Research studies are looking into possible genetic, exposure level, and other risk factors.

The question is: At what exposure level does the risk become significant?

A problem that plagued the group was how to deal with questions of exposure level.

They represent acceptable exposure levels based on eight hours a day for five days a week.

But exposure levels in the Gulf were not high enough to cause such reactions.

Despite this exposure level, from 1985 through 2005, only three deaths were reported.

This is far below the exposure level currently considered to be harmful to human health.

About 4.5 million workers now face exposure levels above the new limits, the department said.

Even so, some people question whether the exposure levels typical of schools make much difference.

But personal exposure levels indoors increased from south to north.