In this study, the risk of death from myeloid leukemia was 78 percent higher among industrial workers with the highest peak exposures compared to those with the lowest peak exposures. Excess risks of death from myeloid leukemia have also been reported among pathologists, embalmers, and other professionals who experience high-intensity peak exposures to formaldehyde. The highest level of increased risk of death from myeloid leukemia in this study occurred early on and has been declining steadily over time. This pattern could be due to chance, but the investigators note that similar patterns of risks over time have been seen for agents that are known to cause leukemia relatively soon after exposure.

"We know that various groups of professionals who may experience high peak exposures to formaldehyde are at increased risk of leukemia, but the evidence from studies of industrial workers, among whom exposure levels and patterns may be more variable, has been conflicting. The fact that we see an excess in this study of industrial workers, which is both the largest and the one with the most extensive exposure assessment, is notable," said Beane Freeman.

Based on the available data, scientists have not been able to identify a mechanism for how normal white blood cells might become leukemic following exposure to formaldehyde, because there is no direct evidence that formaldehyde damages cells in the bone marrow. However, studies of humans exposed to inhaled formaldehyde have shown higher rates of damage to their chromosomes in a type of mature white blood cells compared with rates in individuals who were not exposed to formaldehyde. Although the relevance to the development of leukemia of such chromosomal damage to mature white blood cells is not clear, agents that cause leukemia are also known to be associated with chromosomal aberrations in the peripheral blood cells of humans.

This study is also the first to report a statistically significant association between a chemical exposure and increased risk of death from Hodgkin lymphoma. Although based on a small number of deaths, the finding may warrant further study.

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