, obstructive ventilatory defect in nine (five of them where smokers) and a mixed pattern in two workers. Smokers and former smokers had lower lung function values than non-smokers shown in FEV₁ (3.68±0.75 l vs 3.28±0.67 l; p = 0.0001; and 88.9 % ±13.5 % predicted vs 83.3 % ±13.8 % predicted; p = 0.007) and FEV₁/FVC (81.1 % ±7.4 % vs 76.3 % ±9.2 %; p = 0.0001), while FVC did not differ significantly between the groups.

The results of spirometry and 6MWT of workers with and without silicosis are shown in Table 2. Workers with silicosis had significantly lower lung function values than those without the disease (p < 0.001). Overall, nine workers (4.7%) had a 6MWD lower than 80% of predicted, none of them had silicosis. Furthermore, the distance in meters performed on the 6MWT was not significantly different in workers with and without silicosis (p = 0.09). However, when considering the percentage of the predicted distance, workers with silicosis performed better in the test (p = 0.002). SpO₂ was comparable between groups before and after exercise (p > 0.05). Dyspnea at rest, as well as leg discomfort measured before and after the 6MWT were higher in workers with silicosis (p < 0.05). Of the 193 patients, 47 (24.4%) worked more than 20 years in mine. The comparison between miners working less than 20 years and more than 20 years showed that miners with longer duration of exposure had worse lung function (FVC 84.2±14.8 % predicted vs 93.1±12.9 % predicted, p < 0.0001; FEV₁ 78.7±14.7 % predicted vs 89.8±12.4 % predicted, p < 0.0001), worse performance in 6MWT (100.5±11 % predicted vs 94±11 % predicted, p < 0.0001) and a higher prevalence of silicosis (55.3 vs 17.1%, p < 0.0001). Lung function and 6MWT data according to the lifetime exposure to silica dust stratified into periods of five years in all workers and in workers with and without silicosis are shown in Table 3. Overall, data for workers with and data for workers without silicosis show a decrease in the percentage of predicted lung function variables and an increase in the percentage of predicted 6MWD related to the duration of silica exposure.

Table 2

Spirometry and six-minute walk test data of mineworkers^a

Variable	Total	Silicosis		p value
	n=193	No	Yes
		n = 142	n = 51
Spirometry
FVC, l	4.42±0.82	4.63±0.72	3.84±0.77	<0.0001
FVC, % predicted	90.9±13.9	94.5±12.0	81.1±14.2	<0.0001
FEV₁, l	3.55±0.74	3.79±0.65	2.87±0.55	<0.0001
FEV₁, %	87.1±13.8	91.5±11.3	74.9±13.1	<0.0001
FEV₁, / FVC	80.2±8.5	81.6±7.9	76.3±8.7	<0.0001
6MWT
Distance, m	632.9±58.3	637.3±58.7	621.1±55.6	0.09
Distance, % predicted	95.1±11.3	93.6±11.1	99.1±10.6	0.002
SpO₂ %
Before the test	97.9±1.1	98.1±1.1	97.8±1.1	0.06
At the end of test	97.7±1.7	97.7±1.7	97.5±1.8	0.42
Dyspnea, Borg scale
Before the test	0.16±0.78	0.10±0.61	0.34±1.13	0.003
At the end of test	1.79±1.68	1.62±1.45	2.27±2.05	0.07
Leg discomfort, Borg scale
Before the test	0.21±0.74	0.10±0.51	0.51±1.10	0.003
At the end of test	1.73±1.81	1.47±1.63	2.46±2.08	0.001

^aData are presented as mean±SD. Abbreviations: FVC: Forced vital capacity; FEV₁: forced expiratory volume in 1 second; 6MWT: six-minute walk test; SpO₂ % : peripheral oxygen saturation. Tests: Unpaired Students’ T test or Mann-Whitney test.

Table 3

Lung function and 6-minute walk distance according to lifetime exposure to silica dust^a

