More Chemicals May Pose Higher Risk for Humans than Previously Thought

Scientists publishing in the 13 July issue of Science believe that several organic chemicals like pesticides may pose higher environmental and human-health risks than previously thought.

Because regulatory agencies currently classify chemicals as hazardous according to how they accumulate in aquatic food webs, the authors call on the agencies to expand the number of chemicals monitored for posing threats to humans. That number could reach as high as one-third of the organic chemicals in use.

When Barry C. Kelly, a researcher at Simon Fraser University in British Columbia, and colleagues studied terrestrial food chains in northern Canada, they found several accumulated organic chemicals were nearly absent in marine animals.

For example, the lichen-caribou-wolf food chain showed high levels of the pesticides chlorobenzene and lidane—two chemicals that did not accumulate to dangerous levels in surrounding aquatic environment.

As organisms are exposed to hazardous chemicals like the pesticide DDT through eating and breathing, some of the toxic substance is stored in adipose, or fat, cells. Over time, these chemicals build up through bioaccumulation.

Animals higher on the food chain, like humans, are more vulnerable to the slow build-up as they consume organisms lower on the food chain that contain the toxic chemicals. This process is called biomagnification.

The researchers believe the difference in chemical accumulation between aquatic and terrestrial animals has to do with how they breathe.

While both terrestrial and aquatic organisms release the chemicals through respiration, breathing in water appears to be more efficient at removing toxins like chlorobenzene and lidane, causing the chemicals to accumulate in the terrestrial food webs but not in aquatic environments.

Calling for increased government attention to the issue, the authors believe these chemicals "constitute an unidentified class of potentially bioaccumulative substances that require regulatory assessment to prevent possible ecosystem and human-health consequences."

Evelyn Brown and Benjamin Somers

13 July 2007