Research in industrial workers and professionals exposed to formaldehyde suggests that occupational exposure to this important chemical is associated with increased risk for myeloid leukemia. The significance, however, of these observations for occupational and environmental health is uncertain because of inconsistencies among epidemiologic studies and lack of a demonstrated mechanism through which formaldehyde can cause leukemia. Cytogenetic damage is one potential leukemogenic mechanism, but there are few studies of formaldehyde-exposed humans. Some experimental data suggest that epigenetic changes in myeloid cells could also be involved. We plan to study 40 workers exposed to high levels of formaldehyde and 40 unexposed controls to examine the hypothesis that formaldehyde is associated with these changes. To determine formaldehyde exposure, we will incorporate a number of methods, including questionnaires to determine potential past exposure and on-site monitoring to determine current average and peak intensities of exposure. We will then examine differences in aneuploidy and structural abnormalities in myeloid progenitor cells cultured from peripheral blood. We will specifically look for differences in genes associated with myeloid leukemia such as monosomy 7 and trisomy 8 using interphase and Octochrome FISH. We will also determine whether aberrant methylation in progenitor cells, specifically hypermethylation of genes associated with myeloid leukemia, is higher in those exposed to formaldehyde. This study will substantially contribute to our understanding of the leukemogenic potential of formaldehyde, which has important public health and regulatory implications.