The companies received letters from the U.S. Environmental Protection Agency asking for air-emission tests for soot, carbon monoxide and other compounds.
The Illinois plants are Rock Road Companies in Rockford and Chester Bross Construction in Loraine (about 20 miles northeast of Quincy).
Rock Road vice president Stephen Kennedy says his plant is one of 200 or so in Illinois, and he doesn't understand why he's being asked to run the costly tests and his competitors aren't. Chester Bross officials didn't return calls seeking comment.
A spokesman for the National Asphalt Pavement Association says the letters are unprecedented, and the group plans to talk to EPA officials.