A Minnesota television station's investigation has found that most schools in the state don't test school buildings for radon.

While state law doesn't require radon testing, state and federal guidelines suggest schools test for radon every five years, KARE-TV reported.

Radon is colorless and odorless gas that's a known carcinogen. Children are particularly vulnerable to the chemical, according to medical research.

About 50 of the state's roughly 330 public school districts have reported conducting tests since 2012, according to figures from the Minnesota Department of Health.

The state recommends schools with radon levels of 4 picocuries per liter to create plans to reduce levels. Data shows that only half of the nearly 170 classrooms where high radon levels were detected documented follow-up testing.

Most Stillwater and Elk River schools go about a decade between testing, while most Minneapolis public school buildings haven't been tested in 20 years, according to the TV station's analysis of figures acquired through public records requests.

"Potentially that leaves children and teachers and staff . at increased risk of lung cancer," said William Angell, a University of Minnesota professor and expert on radon testing.

About 80 percent of the state's counties are labeled as radon "hot zones," according to experts.

St. Paul Public Schools tests all ground level rooms every five years for radon, lead, mercury and asbestos.

Testing during the 2013-14 school year found about 30 classrooms with radon levels exceeding the EPA's action level. The district has adjusted air-handling units, installed mitigation systems and retested rooms to keep radon levels in check.

"We're going to get the data, we're going to find out what's a problem and we're going to fix it where we need to or prove that our spaces our safe," said Jeff Connell, the district's assistant director of facilities.

The district is preparing to test buildings for the 2018-19 school year and has budgeted about $180,000 for the efforts.