Radon gas causes cancer, no question about it. And while 40 percent of homes in Minnesota have dangerous levels of the odorless, colorless, radioactive gas, which occurs naturally in soil, only 1 percent have been tested for it in the past five years, the state Health Department (MDH) warned — again — on Thursday.

The good news is this: Thanks to building code requirements enacted in 2009, there are fewer radon gas problems in new construction homes, and a 2014 state law requires home sellers to disclose and notify buyers about whether a home has been tested for radon gas and, if so, what has been done to mitigate the problem, said Dan Tranter, indoor air specialist at the MDH.

That hasn’t solved the problem, though, Tranter said. While about 40 percent of the overall housing stock in Minnesota has dangerous levels of radon, that number drops to 20 percent for new homes.

The Centers for Disease Control estimates more than 21,000 lung cancer deaths each year in the United States are linked to radon gas. Of those, perhaps 600 deaths in Minnesota are linked to radon, Tranter said.

The MDH said that a dangerous level of radon is defined as 4.0 picocuries per liter of air (pCi/L), defined by a test kit, and the average level of radon in Minnesota homes is about 4.6 pCi/L, more than two times the national average. The Health Department recommends installing a radon mitigation system for anything above 4.0 pCi/L and even for homes in the range of 2 to 4 pCi/L.

The only way to test for radon gas is with a kit. The kits are available free or at low cost from some city and county health departments, online from laboratories and at hardware stores. The MDH has links on its website to sites where kits are available as well as radon mitigation specialists.

Kits are placed on the lowest, most-lived-in level of a home for three to seven days, then sent to a lab, which will send the homeowner or renter the results. A mitigation system involves installing a pipe that runs underground and into the house, with a fan that continually operates. Tranter estimated the cost at $1,500 to $2,000.

There really is no do-it-yourself solution to radon gas, Tranter said. Sealing up cracks and holes in the basement or foundation “in most cases will not work,” Tranter said. “We don’t want people to do that and think they’re done.”

Tranter said the MDH does recommend a second test to confirm high levels of radon before spending that amount of money.

“You can have a system installed that is highly effective in almost all cases,” he said. “The hard part is writing a check.”

Why does Minnesota have such high levels of radon?

“It has to do with the soil and what’s present in the soil,” Tranter said. “It has to do with homes being closed up for most of the year. It’s just the way our houses are operated. Those factors combine to cause high levels in Minnesota as well as in much of the Midwest.”

It’s also not clear whether tight, well-insulated homes are more at risk than older, drafty homes, he said.

“The bottom line is you need to test, and without the test, you just can’t know,” he said.

Radon gas is created by the breakdown of uranium in the soil. It floats to the top of the soil and is drawn into homes.

Gov. Mark Dayton has proclaimed January as Radon Action Month in Minnesota. The MDH will be sponsoring TV, radio and internet ads in the Twin Cities and outstate to encourage people to test their homes.