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

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.

"There is no science on these levels of exposure in children," he said.

Some say that levels of exposure generally considered safe for most people can have an effect on a few.

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

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

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

Therefore, in some cases there could be a potential for higher levels of exposure to persons on the ground.

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