Low-level environmental exposures--more dangerous than you thought?

Low levels of common environmental toxins may pose a more significant concern for human health than is generally recognized, according to "Human Health Risks from Low-Level Environmental Exposures: No Apparent Safety Thresholds," an article that appeared in the December 2005 issue of PLoS Medicine (Volume 2, Number 12, page e350). Authors Donald Wigle, of the McLaughlin Centre for Population Health Risk Assessment at the University of Ottawa, and Bruce Lanphear, of the Cincinnati Children's Environmental Health Center at Cincinnati Children's Hospital, consider the cases of four common environmental hazards: lead, secondhand tobacco smoke, radon, and chlorination disinfection by-products in drinking water. Their conclusions are based on pooled analyses of epidemiological studies.

For both lead and secondhand smoke, these analyses found that dose-response relationships between exposure and cognitive impacts were actually stronger at low blood or serum concentrations of the contaminants.

With respect to lead, the authors write: "In a pooled analysis of seven of the eight prospective longitudinal studies, the investigators reported that the average IQ deficit associated with an increase in concurrent blood lead concentration from less than 0.048 [micro]M to 0.48 [micro]M was about 3-fold higher than the IQ deficit associated with an increase in concurrent blood-lead concentration from 0.48 [micro]M to 0.96 [micro]M."

With respect to radon, they cite findings by the National Council on Radiation Protection and Measurements that "the most plausible relationships between low-level ionizing radiation and mutations, chromosome aberrations, and cancer are linear, with no threshold." They also point out that "a pooled analysis of eight epidemiologic studies of underground miners showed that the excess risk of lung cancer per unit of cumulative radon exposure was greater at lower exposure levels. Among men with the same cumulative radon exposure," they conclude, "prolonged exposure at low levels is more hazardous than shorter exposures at higher levels."

With respect to chlorination by-products, they observe that a pooled analysis of six studies showed "a lifetime bladder cancer risk of about seven per 1,000" in men exposed to trihalomethanes (THMs) at levels above 1 [micro]g/L.

In general, Wigle and Lanphear believe that epidemiological studies should be preferred over animal studies in the development of environmental standards. They also argue that "risk assessments should not assume thresholds for noncarcinogens as well as carcinogens, especially for toxins shown in epidemiologic data to exhibit no apparent threshold and those not yet adequately tested for developmental toxicity."

Readers can find the complete article at http://medicine.plosjournals.org/perlserv/?request=get-document&doi=10.1371/journal.pmed.0020350.