Radon, released in the breakdown of soil and rock, can seep into buildings and the air we breathe and with chronic exposure can be deadly, experts said.
At one school in Pennsylvania, tests showed nearly double the EPA's accepted limit for radon gas, "Today" reported Wednesday.
Radon exposure has been linked to more than 20,000 deaths every year, the second leading cause of cancer after smoking, the U.S. Environmental Protection Agency said.
"Of all the environmental exposures you get, this is the one that causes the most deaths," said R. William Field of the University of Iowa, a leading expert on radon.
"If a student's exposed, even at the EPA's action level, 4 picocuries per liter, that's equivalent to smoking half a pack of cigarettes per day," he said.
Most U.S. schools don't test because districts can't afford it, experts say, even though the EPA estimates that more than 70,000 classrooms nationwide are at risk.
Only five states require radon testing and there's no federal law mandating it, "Today" reported.