ABSTRACT

This is a review of one hundred and ten cases of silicosis certified for workers' compensation by the Western Australian Pneumoconiosis Medical Panel in the ten year period from 1984 to 1993 to determine the relationship to their exposure to airborne silica in mining in Western Australia. Three of the cases had commenced exposure to silica in mining in the 5 year period 1965 to 1969 when exposure level for respirable crystalline silica was 0.4 mg/m³ and 2 cases had commenced in the 5 year period 1970 to 1974 when the exposure level was not monitored. There have been no cases which commenced mining work after 1974 when the exposure level has been 0.2 mg/m³. There have been no cases of silicosis in Western Australia since implementation of the current exposure standard of 0.2 mg/m³ for respirable crystalline silica.

Footnote
This poster is based on the paper presented at the Australian National Scientific Forum on Crystaline Silica, 9 November 1993 in Sydney and the 14th Asian Conference on Occupational Health, 16 October 1994 in Beijing, China which has been submitted to the Australian Journal of Occupational Health and Safety for publication.

INTRODUCTION

This review has been conducted to verify the incidence of new cases of silicosis in relation to their commencement of work exposure to silica. The objective of the review is to determine the actual occurrence of cases in relation to their past exposure to airborne silica. This review has been conducted because the number of cases published in the Western Australian Workers' Compensation and Rehabilitation Commission (WCRC) Annual Report reflect only the cases recorded by year of certification and not their exposure to silica at the year of certification. The review will show whether there have been new cases of silicosis since the implementation the current exposure standard of 0.2mg/m³ for respirable crystalline silica in Western Australia.

RESULTS

One hundred and ten cases of silicosis were certified by the Pneumoconiosis Medical Panel for the ten year period between 1984 and August 1993. The 110 cases who were all males, were aged between 29 and 87 years at the time of certification by the Pneumoconiosis Medical Panel. Their mean age was 62 years (SD 9.4). The time interval between first dust exposure at work and certification by the Pneumoconiosis Medical Panel was between 12 to 62 years with a mean of 38.3 years (SD 10.8). Gold mining was the most common type of mining activity.

One hundred and one (91.8%) had been employed in gold mining at some stage in their working life. Of these, 49 had also been engaged in mining for nickel, coal, copper, tin, zinc, lead, mineral sands and uranium. Sixty-eight silicosis cases (61.8%) worked underground only. Thirty-seven (33.6%) worked in both underground and surface mines. The number of years worked underground ranged from 5 months to 47 years with a mean of 21.1 years (SD. 12.4). The earliest exposure to silica dust in the 110 cases occurred in 1923. Only 3 cases commenced after 1968 but none after 1974. In Western Australia, air sampling has been conducted by the Department of Minerals and Energy (DOMEWA) and the mining companies. This air sampling data has been used to estimate the levels of respirable silica in the mines from the 1920's through to 1990's. The average measured atmospheric dust levels in the mines contained respirable silica of about 1mg/m³ in the 1920's. No data was available for much of the 1930's. From the late 1930's through to 1950, measured respirable silica levels averaged 0.6mg/m³. No data was available for the early part of 1950's. During the late 1950's, the average respirable silica level was 0.5mg/m³, and throughout the 1960's, 0.4mg/m³. In the 1970's no data was available. Respirable silica levels in the 1980's and 1990's were 0.2 and 0.1mg/m³ respectively. Fifty-five of the 110 cases had profusion 1/0 and 42% had chest X-ray opacities exceeding profusion 1/0 of the 1980 ILO International Classification of Radiographs of Pneumoconiosis (2). In the assessment of fitness for work, 38.2% were assessed to be unfit for work, 25.5% were fit for light work, 30% were fit for moderate work and 6.4% were fit for heavy work. All cases exceeding 70% impairment were unfit for work.