Silica exposure, Years	Number	FVC, l	FVC, % predicted	FEV₁, l	FEV₁ % predicted	FEV₁/FVC	6MWD, m	6MWD, % predicted
All workers (n = 193)
<5	31	4.72±0.50	100.3±9.9	4.09±0.50	96.6±9.4	85.5±7.8	651.5±54.6	87.9±8.2
5–10	47	4.55±0.87	92.6±13.1	3.74±0.72	89.7±11.8	82.3±6.9	640.7±59.7	94.4±11.1
10–15	40	4.64±0.72	92.7±11.9	3.70±0.67	89.7±12.3	80.4±6.0	641.5±54.0	95.8±10.6
15–20	28	4.27±0.82	86.7±14.8	3.31±0.65	82.5±12.9	77.5±8.9	629.7±59.0	97.3±10.4
>20	47	4.01±0.85	84.2±14.8	3.03±0.66	78.7±14.7	76.2±9.5	607.9±56.0	100.5±11.0
p value		<0.001	<0.001	<0.001	<0.001	<0.001	0.008	<0.001
Workers without silicosis (n = 142)
<5	30	4.72±0.45	100.0±9.9	4.09±0.50	96.7±9.5	86.1±7.3	652.2±55.4	87.6±8.1
5–10	42	4.69±0.79	94.6±11.6	3.88±0.62	92.0±9.9	82.9±6.5	640.8±62.4	92.8±9.8
10–15	33	4.70±0.66	94.8±11.4	3.81±0.65	91.7±11.5	80.5±6.3	641.0±55.4	94.5±10.0
15–20	16	4.61±0.80	91.7±13.0	3.54±0.65	87.3±10.8	76.6±9.9	634.4±57.6	96.9±10.9
>20	21	4.35±0.93	88.1±13.3	3.37±0.66	85.8±13.2	78.4±8.9	605.2±55.4	97.9±11.8
p value		0.44	0.009	0.001	0.006	<0.001	0.07	0.03
Workers with silicosis (n = 51)
<10	6	3.77±1.14	81.5±19.0	2.82±0.70	74.3±12.1	76.0±9.1	638.3±28.6	105.2±17.2
10–15	7	4.33±0.96	82.9±9.3	3.15±0.51	80.6±13.2	79.7±4.8	643.1±50.4	101.6±11.8
15–20	12	3.81±0.61	80.0±12.4	2.99±0.54	76.1±13.0	78.8±7.6	623.4±62.9	97.2±9.8
>20	26	3.73±0.67	81.0±15.5	2.75±0.52	72.9±13.5	74.4±9.6	609.9±57.5	101.7±10.6
p value		0.34	0.98	0.29	0.59	0.35	0.43	0.53>

^aData are presented as mean±SD. Abbreviations: FVC – forced vital capacity; FEV₁ – forced expiratory volume in 1 second; 6MWD – six-minute walk distance.

The single correlation between the percentage of predicted FEV₁ and predicted 6MWD and additional parameters are shown in Table 4. Regarding FEV₁, the best correlations were observed with age, education, time working in mine and presence of silicosis. In the multivariate analysis, only time working in mining, diagnosis of silicosis and education remained significant (r = 0.60; r² = 0.36; p < 0.001). The correlations between 6MWD and lung function parameters were weak. The best correlations were observed between 6MWD and age, body height, years working in mining and presence of silicosis. However, in the multivariate analysis, only age and height remained significant, explaining 49% of the variability of the distance covered on the 6MWT (r = 0.71; r² = 0.49; p < 0.001). Figure 1 shows the relationship between length of time working in mine and 6MWD and FVC, as well as between 6MWD and FVC.

Table 4

Relationship between predicted 6-min walk distance (%) and further variables

Variable	Univariate analysis
	r	p value
Age, years	0.50	<0.001
Weight, kg	–0.22	0.002
Height, m	–0.52	<0.001
BMI, kg/m²	0.03	0.65
Smoking, years	0.13	0.08
Cigarretes, per day	0.09	0.22
Smoking, pack-years	0.12	0.10
Dyspnea, yes or no	0.18	0.02
MMRC, nominal scale	0.07	0.32
FVC, % predicted	–0.11	0.13
FEV₁, % predicted	–0.18	0.01
FEV₁,/ FVC	–0.20	0.005
Time working in mine, years	0.40	<0.001
Silicosis, yes or no	0.33	<0.001

Abbreviations: r: correlation coefficient; MMRC: Modified Medical Research Council; FVC: forced vital capacity; FEV₁: forced expiratory volume in 1 second.

Fig. 1

Correlation between time working in mining and 6MWD % of predicted (A) and FVC (B) and between 6MWD and FVC (C and D). Abbreviations: 6MWD: 6 minute walk distance; FVC: Forced vital capacity.

4 Discussion

The present study evaluated the effect of mining work on lung function and functional exercise capacity among current workers engaged in underground semi-precious stone mining in southern Brazil. Our results show that: 1) although about one quarter of all workers presented abnormal spirometry, functional exercise capacity was preserved; 2) overall, there was a “time on job effect” with worsened lung function and improved functional exercise capacity in relation to increased length of time working in mining; 3) while pulmonary function was worse in subjects with silicosis compared to those without the disease, functional exercise capacity was preserved; 4) there was only a weak relationship between lung function parameters and functional exercise capacity.

