The city will spray to kill mosquitoes on the Northwest and Southwest sides after mosquitoes tested positive for the West Nile virus in those areas.
There have been no human cases of it in Cook County, but a bird in downstate Illinois that tested positive for it.
Crews will spray in portions of the Auburn Gresham, Forest Glen and Washington Heights neighborhoods, where traps recently captured mosquitoes carrying the virus, according to the Chicago Department of Public Health.
Licensed mosquito abatement technicians will begin dispensing "ultra-low-volume spray" at dusk Wednesday until about 1 a.m. Thursday, weather permitting, according to a statement from CDPH. City Streets & Sanitation workers will guide the crews.
They will be spraying Zenivex, a chemical approved the U.S. EPA to control mosquitoes in outdoor areas, the statement said. The chemical is not harmful to people or pets and degrades rapidly.
"When our mosquito traps indicated that West Nile virus may threaten human health in a community, we take decisive action," city Health Commissioner Bechara Choucair said in the statement. "By spraying we are doing all that we can to help keep Chicagoans virus-free."