MINNEAPOLIS (AP) — 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.

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 reports.

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.

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