Workers are testing the air inside 40 homes and businesses in St. Louis Park after chemical vapors were discovered in the soil under their basements, the U.S. Environmental Protection Agency said Wednesday.

Those properties were among more than 200 homes and businesses that were checked for potential risk from underground solvent contamination.

Sonia Vega, the EPA's on-scene coordinator, said there's no evidence that people living or working in the buildings are being exposed to contaminants, but that's the focus of additional tests that began this week.

"Until we see the indoor air data, we really don't know what we're looking at," she said.

Each of the 40 properties will be visited by a mobile lab, Vega said. Technicians are going room to room in the homes, using special hoses to pump air into the lab to see whether the vapors are present in high concentrations. They will also leave 24-hour sampling canisters in each building to test the air in the basement and first-floor levels.

The main chemicals of concern, trichloroethylene and perchloroethylene, have been used for decades as industrial degreasers, metal cleaners and dry-cleaning fluids. Long-term exposure to them at certain levels has been linked to cancer, liver problems and other adverse health effects, according to state health officials.

The EPA's air testing began after a continuing effort to monitor groundwater pollution in the suburb discovered solvent fumes in shallow soil samples last year. Officials suspected that the vapors could be a potential problem if they were seeping into homes through cracks in basements.

The affected groundwater is not used for drinking, according to city officials. State pollution control authorities are working with the EPA to identify and to investigate the sources of the chemicals.

The properties being tested are on both sides of Hwy. 7 near Wooddale Avenue. By last week, workers had pulled air samples from beneath the basement floors of 184 residences and 29 commercial or industrial buildings.

Vega said the vapors measured beneath the 32 homes and eight commercial buildings ranged from slightly above health guidelines to more than twice what is considered safe. The buildings with the higher vapor concentrations in their soils were clustered, Vega said, but she could not provide more details until the test results are completed and mapped.

She said it's not clear whether the testing area will need to be expanded until the data are analyzed.

If homes are determined to have unsafe levels of contaminants, Vega said the EPA has contractors on standby who could install a simple piping and fan system that would vent any vapors seeping into the buildings to the outdoors.

The system, similar to what's done for homes with radon gas in their basements, would take about one day to install, would cost about $1,500 and would be paid by the federal Superfund, she said.

The EPA expects to release results of the indoor air sampling in a week to 10 days, said spokesman Mick Hans. "We're still in the snapshot phase of the project," he said.

Tom Meersman • 612-673-7388