Seventy-five percent of the workers did not have silicosis, so we were able to address the effect of silica exposure on spirometry results in the absence of the disease. Our study showed an overall negative effect on pulmonary function related to the length of time in mining. The negative impact on lung function of inhaling silica and other dusts that are toxic to the lungs or cause pneumoconiosis has been reported in a number of studies [12 , 27]. However, another study of 1,072 individuals exposed to silica, but without evidence of pneumoconiosis, found an association between obstructive abnormalities and cumulative silica exposure only in smokers [9].

Forty-three percent of the workers with silicosis in our study had normal spirometry. Similar results have been observed in a study comparing foundry workers with a control group [28]. Although the restrictive pattern predominated among the workers with silicosis and altered lung function, nine workers, four of whom where non-smokers, had an obstructive pattern. It is to note that silicosis is classically described as a restrictive lung disease. However, an obstructive pattern in subjects with silicosis has been previously described. Rosenman et al. (2010) studied the results of spirometry in 526 subjects listed in a silicosis registry and found restrictive, obstructive or a mixed restrictive and obstructive pattern in 21.4%, 25.3% and 24.9% of the cases, respectively. An obstructive pattern was detected in 28.5% of smokers and 17.4% of never smokers. In addition, the proportion of abnormal spirometry was higher than that in our study (71.6% vs 57%). This is probably due to the fact that in the referred study the population was older and, in most cases, retired [7]. Moreover, the spirometric data were obtained from a disease registry database rather than a cross-sectional study involving workers in activity, as in our study.

In the present study the 6MWT was used to measure functional exercise capacity because the response elicited by the test is close to daily life activity [29]. The normal functional exercise capacity observed in most of the workers in our study may have resulted from the intense physical activity required in mining and could be related to the fact that miners temporarily absent due to illness or retired because of silicosis did not agree to participate on the current study. It can also explain the low intensity of dyspnea and discomfort in lower limbs at the end of the 6MWT and is probably one of the reasons for the weak relationship between lung function parameters and functional exercise capacity. Similarly, preserved functional exercise capacity was previously reported in a study evaluating 153 workers exposed to silica and 62 patients with silicosis using ergospirometry. All the subjects had been engaged in manufacturing fire-proof bricks in an iron-steel plant in China. The mean exercise time and exercise amount performed were not significantly different between the workers (5.6 minutes and 110 watts) and the patients with silicosis (5 minutes and 97 watts) (p > 0.05) [17].

The strength of our study is that, to the best of our knowledge, it is the first to evaluate the functional exercise capacity of current semi-precious stone miners using spirometry and the 6MWT. While there are two other studies in which the 6MWT was performed in subjects with silicosis, the data cannot be compared with those from our study, because in one of these studies different occupational lung diseases were evaluated together [16] and on the other, the sample consisted of hospitalized patients with silicosis [15].

Our study has also limitations that should be listed. First, we only performed a single 6MWT. Although a practice test is not needed in most clinical settings it should be considered in research settings, since a learning effect has been previously described [14]. However, the effect of the lack of a practice 6MWT was minimized by the workers’ good exercise capacity resulting from the high physical demand in mining work. Moreover, a multicenter study with 470 highly motivated chronic obstructive pulmonary disease (COPD) patients, who performed two 6MWT one day apart, showed a difference of only 5.8% between the two tests [30]. Second, despite care to avoid sample selection bias healthy worker survival bias may occur in occupational studies because of the tendency for unhealthy individuals to leave work early or to select healthier individuals to work in the mining industry. In addition, in our study, miners who were in sick leave or retired due to silicosis did not agree to perform the walk test. Third, we did not measure the environmental concentration of silica inside the mine to characterize the degree of worker exposure. Likewise, it was not possible to relate the type of activity performed in the mine with pulmonary functional loss or with the presence of silicosis, since the miners constantly varied their work activities. Finally, as the design of our study was cross-sectional, data on functional changes over time were not available.

In summary, our study suggests long term working in semi-precious stone mining negatively affects lung function, regardless of the presence of silicosis, while functional exercise capacity is relatively preserved in workers still working in mining. The relationship between lung function parameters and exercise functional capacity was weak. Follow-up studies would be useful to evaluate changes in exercise capacity of the mineworkers over time and the potential role of the 6MWT to determine prognosis and identify disability in this group of workers.

Conflict of interest

All authors stated they have no conflicts of interest.

Footnotes

Acknowledgments

The authors thank the participants and the Occupational Health Center of Ametista do Sul for allowing access to the mineworkers. This study was supported by the Universidade Federal do Rio Grande do Sul. The first author had a scholarship from Coordenadoria de Aperfeiçoamento de Pessoal de Nível Superior (CAPES).

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