A new state building code that took effect a few years ago was supposed to reduce the risk of radon exposure in Minnesota, where roughly 40 percent of homes tested come back at unsafe levels.

Still, health officials were concerned that the code left some homeowners exposed to potentially deadly levels of the cancer-causing gas.

It turns out that the mitigation systems required in new homes by the 2009 code change aren’t always effective at reducing radon levels below the federal health safety standard.

Roughly one in five new homes with the passive radon mitigation systems had radon levels above 4.0 picocuries per liter, according to preliminary results of a state Health Department study launched last fall. That’s above the level that the U.S. Environmental Protection Agency considers safe.

“That’s still a quarter of our population being highly exposed to something that gives lung cancer,’’ said Joshua Miller, manager of the department’s radon program.

As part of the study, the Health Department tested 770 new homes. If tests showed high radon levels, the department offered homeowners fans to upgrade their passive mitigation systems. (Passive systems consist of a pipe dug into a home’s subbasement area to vent the radioactive gas; a fan gives added suction.)

The fan-driven systems are costlier, but are what most radon mitigation contractors install.

The Health Department handed out about 50 fans and then retested the homes. The average test score after the fan was installed was about 0.3 picouries per liter.

“You can’t get any better,” Miller said. “It’s essentially outdoor air. There’s very little radon getting into homes with the fan installed.”

Health officials encourage Minnesotans to test their homes for radon because it is the second-leading cause of lung cancer deaths in the United States, killing an estimated 21,000 people a year.

Because of local geology and other factors, Minnesota and the Midwest have some of the highest radon levels in the country.

For more information on radon and radon testing, visit the department’s website (www.health.state.mn.us). Or e-mail Miller at joshua.miller@state.mn.us if you are a new homeowner interested in participating in the study.

To look up radon levels in your area of the state, visit startribune.com/radon.