Smoking, Chemical Exposure, and ALS Risk

A prospective study of 5 large cohorts (N > 1 million), presented Wednesday at the annual meeting of the American Academy of Neurology, showed that the relative risk of amyotrophic lateral sclerosis (n = 633) increased significantly with the duration of smoking and the number of cigarettes smoked per day (AAN abstract P04.076). Investigators reported that the multivariate-adjusted RR increased from 1.2 for 1-19 pack-years to 1.4 for ≥20 pack-years (P = .03), and that the risk did not vary by sex.

Investigators also reported the risk of ALS mortality as a function of chemical exposure in more than 1 million participants in the Cancer Prevention Study II (AAN abstract S25.005). From 1989 to 2002, the risk of death due to ALS (n = 937) was not associated with regular exposure (self-reported) to pesticides or herbicides (RR = 1.1). However, regular exposure to formaldehyde did increase the risk of ALS-related mortality (RR = 1.5), and this risk increased with more years of exposure.

The investigators advised that this finding is unlikely the result of bias, because of the longitudinal design of the study. (It is important to remember that increased relative risks do not indicate cause-and-effect relationships.)

According to OSHA, formaldehyde exposure occurs most often by means of gas inhalation, but absorption of formaldehyde liquid through the skin is also possible. High-risk exposure is observed most commonly in the health care professions (particularly pathology and histology), in mortuary work (duh), and among students or teachers who handle preserved specimens.