The National Institute of Environmental Health Sciences (NIEHS) convened the three-day meeting, which will look at indoor and outdoor pollutants' role in asthma, brain and reproductive system disorders, behavioral problems like autism and attention deficit hyperactivity disorder and cancer.

NIEHS director Kenneth Olden said the discussions are "important for the public health mission of this nation."

Human disease is caused not just by genetic susceptibility, but by the interaction of genes, age, development stage, behavior and environmental exposures, Olden said.

Children are especially vulnerable to pollutants because they breathe in more air and take in more food and liquid, proportional to their size, than adults, said Phil Lee, a senior scholar at the University of California, San Francisco, and former assistant secretary at the Department of Health and Human Services.

Also, children's quickly dividing cells and rapidly growing organs are more heavily impacted by toxins, Lee said.

Researchers believe that limiting exposure to smog, tobacco smoke, pesticides, lead, mercury, agricultural and industrial chemicals and other substances found indoors and outside can help head off many diseases.

But there is still only a small body of evidence proving that keeping children away from pollutants can prevent asthma, cancer or behavioral problems, Lee and others noted.

For instance, children and adults with ADHD have high levels of manganese in their bodies, Lee said. Manganese is a trace element essential for survival, but is also a known neurotoxin when it is highly concentrated. Infant formulas--both cow's milk and soy--have high manganese levels. But no one has been able to link manganese intake in infants and small children to later brain disorders, Lee said.

"The lack of research in this area has really been remarkable in light of the facts," he said.

It has also been shown that second-hand smoke and close proximity to automobile exhaust exacerbates asthma in children who already have the condition and can slow lung development. But it is not known if pollutants cause asthma, said Jonathan Samet of Johns Hopkins University.