Mineral dusts and prevention of silicosis, vol 4; No.2, September 1997Clinical cases of silicosis in Thailand,by P. Youngchaiyud, A. Nana Thailand |
Introduction
During the past decade, economic growth in Thailand, has been rapid. This rapid economic expansion has meant an increasingly high level of industrialisation. Stone-grinding is one industry that has expanded vigorously. Recent estimates suggest that Thailand now has about 6,700 stone-grinding factories which employ more than 1.7 million workers.
Silicosis is a fibrotic disease of the lungs caused by inhalation of dust containing crystalline silicon dioxide or silica, a component of rock and sand. Industries in which silicosis may occur include quarrying, stone cutting, mining, tunnelling, foundry work, ceramic, sandblasting, glass-making etc. The relationship between exposure to silica and silicosis was demonstrated by Hnizdo and Sluis-Cremer (1). These authors found that over an average exposure period of 25 years, the cumulative exposure for the whole cohort was about 2 mg/m3 per year. No silicosis occurred when the cumulative exposures were below 0.9 mg/m3 per year. In contrast, the cumulative risk of silicosis was approximately 25% at a cumulative exposure of 2.7 mg/m3 per year and 77% at the highest observed cumulative exposure level of 4.5 mg/m3 per year.
Prevalence
N. Chinachoti reported the first case of silicosis in Thailand in 1954 (2). During 1973 1977, 35 silicosis cases were reported, most of them among miners. A series of cross-sectional studies was then carried out among various occupational groups exposed to silica (Table 1). The prevalence of silicosis in refractory brick factories and ore mills was reported at 9.5% and 12.5%, respectively. A study of 266 mortar and pestle workers was carried out in Northern Thailand by Metadilogkul et al. (3); it revealed that 21.1% of workers were affected with silicosis. A relationship was also observed between the duration of exposure and silicosis. Aungkasuvapala et al. (4) studied workers at the 33 stone-grinding factories in Saraburi, a province about 100 kilometres north of Bangkok. Thirty-one of the factories (93.6%) had levels of either total dust or respirable dust exceeding the hygienic limit values, the average levels of total dust and respirable dust being 24.3+34.6 and 2.4+1.6 mg/m3, respectively. The prevalence of silicosis in this study was 9%. The association between silicosis and tuberculosis, too, is well-recognized. (5) The prevalence of silicotuberculosis in Thailand ranges from 1% to 25% (Table 1).
Clinical features
The three clinical forms of silicosis chronic, accelerated, and acute are determined primarily by the intensity and the length of exposure to silica. Long-term exposure to low concentrations of silica is likely to be associated with a slow progressive nodular infiltration on both lungs, predominantly in the upper lobe. This radiographic abnormality of chronic silicosis usually appears more than 15 years after the onset of exposure. The lesion usually marks a progressive disease, even in the absence of further exposure to silica, and will frequently develop into progressive massive fibrosis. Most patients with chronic silicosis have no symptoms. Among symptomatic patients, dyspnea is common but is usually severe only if there is progressive massive fibrosis.
In accelerated silicosis, exposure to high concentrations of silica over a period of as little as four years results in a more rapidly progressing form of silicosis. The symptom of breathlessness presents early and the patient's condition rapidly deteriorates to hypoxic respiratory failure. This type of silicosis is common in certain occupations, e.g. sandblasting and stone masonry.
Acute silicosis can develop within a few weeks or months of exposure to very high concentrations of silica. The lungs show a ground-glass appearance, similar to that of pulmonary edema. The lesion may consolidate into appearances more characteristic of massive fibrosis over a short period of time. The clinical presentation of this group is usually rapidly progressive dyspnea, cough and weight loss. Death occurs within a short period of time despite intensive treatment.
Diagnosis
The keys to diagnosis are a detailed occupational exposure history if the patient has worked for several years in an environment where stone dusts was inhaled, and a chest X-ray that is abnormal according to ILO's international classification of pneumoconiosis from 1980 (6). Silicosis should be distinguished from other causes of diffuse parenchymal lung infiltration such as sarcoidosis, pulmonary tuberculosis, idiopathic interstitial pulmonary fibrosis, etc. Since these diseases may be treatable, it is important to exclude them by histologic diagnosis. This usually requires transbronchial, video-assisted thoracoscopy or open lung biopsy. In some cases, evaluation of the amount of silica in lung tissue will give a final diagnosis. An elevated level of alveolar macrophages in bronchoalveolar lavage fluid and detection of silica particles in the cytoplasm of alveolar macrophages under phase contrast light microscopy may be an alternative and less invasive diagnostic technique. (7) Abnormalities in pulmonary function are not diagnostic criteria and should be used only to evaluate disability or for medicolegal purposes.
Complications
Pneumothorax occurs more frequently among patients with accelerated or acute silicosis. Patients with silicosis are at increased risk of infection with mycobacterium tuberculosis, and some eventually develop lung cancer (8).
Treatment
There is no known effective treatment for silicosis. In acute silicosis, however, sequential whole-lung lavage may be beneficial. Workers with silicosis and who have a positive result to the tuberculin but who show no signs of active tuberculosis should receive antituberculosis chemoprophylaxis (9). A recently published study suggested that treatment for silicotuberculosis should last longer than the standard six months. (10)
Conclusion
At present, silicosis continues to be an occupational disease of considerable importance in Thailand. Over the past five years, the number of claims for compensation of silicosis has been increasing. These findings indicate that silicosis is a serious social and economic problem in Thailand.
References
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2. Chinachoti N. Medical records: pneumoconiosis. J Med Assoc Thai 1954;376:369 80.
3. Metadilogkul O, Limpakarnjanarat K, Ittiravivongs A, et al. Silicosis among mortar and pestle workers in Northern Thailand: cross-sectional study. J Med Assoc Thai 1988;71:533 6.
4. Aungkasuvapala N, Juengprasert W, Obhasi N. Silicosis and pulmonary tuberculosis in stone-grinding factories in Saraburi, Thailand. J Med Assoc Thai 1995;78:662 9.
5. Snider DE. The relationship between tuberculosis and silicosis. Am Rev Respir Dis 1978;118:455 60.
6. International Labour Organisation. Guidelines for the use of ILO international classification of radiographs of pneumoconioses. Geneva: International Labour Office (Occupational Safety and Health Series, No. 22), 1980.
7. Saenghirunvattana S, Boonpucknavig V, Charoenpan P, et al. Silicosis in ceramic-industry workers with particular reference to the diagnostic value of bronchoalveolar lavage. J Med Assoc Thai 1991;74:358 62.
8. Weill H, McDonald JC. Exposure to crystalline silica and risk of lung cancer: the epidemiological evidence. Thorax 1996;51:97 102.
9. Hong Kong Chest Service /Tuberculosis Research Centre: A double-blind placebo-controlled clinical trial of three antituberculosis chemoprophylaxis regimens in patients with silicosis in Hong Kong. Am Rev Respir Dis 1992;145:36 41.
10.Hong Kong Chest Service/Tuberculosis Research Centre/Madras/British Medical Research Council. A controlled clinical comparison of 6 months and 8 months of anti-tuberculosis chemotherapy in the treatment of patients with silicotuberculosis in Hong Kong. Am Rev Respir Dis 1991;143:262 70.
P. Youngchaiyud, A. Nana
Siriraj Hospital, Mahidol University
Bangkok 10700, Thailand