MANKATO — Melissa Matthies regrets it now. But when it came time to set the closing date for the purchase of her home, she hadn’t thought to wait until the results of the radon tests came back.

Of course, after the sale was finalized and it was too late to back out, the results came back with bad news.

The Environmental Protection Agency says a safe level of radon in your home is 4 picocuries per liter. The radon in Matthies’ home measured 56.5 pCi/L.

She called her dad.

“He told me it was really high,” she said.

So she bit the bullet and called a professional radon mitigator to take care of the problem.

Andrew Kelley, owner of Radon Solutions, came to her aid.

It was a common call for Kelley, the kind he’s gets all the time these days.

“It used to be seasonal,” Kelley said. “Now, I’m busy year-round with it.”

They call carbon monoxide the silent killer. But that title could also apply to radon.

You can’t see it, smell it or taste it. And unlike carbon monoxide, it won’t hit you immediately. But it can kill you Ñ eventually.

Long-term exposure to radon, the Minnesota Department of Health said, can lead to lung cancer. Each year radon causes more than 21,000 lung cancer deaths in the U.S.

Such deadly facts have lead the Department of Health to spread the word more and more each year about radon and its deadliness.

Here’s how radon works, according to the health department.

Radon comes from the soil. It occurs as a product of the natural decay of uranium that is found in small amounts in most soil. Uranium breaks down to radium. As radium disintegrates, it turns into a radioactive gas called radon. Radon moves up through the soil and into the air you breath.

This simple fact of nature would be harmless except for the fact that radon can accumulate in dangerous levels in dwellings. The air pressure inside your home is usually lower than the pressure in the soil. This creates a vacuum effect, causing radon to push into your home wherever there is a crack or opening.

How much radon is in a house can vary from one house to the next. Your house may be safe while your neighbor’s house may have dangerous levels.

One thing the health department knows for sure: In Minnesota, the counties with higher levels of radon create an L shape on a map. Southern Minnesota is well within the area where higher levels are more common.

Determining how high radon levels are in your house depends begins with a test kit, which can be purchased at a hardware store or obtained through the health department. (In some cases, counties or county partnerships have made kits available to residents.)

Radon can come into your house in many ways: cracks in concrete basement slabs, spaces behind brick veneers, floor-wall joints, exposed soil, such as a sump pump or crawl space, mortar joints, drainage tiles, open tops of block walls and many others.

If the kit comes radon levels as high, then it’s time to contact a mitigation specialist, such as Kelley.

They’ll run their own tests and figure out the best possible course of action. Most cases, though, involve a radon-removal system. The systems run on a simple premise: Figure out where the epicenter of radon seepage into the house is and provide a place for it to funnel out of the house (with the help of a fan).

In Matthies’ case, getting the radon in her house down to safe levels will run her $1,380.

“The majority of these houses are mitigated really easily,” Kelley said.

The fix in this case involved running a radon gas collection pipe from the sump pump hole in the basement floor to the roof. In this case, drainage tiles that went around the home emptied into the sump. Those drainage tiles are prime spots for radon to enter the home.

Kelley began installing radon mitigation systems in 2007. That year he installed 20 of them. Now, he installs about 100 per year. He said he’s going to need a second truck to keep up with all the business coming in.

Luckily, his son, Mitch, is about to take the radon mitigation certification test.