Worker Health Study Summaries
Research on long-term exposure
Gold Miners (Silica Exposure) (1)
NIOSH studied gold miners silica dust exposure and the risk of kidney disease.
NIOSH’s first study was of gold miners who worked underground for at least one year between 1940-1965. The study examined deaths due to lung cancer, kidney disease, tuberculosis, and diseases of the immune system. NIOSH next studied gold miners who worked underground for at least one year between 1940-1965 and who developed end-stage renal disease, the most serious type of kidney disease. For more information please call the NIOSH toll-free number at 800-356-4674.
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.
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.
Lymphoma was increased in those in the highest dust level. However, it was not significantly elevated in any other dust level group.
Results Using Multiple Causes of Death
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.
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.
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.
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.
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.
|Year Hired||Percent Who had Silicosis by 1990||No. With Silicosis/Total No. of Miners|
In our study, exposure to dust was not associated with an increase in lung cancer. However, we found an increase in auto-immune diseases, other lung diseases including silicosis and tuberculosis. The increase in silicosis and TB was higher in those with more dust exposure.
Most of the diseases showing an increase above expected occurred in workers hired before 1930. There have been no deaths from TB between 1976 and 1990.
Thus, many of the illnesses caused by gold mining no longer occur or occur only in people who started mining before 1930.
We wanted to know if miners had an increase in end-stage renal disease or ESRD. ESRD is the most serious type of kidney disease. With ESRD, the kidneys no longer work. We also wanted to know if ESRD was linked to the number of years a miner worked or to how much silica dust the miners worked around.
What did the Gold Miners Kidney Study find?
We found that gold miners who worked underground at least one year between 1940 -1965 had a slightly higher risk of developing ESRD than other people. These gold miners had a high risk of getting glomerulone phritis (glo-MARE-u-lo-na-FRITE-is), one type of ESRD.
The study found that a miner’s risk of getting ESRD increased:
- The more silica dust a miner worked around in his lifetime
- The more years a miner worked underground
Healthy kidneys work like a car’s oil filter. They remove waste materials from your blood and the rest of your body. But your kidneys also do other important things.
- They help control blood pressure.
- They tell the bone marrow to make red blood cells.
- They control bone strength and the calcium levels in the blood.
- They control the amounts of salt, water, and other chemicals in the bloodstream.
When you have kidney disease, extra waste builds up in your blood. This waste poisons your body. ESRD, the disease studied in the gold miners, is the most serious stage of kidney disease, when your kidneys no longer work. To live, people with ESRD need dialysis or a kidney transplant.
The Good News! If you find kidney disease early, it can be treated and ESRD can be prevented!
Milder kidney disease can be treated to prevent progression to ESRD. If you are concerned about your risk for kidney disease, share this information with your doctor.
He or she will decide what screening tests you may need to detect any ESRD as early as possible. Your doctor also may decide to do some tests to detect any silica-related disease as early as possible.
Damage to your kidney may happen slowly. Thus, your doctor should decide which screening tests are best for you
Tell Your Doctor
If you are concerned about your risk for kidney disease, tell your doctor that:
- You worked around silica dust
- A study has shown that workers exposed to silica may have an increased risk of kidney disease
What Else You Can Do
Our studies suggest that maintaining silica air concentrations below the OSHA/MSHA silica dust standard may not prevent the development of silicosis and silica-related kidney disease.
If you are still potentially exposed to silica dust, we recommend that you follow work rules and your training to achieve safe exposure levels.
What is the Risk for Gold Miners?
These results are based on small numbers. Among the 2412 miners we studied, we found 11 cases of ESRD (all types), but expected only 8. We found 5 cases of glomerulonephritis but expected only 1. Comparatively, for every 33,000 U.S. residents, 1 gets glomerulonephritis each year.
Although the results in this study are based on small numbers, NIOSH believes that there is an increased risk of glomerulonephritis among workers exposed to silica. We believe this because the risk of ESRD caused by glomerulonephritis was higher in workers with longer or higher exposure to silica, and because other studies have reported kidney disease in workers exposed to silica.
Even though the risk for ESRD was shown to be increased in our study, ESRD is still a rare disease.
For More Information About Kidney Disease Contact:
American Kidney Fund
6110 Executive Boulevard Rockville, MD 20852
National Kidney Foundation
30 East 33rd Street New York, NY 10016
National Kidney and Urologic Disease Information Clearinghouse (NKUDIC)
3 Information Way
Bethesda, MD 20892-3580
NIOSH is the National Institute for Occupational Safety and Health. It is the federal agency that does research on worker safety and health. NIOSH is part of the Centers for Disease Control and Prevention.
Miners have an increased risk of silicosis and tuberculosis and other lung diseases. However, it appears that these risks are slight for miners hired after 1960.
Even though your risks may be low, every year you should see a doctor who knows about silicosis (e.g. a pulmonary (lung) medicine specialist or an occupational medicine specialist).
When you go to see the doctor, the visit should include an interview and an examination. You may also need a periodic chest x-ray and tuberculin skin test.
Miners who have lung disease may be more susceptible to flu, pneumonia and other lung infections. They also may become very sick from these infections.
Thus, if you have lung disease or are over 65, you should be vaccinated for pneumonia once to prevent pneumonia. People who have lung disease or who are 65 or older should also get a flu shot every year.
If you have lung disease, at the first sign of lung infection, you should contact your doctor so that treatment starts early. The early signs of lung infection can include fever, chills, lung pain, and coughing up of yellowish or greenish material.
You may have an increased risk of kidney disease. Creatinine is a chemical that is increased in your blood when your kidneys do not work well. Therefore, the creatinine level in your blood should be measured periodically. If your creatinine level is increased, your doctor may provide recommendations to reduce or prevent further damage to your kidneys.
We believe that the current Occupation Safety and Health Administration (OSHA) and the Mine Safety and Health Administration (MSHA) standard may not protect workers from silicosis over a 45 year working lifetime. Of course most workers work less than 45 years. Therefore, they have less risk.
However, the level of silica dust in the gold mine since the 1950s has generally been much lower than the MSHA/OSHA standard.
We did not see any silicosis in workers hired between 1960-1965. Thus, it appears that current dust levels are low enough to protect workers from silicosis.
Avoiding excessive or unnecessary dust exposure will help prevent occupational lung disease. Use of a dust filtering mask will help to lower exposures if it is not possible to avoid exposure in any other way.
Calvert G, Steenland K (1997). End-stage renal disease among silica-exposed gold miners. A new method for assessing incidence among epidemiologic cohorts. Journal of the American Medical Association 277:1219-1223. (Study Report)
Steenland K, Brown D. (1995). Mortality study of gold miners exposed to silica and nonasbestiform amphibole minerals: an update with 14 more year of follow-up. American Journal of Industrial Medicine. 27:217-229.
Steenland K, Brown D (1995). Silicosis among Gold Miners: Exposure Response Analyses and Risk Assessment. American Journal of Public Health 85 (10): 1372-1377.
Brown D, Kaplan K, Zumwalde R et al. (1986). Retrospective cohort mortality study of underground gold mine workers. In Goldsmith D, Winn D, Shy C (eds): Silica, Silicosis, and Lung Cancer.” New York: Praeger:335-350.