Why was this study done? Previous studies suggest exposure to high doses of ionizing radiation may cause leukemia. However, it is still uncertain if low doses of ionizing radiation, which are common in some workplaces, may also cause leukemia. Leukemia is a type of cancer, an illness that starts in blood-forming tissue such as bone marrow. Leukemia causes large numbers of abnormal blood cells to be produced. The cancer cells interfere with the body's production of healthy cells, making the body unable to fight off infections. Leukemia is either chronic (slowly progressing) or acute (rapidly progressing). There are many types of leukemia. The four main types are: 1. Acute lymphocytic leukemia; 2. Acute myeloid leukemia; 3. Chronic lymphocytic leukemia; 4. Chronic myeloid leukemia. Who did this study? The National Institute for Occupational Safety and Health (NIOSH) is a federal research agency that works to improve the health and safety of workers. NIOSH researchers looked at a group of nuclear workers to see if being exposed to low doses of ionizing radiation in the workplace may or may not be linked with fatal leukemia (death caused from leukemia). What nuclear facilities were involved in this study? Five U.S. nuclear facilities were involved: 1. Hanford Site; 2. Los Alamos National Laboratory (including Zia workers); 3. Oak Ridge National Laboratory; 4. Portsmouth Naval Shipyard; 5. Savannah River Site. Between the 1940s and 1990s, a total of 94,517 workers were monitored for radiation exposure at these facilities. Using death certificates, we found 257 of these workers died from a type of leukemia. Who was included in the study? Because this study focused on fatal leukemia we wanted to learn more about the workers who died from leukemia by comparing them to workers who did not die from leukemia. This study included 249 of the 257 workers who died from leukemia (eight workers were not included because it was not clear what type of leukemia they died from). These 249 workers died from different types of leukemia. We compared the 249 workers to a sample of 995 workers from these sites who did not die from leukemia and were the same age as the workers who had died from leukemia. How was the study done? Using information and records gathered from the study sites, we estimated radiation doses for the 249 workers who died from leukemia. Radiation doses were also estimated for the comparison group of 995 workers who did not die from leukemia. We noted if any of these workers smoked or were exposed to other possible cancer-causing agents in the workplace, including benzene and carbon tetrachloride. We compared the radiation doses of workers who died of leukemia to the radiation doses of the comparison group. From this, we were able to examine the possible link between fatal leukemia and the amount of radiation exposure in the workplace. This is called the "dose-response" relation. What did the study find? The death rate for chronic myeloid, acute lymphocytic, acute myeloid and the other, less common types of leukemia combined was slightly higher in workers exposed to more than one rem of radiation total during their employment at one of these sites compared to workers who received lower doses. No increase in death rate for the chronic lymphocytic type of leukemia was seen among these workers. On average, workers with fatal leukemia were exposed to a total of three rems of radiation during their employment at one of these sites. The dose response relation in our study was similar to earlier studies of radiation-exposed populations. What does this mean? After taking into account smoking and exposure to benzene and carbon tetrachloride, we found the risk of some types of leukemia was slightly higher in workers exposed to more than one rem of radiation. One way this can be explained is by comparing two groups of men in the U.S.: one group not exposed to radiation and a second group exposed to three rems of radiation (the average total exposure in the workers with fatal leukemia). The normal lifetime chance of dying from leukemia in the first group is 71 men out of 10,000. The lifetime chance of dying from leukemia in the second group is 77 men out of 10,000. If you were exposed to this level it does not mean you will get leukemia. It is important to remember that our study looked at these workers as a group, and therefore cannot predict the future health of any one person. What were the limitations? Because methods to record radiation exposure have improved over time, it is likely exposure estimates are more accurate for later years compared to those from the 1940's. When we looked at workers with the same amount of radiation exposure, we found workers hired after 1952 seemed to have a higher chance of leukemia than workers hired earlier. Also, this study only included workers who died from leukemia. Workers who currently have, or did not die from leukemia were not included because information on cancer illness was not available in all states until recently. What should you do? Average radiation exposures are generally lower in workers today compared to 40 years ago because of improved technology and changes in work practices. If you currently work with radioactive materials or non-radioactive hazardous materials, contact your health and safety representative if you have any questions about your exposure or how to best protect yourself.