Most parents expect school air to be safe. They worry about grades, lunches, bus delays, and the occasional hallway cough, not an invisible radioactive gas rising from the ground beneath the building. Yet radon can collect inside classrooms, cafeterias, offices, and libraries without leaving a smell, color, or taste.
The U.S. Environmental Protection Agency says a nationwide survey estimated that nearly one in five schools has at least one schoolroom with a short-term radon level above the action level. The agency also estimates that more than 70,000 schoolrooms in use today have high short-term radon levels, making testing less of a paperwork issue and more of a basic health safeguard.
Why radon matters
Radon is a radioactive gas that forms naturally underground. It comes from the breakdown of uranium in rock and soil, then moves upward through cracks, gaps around pipes, construction joints, and other openings in a foundation.
The health concern is long-term breathing exposure. The agency estimates that radon is the leading cause of lung cancer among nonsmokers and is linked to about 21,000 lung cancer deaths each year in the United States.
How it enters schools
A school can act a bit like a vacuum. When indoor air pressure is lower than the pressure in the soil, radon can be pulled into the building through the foundation and into occupied rooms.
That is why two classrooms in the same hallway can show different results. The agency’s school guidance says radon levels can vary significantly from room to room, so one safe room does not prove that the room next door is safe.
Old maps can miss risk
Radon maps are useful, but they are not a substitute for testing. The agency developed its national radon zone map in 1993, and warns that it should not be used to decide whether an individual building needs testing.
A 2025 study in Proceedings of the National Academy of Sciences led by Longxiang Li, with researchers from Harvard T.H. Chan School of Public Health and Emory University, compiled more than six million radon measurements from 2001 to 2021.
The team found that current national maps lack up-to-date, high-resolution detail, and estimated that more than 83 million people live in residences where screening-floor radon is above the recommended action level.
Testing every used room
The agency’s school measurement guide says schools should test all frequently occupied rooms that touch the ground. That includes classrooms, offices, laboratories, cafeterias, libraries, and gymnasiums, while spaces like restrooms, hallways, and utility closets are usually not part of the first round.
Short-term tests are the quickest starting point and can run from 2 to 90 days. Long-term tests stay in place for more than 90 days and can give a better picture of average exposure during the school year.
When schools should test
Cooler months are often the best time for short-term school testing. Windows and exterior doors are more likely to stay closed, heating systems are usually running, and the results may better reflect the average radon level during the school year.
Tests should be done under normal school conditions, not during unusual weather or with unusual ventilation patterns. A fan schedule, an open window, or a recent renovation can change the numbers more than people might expect.
What the numbers mean
A picocurie per liter is the unit used in the United States to describe radioactive decay in a volume of air. In plain English, a higher number means more radioactive particles may be available to breathe into the lungs over time.
The agency recommends that schools reduce radon when a room reaches 4 picocuries per liter or higher. It also says any radon exposure carries some risk, so lower is better when practical.
Fixing high levels
A high first result should trigger follow-up testing, not panic. The agency does not recommend making a mitigation decision based on a single short-term test, because indoor radon can move up and down from day to day.
When mitigation is needed, professionals often look for entry points and pressure problems before choosing a fix. A common approach is to draw soil gas from beneath the slab and vent it above the roof, where it can disperse safely.
Boise showed the process
Boise School District reported elevated radon levels in select basement classrooms at North Junior High School and temporarily relocated the affected classrooms as an extra precaution. The district later said mitigation had reduced levels to safe limits and that it would continue regular monitoring.
That kind of public update matters. Families do not need vague assurances when a school finds radon. They need numbers, follow-up plans, and a clear explanation of what changed after repairs.
The data gap
The agency recommends that all schools be tested, but says only about 20% of schools nationwide have done some testing. That leaves many districts depending on local budgets, local rules, and local attention.
A pooled European study of residential radon and lung cancer found increased risk across home exposures, reinforcing the idea that radon should be treated as a long-term prevention issue rather than a one-time scare. The point is not to frighten families, but to keep routine testing from slipping off the calendar.
What families can ask
Parents can ask when their child’s school was last tested, which rooms were included, what the results were, and whether follow-up tests were done. They can also ask how renovations, heating and cooling changes, or basement classroom use will be handled in future testing plans.
Homes near a school can share the same geology, so a simple home radon test can also be useful. At the end of the day, the only way to know the level is to measure it.
The official guidance was published by the U.S. Environmental Protection Agency.
