IREDELL COUNTY, North Carolina — Considered one of the United States' most serious environmental health problems, radon testing in schools is recommended by the Environmental Protection Agency. A Defenders investigation found few schools take that advice.

NBC Charlotte surveyed 45 school districts in the Carolinas and found just seven that test for the radioactive gas. Our questions prompted a new test in one Western North Carolina district, which found elevated levels and led to a plan addressing the problem.

Nearly one in five schools has at least one room with a short-term elevated level of radon, which is the second leading cause of lung cancer in the U.S., according to the EPA. The federal government warns concerns are highest during the winter months.

The Iredell-Statesville Schools district is one of few districts that took the federal government's advice. ISS paid an environmental firm $13,000 to test 10 schools last month with buildings that have basements.

"We just wanted to be proactive," ISS Communications and Development Director Dr. Boen Nutting said. "We're very interested in making sure that our children are safe."

ISS joins a small group of school districts, including Charlotte-Mecklenburg, Kannapolis City, Catawba and Lincoln, that have tested for radon in recent years.

Dozens of others district have not tested. Some are in what are considered radon hot zones, highlighted in red on EPA radon maps and known for radon readings higher than the federal action benchmark. Others highlighted in orange on federal radon maps are known for levels near that action level. 

WCNC's Defenders found districts in both red and orange that don't test, mostly, because it isn't required by law.

"Testing is not required through any regulatory agency in North Carolina," Union County Schools Assistant Superintendent Tahira Stalberte said. "Effective practices of building management, such as but not limited to, sealing foundation cracks and maintaining adequate building ventilation, are utilized to manage any potential existence of, or mitigate any exposure to existing radon in the surface soils."

Communications Director Ronnye Boone told NBC Charlotte Cabarrus County Schools complies with local, state and federal laws. 

"Radon testing is not required by any regulatory agency in North Carolina," Boone said. "Our district's health and safety professionals work hard to employ building and facility management practices to identify or mitigate any known hazards."

A lawmaker is now asking questions.

"We're going to ring the alarm about this and hopefully get some answers," said Representative Chaz Beasley (D), NC-District 92.

Rep. Beasley said the Defenders' discovery surprised him -- now he plans on taking action to make sure kids are safe.

"That could have been me, that could have been my family who is from Buncombe County," Beasley said. "We want to be proactive about these things." 

Radon is an odorless, colorless and naturally-occurring gas that seeps into buildings and is the number one cause of lung cancer for non-smokers. It's responsible for an estimated 21,000 deaths every year, according to the EPA.

Because of that, the EPA and other experts recommend tests. Our investigation found some schools last tested more than 20 years ago -- including Watauga County, which is in the red zone. 

District administrators said that test found three rooms above the acceptable threshold for radon, but even more surprising, no records currently exist to show what action, if any, the district took. 

As a result of Defenders' questions, Watauga County retested and found a handful of rooms with levels above the federal action level at Blowing Rock School.

"As a result of the new tests, we consulted with a radon mitigation contractor who recommended a course of action to deal with the remaining issues -- which according to my understanding, called for improvements in the way rooms are ventilated," Communications Director Garrett Price said. 

Based on that recommendation, they contacted an architectural firm handling facility renovations across the district. 

"The firm recommended upgrades to our HVAC system that we have now built into our upcoming facilities improvement budget request to our county commissioners," Price said. "It is our intention to make the necessary facility upgrades as part of ongoing renovations to some of our aging facilities."

Back in Iredell County, records show tests there found levels below the federal action level, which is the same as the most recent tests in CMS and Lincoln County.

Rowan-Salisbury told NBC Charlotte the district is putting a radon testing program in place. Rock Hill Schools confirms the district has secured quotes for radon tests in seven locations, but has not voluntarily completed any testing at this time.