A “significant public health threat” is brewing in Michigan because of chemical vapors that percolate up from the ground of former industrial or commercial sites and can leak into nearby homes, Department of Environmental Quality Director Heidi Grether said. Grether told lawmakers during a Tuesday budget presentation that there could be 4,000 sites in Michigan, where potentially harmful vapors could pose a problem to human health. She asked legislators to consider beefing up the department’s 2018 budget request by $2.6 million to dedicate eight employees to the issue full time.Metal de-greasers and a dry cleaning chemical contaminated soil and groundwater at former commercial and industrial sites and can result in chemical vapors filtering up into basements.