Officials from the DHHR’s Bureau for Public Health, the CDC and the West Virginia University School of Public Health will go door to door to randomly survey selected households about public-health concerns during the leak, the DHHR said. Surveyors will carry official credentials to identify themselves as public-health staff. All responses will be voluntary and will be kept confidential, the DHHR statement said.

While the hospital-chart review provides information about the more than 500 residents who sought medical treatment at hospitals after exposure to the coal-cleaning chemical Crude MCHM, the door-to-door survey will give scientists information about a more random selection of residents.