Government agencies planned to conduct air monitoring tests to gauge the hazard, refinery manager Kollin Schade said during an evening press conference.
The duration and extent of the toxic hazard depends on a variety of factors, such as wind direction and speed, proximity to the refinery and weather events that can trap pollution close to the ground, said Elena Craft, senior health scientist for the Environmental Defense Fund.
Federal officials did not immediately respond to questions about the health risks from the smoke. A spokeswoman for the Wisconsin Department of Health Services referred questions to local officials in Douglas County, who could not be reached for comment.
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