Ramping up their campaign to reduce health hazards from radon, state officials released an online map Tuesday that Minnesotans can use to check risk levels in their area.

The Minnesota Department of Health hopes the map will encourage homeowners to test for radon, an odorless gas that is the leading cause of lung cancer in nonsmokers.

About two of every five homes in Minnesota have levels of radon so high that homeowners should take steps to eliminate the health danger.

High radon levels have been detected in 78 percent of Minnesota counties.

“Radon is a problem all across Minnesota, but it can be hard to know if you have a problem because you can’t see it or smell it,” said Dan Tranter, indoor air program supervisor at the state Health Department.

“With the housing market heating up this summer, it is important for home buyers to know their rights and get potential homes tested for radon so any problems can be fixed before buying a house.”

Under a state law passed in 2014, home sellers must inform buyers about any radon testing that has been done. However, state law does not require homeowners to test for radon or to mitigate the risks.

Radon is a radioactive gas that seeps up through the soil. It can enter homes, both new and old, through many pathways, including cracks in the foundation, exposed soil in a crawl space or mortar joints.

More than 86,000 homes were tested in Minnesota between 2010 and 2014. Of those, 45 percent had radon levels reaching or exceeding dangerous levels. Although high radon levels were detected statewide, they were most prevalent in the state’s southwestern counties.

Radon test kits are available at city and county health departments, hardware stores and radon testing laboratories.