Silicosis, the most prevalent form of pneumoconiosis, is a preventable, but untreatable, environmental pulmonary disease that can be fatal through the impairment of overall health status.1 The disease occurs when workers are exposed to quartz-containing dust; common professions at risk for silicosis include tunneling, quarrying, mining, and foundry work.3

Although sandblasting was reported as a source of silicosis in Brazil, the use of sandblasting has been banned by the Ministry of Labor and Employment since 2005.4 However, many former shipyard workers in the city of São Gonçalo of Rio de Janeiro State still live with the consequences of silicosis.5,6 Exposure through sandblasting appears to be more hazardous compared to many other known sources of silica dust because of the intense exposure during long work hours under very poor hygienic conditions without any efficient respiratory protection.1,7

In Brazil, silicosis is also observed among stone carvers.8 In the region of the Baixada Fluminense of Rio de Janeiro State, various workers produce stone souvenirs, frequently for export. All belong to the informal economy and work in small and rudimentary workshops.9

The use of CPET in clinical medicine has become established, primarily as a tool for understanding the mechanisms of exercise intolerance and fatigue. In patients with silicosis, dyspnea on exertion is frequently reported by these patients, even when resting cardiopulmonary measurements (including PFTs and arterial blood gas levels) are within the reference ranges. Thus, CPET may constitute a more sensitive way of assessing pulmonary impairment than resting functional evaluation.10

Given the different activities that lead to silicosis, the objective of this study was to compare the imaging findings, lung function variables and CPET results between sandblasters and stone carvers with silicosis.

The median age of the 41 patients was 48 years (range, 23–74 years); except for two artisans, all participants were men. The median duration of silica dust exposure and latency period was 96 months (range, 9–244 months) and 132 months (range, 12–257 months), respectively. The median follow-up period of the patients was 72 months (range, 12–122). Most of the patients reported working without a breathing-mask. No sandblasters were still working in occupations with silica dust exposure at the time of the study. However, half of the artisans were active in work environments with silica-dust exposure at the time the study was being conducted. Case characteristics separated according to occupation (sandblasters vs. stone carvers) are presented in Table 1.

Among the sandblasters, 21 (84%) had ventilatory impairment: restrictive defect was identified in 8 (32%), obstructive in 6 (24%), and mixed in 7 (28%). Among the stone carvers, 11 (68.7%) had ventilatory impairment: restrictive defect was identified in 3 (18.8%), obstructive in 5 (31.2%), and mixed in 3 (18.8%). The carbon monoxide diffusion capacity (DLCO) was at the lower limit of normality in 22 (88%) sandblasters and 7 (43.7%) stone carvers.

Of the 41 chest radiographs, the inter-reader agreement for small opacity profusion and of the large opacity type revealed kappa coefficients of 0.38 (P<0.001) and 0.72 (P<0.001), respectively. The distribution of sandblasters and stone carvers in the ILO radiological categories and HRCT findings are presented in Table 2.

The CPET results for each patient population (sandblasters vs. stone carvers) are summarized in Table 3 and Fig. 1. While 92% of sandblasters failed to reach at least 80% of their predicted peak VO2, this was observed in only 43.7% of stone carvers. A breathing reserve of less than 25% was observed in 40.5% of sandblasters; this was not observed in any stone carver. Additionally, 48% of sandblasters had P(A-a)O2 values greater than 35mmHg; this was not observed for any stone carver.

Figure 1.

Box plots (median, 1st and 3rd quartiles, minimum and maximum) of peak oxygen uptake (peak VO2) according to the type of workers exposed to silica dust. Significant difference (P<0.0001) was found between the sandblasters and stone carvers.

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This study shows that sandblasting causes worse clinical outcomes although sandblasters have less exposure duration and shorter latency period compared to stone carvers. Differences in results can be observed in imaging findings, lung function variables and CPET measurements.