CONCLUSION

The results and Figure (1) show that of the 110 cases in Western Australia only 3 cases had commenced after 1968 and none after 1974. The absence of cases for the past 19 years corresponds to the implementation of the 0.2mg/m³ respirable crystalline silica exposure standard in Western Australia. The experience in Western Australia is similar to China where 557 pneumoconiosis cases in metal mines in China were diagnosed in an epidemiological survey from 1982 to 1986. Only 10 cases (1.8%) had commenced after 1960 of which only 1 case (0.2%) had commenced after 1965. Since dust control measures were implemented in China in 1963, total dust levels was reduced from 94.4-428.6mg/m³ to 1.9-17.5mg/m³ and since 1973 it has been 2.7-4.8mg/m³(4). The risk assessment model which predicts 20 silicosis cases per year averaged over the next 40 years is therefore not upheld by actual experience in Western Australia (5). No miner has developed silicosis since the advent of the Ventilation Board in 1974 (6). The Mines Regulation Act before amendment in 1993 specified that dealings with all matters relating to ventilation or environmental atmospheric control and health of persons shall be referred to the Ventilation Board which is chaired by the West Australian State Mining Engineer comprises relevant scientists and experts in occupational health from government, industry and the workforce (7). The Ventilation Board has recently been replaced by a Mines Occupational Health and Safety Advisory Board (MOHSAB). In Western Australia a large amount of exposure data is contained on the CONTAM database. Greater than 90 per cent compliance has been achieved by having the sampling protocol overseen by the Ventilation Board and monitored by the Mines Department Inspectorate.

The incidence of silicosis in Western Australia shows a progressively declining trend. When the new cases still arising as a legacy of the past have all been accounted for, new incidences of this disease will have been virtually eradicated.

The gradient of this decline will not become steeper by lowering of the Exposure Standard because of compulsory medical examination for miners required by law in Western Australia and the absence of cases in the last 19 years.

The confidence level is 93% according to cumulative probability and the mean time interval between first exposure and certification by the PMP of 38.3 years and SD 10.8 years. Indeed the stricter standard may have a detrimental effect. At the present Standard (0.2mg/m³), it is possible to direct surveillance at a small group of companies that have difficulty in meeting that Standard. With the proposed Standard (0.1mg/m³), direct surveillance will have to be spread over a wider spectrum since more companies will then not comply. This, in effect, means that, for the same resources, one is getting a shallower degree of surveillance which inevitably will lead to lowered compliance.

While it is realised that these Exposure Standards are meant to be guidelines to be used by appropriately qualified officers to assess a working environment, it is inevitable that these standards will be used as a measure of compliance with relevant legislation and as fine lines between acceptability and non-acceptability. The ALARA principle should still prevail. Unless there are sound scientific reasons, exposure standards which cannot be met or can only be met with great difficulty should not be set since this is bound to lead to unnecessary industrial conflict and disharmony without significantly increasing the protection for workers.

The Western Australian experience is that high compliance of 90% or more with the current     0.2mg/m³ respirable crystalline silica standard rather than lowering the Exposure Standard will prevent silicosis.

1. Wan, K.C. and Street, N.R., Criteria for determining work related heavy asbestos exposure, Proceedings of the VII International Pneumoconiosis Conference, Pittsburgh, Pennsylvania, USA, (1988).
2. ILO Guidelines for the use of ILO International Classification of radiographs of pneumoconioses. Revised edition 1980 (1987).
3. Guides to the evaluation of permanent impairment, American Medical Association. 4th Ed. (1993).
4. Liu, Z.Y. et al., Exploration into prevention of pneumoconiosis in metal mines. Minesafe Conference, Perth (1993).
5. Nurminen, et al, Prediction of silicosis and lung cancer in the Australian labor force exposure to silica, Scan. J. Work Environ. Health (1992).
6. McNulty, J.C., Unpublished Communication (1990).
7. Mines Regulation Act 1946 and Regulations Western Australia (1991).

ACKNOWLEDGMENTS

The authors thank the Chief Executive Officer, WorkSafe Western Australia and the Workers' Compensation and Rehabilitation Commission for permission to publish this paper. The work of the Secretary of the Pneumoconiosis Medical Panel Mr Hans Frederics in coordinating the case notes, help with statistical analysis from WorkSafe Western Australia's Statistician Hoo Shang, Karimi and typing of the manuscript by WorkSafe Western Australia's Medical Secretary Jenny Hay are gratefully acknowledged.