By John Gever, Staff Writer, MedPage Today
Published: April 17, 2008
Reviewed by Robert Jasmer, MD; Associate Clinical Professor of Medicine, University of California, San Francisco

CHICAGO, April 17 -- Working with formaldehyde may increase the likelihood of developing amyotrophic lateral sclerosis (ALS), a researcher said here, but pesticides were exonerated as a factor.

Both were major surprises from the largest-ever prospective examination of chemical exposure and ALS, Marc Weisskopf, Ph.D., of the Harvard School of Public Health, said at a press conference at the American Academy of Neurology meeting.

Formaldehyde had not previously been associated with ALS or ALS mortality, he said, but pesticides had been the focus of strong suspicion on the basis of reports from smaller studies.

Among people who reported specific durations of formaldehyde exposure, primarily at work, the adjusted risk ratio for death was 2.47 (95% CI 1.55 to 3.05) relative to those without formaldehyde exposure, Dr. Weisskopf said.

He and colleagues analyzed data from 987,229 persons included in the American Cancer Society's Cancer Prevention Study II. Participants were interviewed about chemical exposures, occupation, and lifestyle and demographic factors in 1982 and then were followed for more than 20 years.

They analyzed causes of death for those who died from 1989 to 2004. They identified 1,156 persons who died of ALS. Of these, 22 had reported exposure to formaldehyde at the 1982 interview and were able to estimate a duration of exposure.

They found an apparent dose relationship between formaldehyde and ALS. Those with more than 10 years of exposure to the chemical had an adjusted relative risk of 4.1 (95% CI 2.1 to 7.0); for those with four to 10 years of exposure, the adjusted relative risk was 2.1 (95% CI 0.9 to 5.5); and for those with less than four years of exposure it was 1.5 (95% CI 0.5 to 4.1).

The risks were adjusted for factors known to affect ALS prevalence, including sex, smoking, military service, education, alcohol intake, and vitamin E supplement use.

However, Dr. Weisskopf pointed out, the numbers in these subcategories were small. Only four of the ALS deaths involved people with formaldehyde exposure if less than four years and five deaths were in individuals with four to 10 years of exposure.

The researchers correlated participants' occupations with their on-the-job exposure to formaldehyde, then determined if there was a relationship between high-exposure jobs and ALS risk.

Beauticians most frequently reported on-the-job formaldehyde exposure (24.6%), followed by pharmacists/morticians/chemists, radiological and lab technicians, doctors and veterinarians, dentists, photographers and printers, and nurses (6.6%).

Dr. Weisskopf and colleagues found that those in high-exposure jobs had an adjusted risk ratio for ALS of 1.3 (P=0.02).

In addition to formaldehyde and pesticides, the researchers also looked at reported exposure to gasoline and diesel exhaust, various toxic dusts, dyes, x-rays, asphalt and related products, asbestos, and solvents. None had any significant relationship to ALS rates.

Dr. Weisskopf identified several limitations of the study, including self-reporting of exposure data and the small numbers of cases in some exposure categories.

He also noted that the study relied on death certificates to identify ALS cases. However, he said death from ALS was a reasonable surrogate for ALS cases because the disease is nearly always fatal within a few years of diagnosis.

Previous studies reported that 70% to 90% of ALS deaths are accurately reported on death certificates, he said.