In a study with 49 silicosis patients, Moreira et al.26 showed that the HRCT is more sensitive than the chest radiography in the evaluation of the intrathoracic abnormalities.26 The superiority of HRCT over chest radiograph for the detection of large opacities and emphysema has been well established.2,24–26 As reported by other authors,25,27–29 we demonstrated that HRCT identifies more PMF when compared with chest radiographs. In some patients with silicosis, it is possible that a confluence of nodules on HRCT is viewed only as a background of diffuse pulmonary nodules on chest radiograph.

Interestingly, the present study identified PMF in 25 sandblasters (92%) and only 7 stone carvers (43.7%). Similarly, Antao et al.29 found PMF in 39.1% of stone carvers. Unlike our results, Alper et al.1 observed PMF in half of the cases of silicosis in denim sandblasters. Although both studies showed similar latency periods (median 84 vs. mean 82 months), the exposure durations, ages of the workers, and criteria for the diagnosis of PMF on HRCT were different; this makes it difficult to reach a conclusion about the different frequencies of PMF between the two studies. Despite these considerations, the identification of PMF is important because it is the replacement of aerated lung tissue with a mass of non-functional fibrous tissue; additionally, it is usually associated with impaired lung function, disability, and premature death.30 A significantly increased relative risk of PMF is associated with higher levels of dust exposure30; in our study, this may be a possible explanation for the higher frequency of PMF observed in sandblasters.

In the present study, there were no significant differences on PFT results when sandblasters were compared with stone carvers. However, while DLCO values were only slightly altered in the artisans, pronounced changes were observed in the sandblasters (P=0.02). Interestingly, DLCO has been found to be fairly correlated with gas exchange abnormalities during exercise, and particularly to be the best predictive and sensitive index of a fall in PaO2.31

CPET allows for the objective determination of exercise functional capacity and for the evaluation of the mechanisms underlying exercise limitation.32 In the present study, on comparing the sandblasters to the stone carvers, the CPET response variables exhibited higher significant differences in relation to measurements taken at rest. This is not necessarily surprising because exercise capacity is the result of the response of multiple factors, including gas exchange across the lung, blood oxygen content, and tissue uptake of oxygen.33

The measurement of peak VO2 remains the best available index for the assessment of exercise capacity.32 In the current study, while almost all the sandblasters failed to reach at least 80% of the peak VO2 (% predicted), this occurred in less than half of the stone carvers. Violante et al.10 who evaluated 45 silicosis patients with different occupational fields including foundry work, coal or metal mining and milling, and sandblasting, observed peak VO2 values (% predicted) below 80% in 35 patients.

In the present study, the sandblasters had a lower breathing reserve compared to the stone carvers (P<0.0001). Similarly, Wang et al.33 found low breathing reserve values in silicosis patients who had been engaged in the manufacturing of fire-proof bricks in China. According to these authors, the increase in ventilatory demand was presumably due to the increase in dead space ventilation secondary to ventilation-perfusion mismatching; further, decreased blood oxygen saturation stimulates the peripheral respiratory receptors to impel breathing rates to increase. We hypothesize that the higher frequency of PMF observed in sandblasting-induced silicosis could explain these functional abnormalities.

Some limitations of this study must be addressed. First, the patient groups may not be representative of the entire population of sandblasters and stone carvers of the regions involved. Second, information on the exact type of silica and the concentrations of silica to which subjects were exposed was not available. Finally, suboptimal subject effort alone could account for the reduced peak VO2 readings. However, all patients were actively encouraged during exercise to reach a maximum, and they showed high blood lactate levels and high respiratory exchange ratios.32

In conclusion, the data indicate that imaging findings, lung function variables and CPET results are strongly influenced by the type of silica dust exposure in silicosis patients. In addition, when the sandblasters were compared with stone carvers, CPET abnormalities were more pronounced compared to measurements taken at rest. To our knowledge, this is the first study comparing two different groups of workers exposed to silica dust using CPET analysis.

The authors have no conflicts of interest to declare.