1991
Why the Gold Miners Studies Were Done
NIOSH conducted 2 studies of gold miners. One looked at all causes of death. The other study focused on just one disease, silicosis. Here we report the result of both studies.
Other studies have shown that lung cancer was increased in miners exposed to silica. However, earlier studies have not shown such an increase. We wanted to see if an additional 14 years of data would change this result.
Thus, one of the main goals of the lung cancer study was to see if exposure to silica at a gold mine is associated with lung cancer.
We also wanted to learn more about silicosis among the gold miners.
How the Studies Were Done
NIOSH scientists had dust measurements of the air in a gold mine taken from 1937 to 1975. For the time before 1937, they estimated the amount of dust in the air. For each job a miner had, NIOSH researchers estimated how many hours a day miners spent in the mine and what the average dust exposure was for each day.
From these data, we estimated the daily dose of dust for each miner. We then added up all of the daily doses to obtain the total called dust days.
We then divided the workers into four levels based on total dust-days. Those groups were:
- less than 8,000 dust-days
- 8,000 to 32,000 dust-days
- 32,000 to 48,000 dust-days
- more than 48,000 dust-days
We studied the risk of many diseases as the total dust-days increased. We also grouped the miners into three groups based on the date of hire. These groups were those hired before 1930, those hired between 1930 and 1950, and those hired after 1950.
Death certificates list the main or underlying cause of death which we usually use for our studies. The certificates also list other diseases that might be present at death. We looked at all causes of death listed to see if other diseases besides the main cause were related to mining.
In other studies, people who have silicosis or have silica exposure have developed immune reactions to parts of their own body. We call this type of reaction an auto-immune disease.
These diseases include lupus which usually damages the kidneys and joints, rheumatoid arthritis, a skin disease called scleroderma, and a rare type of heart disease. Thus, we also examined these diseases in the lung cancer study.
Results for All Diseases Using Main or Underlying Cause of Death
Lung Cancer
Lung cancer was not increased at any dust level. We also compared the dust levels of miners who had lung cancer to the dust levels of miners who did not get lung cancer.
The miners who had lung cancer had somewhat lower levels of dust than those that did not have lung cancer. Thus, dust exposure at the mine did not appear to cause lung cancer.
Tuberculosis, kidney disease and other lung diseases
The highest risk for TB, silicosis and kidney diseases was found in miners hired before 1930 and who had the highest dust exposure.
We found an increase in pneumonia and emphysema which may be a misdiagnosis of silicosis. These diseases may also occur more often in people who have silicosis.
There was also an increase in asthma. People who are exposed to dust have an increase in bronchitis. Asthma often occurs with bronchitis. This may explain the increase in asthma seen in this study.
Non-Hodgkin's Lymphoma
Lymphoma was increased in those in the highest dust level. However, it was not significantly elevated in any other dust level group.
Results for All Causes of Death Listed on the Death Certificates
When we looked at all diseases listed on the death certificates, we saw increases in many diseases which were not always the main cause of death.
Auto-Immune Diseases
There were increases in several auto-immune diseases. We saw 17 cases of arthritis but expected only 8. We found 10 cases of skin diseases but expected about 4. We found 10 cases of diseases of the muscle and skeleton but expected only 5. Most of the increase in these diseases was found in workers hired between 1930 and 1950.
We also found an increase in diseases of the heart muscle. We found 20 cases but expected 6 or 7. These heart diseases may be caused by an auto-immune reaction in the heart muscle. These heart diseases are only 5% of heart disease in the miners. Deaths from heart attacks were not increased. The increase in these rare heart diseases, was seen in workers hired before 1930. In these workers, we found 15 cases but expected 1 or 2. In workers hired after 1930, the risk of these heart diseases was not increased.
There was also an increase in kidney disease. We found 34 but expected 27. The highest risk was in workers hired before 1930. In these workers, we found 10 cases but expected 4 or 5. The risk was not increased in workers hired after 1930. The kidney diseases may also be an immune reaction.
Blood Diseases
We found an increased risk of diseases related to the forming of blood cells. We found 9 cases but expected only 4.
These diseases may be present as an early sign of lymphoma or leukemia.
Alcoholism
We found an increase in deaths from alcoholism in miners. We found 22 deaths but expected only 12. This increase was not linked to exposure to dust.
Results for The Silicosis Study
We found that the earlier a worker was hired, the higher was the risk of silicosis (See Table 1). Those hired before 1920 had a 48% risk of silicosis. However, there were only 4 cases in the 1,400 miners hired after 1950. There were no cases in workers hired after 1960.
Table 1
NlOSH U.S. Department of Health and Human Services