Table 1.

Definitions of exposure levels among World Trade Center Workers.

Group	Definition
Low	Exposed for less than 40 days, not exposed to the cloud and did not work on the pile
Intermediate	Not exposed to the initial cloud, worked between 40 and 90 days or did not work on the pile
High	Exposed to the dust cloud but either worked less than 90 days or did not work on the pile
Very high	Worked more than 90 days, exposed to the dust cloud, and worked in the pile