Federal officials are set to wrap up the first part of their wide-ranging study of pollution levels in the North Texas town of Midlothian.

The U.S. Agency for Toxic Substances and Disease Registry began interviewing residents this week and were to wrap up by Saturday. The agency, charged with protecting public health around toxic sites, has promised a thorough review of the town that’s home to the nation’s largest concentration of cement plants.

Environmentalists for years have complained for decades that the tons of pollutants the plants emit every year cause major health problems.

The ATSDR is expected to finish its full study within a year.