Chronic on-the-job exposure to some solvents may lead to cognitive deficits in workers with less than a high school education, while those with more years of schooling may have protective mental reserves, a large French study found.
The prospective study of more than 4,000 gas and electric workers found that not only was exposure greater among the less-educated, but there was also a dose-response relationship between lifetime exposure to three out of four solvent types and a relative risk of poor cognition, particularly for high exposure to benzene (RR 1.24, 95% CI 1.09 to 1.41) with significant linear trends (P<0.05), researchers reported in Among 765 worker the journal Neurology.
Those men exposed to solvents who completed a cognitive test had a mean score of 48.86 (SD 9.61, range 5 to 93), with a cutoff score for "impaired" of ≤41. Among 765 workers with low educational attainment, 32% were found to be cognitively impaired, compared with 16% of 273 better-educated peers, wrote Lisa F. Berkman, PhD, of Harvard University, and colleagues.
In their introduction, Berkman and co-authors wrote that occupational organic solvents are known to be neurotoxic and are risk factors for cognitive impairment, affecting central nervous system (CNS) functioning, including attention, processing speed, and motor performance.
Berkman's team noted that in the U.S., federally mandated permissible exposure limits for some solvents set by the Occupational Health and Safety Administration may be "insufficient to protect workers against health consequences of exposure."
"It's very difficult to change these standards and current regulatory bodies have varied exposure limits. But this study suggests that education tends to give people an extra layer of protection," study co-author Erika Sabbath, ScD, of Harvard University, told MedPage Today in an interview.
Solvents -- classified as chlorinated, petroleum, benzene, and nonbenzene aromatic -- are used in paints, degreasers, pesticides, and adhesives, with exposure prevalence estimated to be 8% among working populations in industrialized countries.
But, they noted, little is known about potential effect modifiers, and there is "evidence of education's role in shaping cognitive reserve -- the ability of the brain to withstand insults and continue normal functioning."
So they tested 4,134 men who were part of the GAZEL (French national gas and electric workers) cohort, to assess the effects of solvent exposure and whether those effects varied by levels of educational attainment.
They also examined whether any differences might be attributable to more recent exposure among less-educated men.
Participants were given the Digit Symbol Substitution Test (DSST) which measures executive function and cognitive skills in relatively young, healthy populations. The DSST is a subscale of the commonly-used Wechsler Adult Intelligence Scale.
The mean age of the men was 59 in 2002–2004 when they took the DSST. At the time, more than half of the cohort (58%) reported low levels of education.
In fully adjusted models that accounted for age, occupational grade, alcohol consumption, and smoking, among less-educated workers the relative risk (RR) for cognitive impairment was greater among those with greater exposure to the four solvents, compared with those who were not exposed. Specifically:
- For chlorinated solvents (RR 1.14, 95% CI 0.99 to 1.31).
- Petroleum solvents (RR, 1.14, 95% CI 1.01 to 1.28).
- Benzene (RR 1.24, 95% CI 1.09 to 1.41).
- Nonbenzene aromatic solvents (RR 1.36, 95% CI 1.06 to 1.76).
Point estimates showed a dose trend, and trend tests indicated significant or near-significant dose-response relations between each solvent type and cognition:
- Chlorinated solvents (P for trend 0.06).
- Petroleum solvents (P=0.04).
- Benzene (P=0.002).
- Nonbenzene aromatic solvents (P=0.01).
In the fully adjusted models, among better-educated men "we observed no statistically significant associations between high solvent exposure and cognitive impairment, and no significant linear trends between exposure dose and cognitive impairment for any solvent type," Berkman and co-authors wrote.
However, "[w]e found a dose-response relationship between time since last exposure and cognition in the whole population, with the most proximally exposed experiencing significantly worse cognition than unexposed in all categories except nonbenzene aromatic solvents," they wrote. "Tests of interaction between time since exposure and education were not statistically significant (P>0.05)."
When the investigators stratified by education, "among the lesser-educated men there remained a general trend of poorer cognition among the more recently exposed," they added.
The study had several limitations, including use of a one-time cognitive assessment after exposure. "There is risk of selection into the exposed group with respect to the outcome if individuals with poorer cognition tend to stay in higher-exposed (e.g., low-wage) jobs rather than being promoted out of such positions," the investigators noted.
At the same time, studying people with varying levels of education who can advance in their work without higher education allowed researchers a chance to examine the joint effects of education and solvent exposure on cognition. Another strength of the analysis was its use of a validated exposure assessment tool.
"Today, it's possible we are being exposed to chemicals that in 20 years we may find out are bad for the brain," remarked Sabbath. "Taken on a broad level, this paper suggests that the earlier part of life may be a very sensitive time to build up cognitive reserve, and could bolster arguments that keeping kids in school could have additional positive effects on health."
This study is part of a project funded by the Agence Nationale de la Recherche (ANR, French National Research Agency) and Agence Française de Sécurité Sanitaire de l'Environnement et du Travail (AFSSET, French Agency for Environment and Work Health Security), and was also supported by the American Heart Association.
Berkman reported links with the National Institutes of Health (NIH), the National Institute on Aging (NIA), the National Cancer Institute, the MacArthur Foundation, and the Robert Wood Johnson Health and Society Scholars Program. Co-authors reported links with the NIH, the NIA, the National Institute of Mental Health, the National Institute of Child Health and Human Development, the Robert Wood Johnson Foundation Health and Society Scholars Program, the Milton Fund for Harvard University Junior Faculty, and the American Heart Association.
- Reviewed by Zalman S. Agus, MD Emeritus Professor, Perelman School of Medicine at the University of Pennsylvania and Dorothy Caputo, MA, BSN, RN, Nurse Planner