Fresno, California – A dangerous gas could already be building inside homes across Central California as cool spring nights and warming daytime temperatures keep windows closed, allowing radon—known as the “invisible killer”—to accumulate indoors within hours. The risk can increase quickly when airflow is limited during overnight temperature swings common in the Central Valley.
According to the U.S. Environmental Protection Agency, radon is a radioactive gas that forms naturally in soil and can enter homes through foundation cracks, crawl spaces, and slab floors. While California is generally considered a lower to moderate radon-risk state, elevated levels have been detected in parts of the Central Valley, including Fresno, Kern, and Tulare counties, where some homes exceed the EPA action level of 4 picocuries per liter.
In Fresno and surrounding communities including Bakersfield, Visalia, and Madera, homes with slab foundations and crawl spaces are especially vulnerable during seasonal transitions. Residents often keep homes sealed overnight to maintain indoor temperatures, which can trap radon indoors. The California Department of Public Health reports radon is the second leading cause of lung cancer in the United States, linked to about 21,000 deaths annually.
Radon exposure produces no immediate symptoms, making testing the only reliable way to detect it. Short-term test kits can return results within 48 to 96 hours, and mitigation systems can reduce indoor levels by up to 99 percent when properly installed.
Residents are urged to test homes now as conditions through April continue to support indoor buildup. Additional advisories may be issued as risks persist into early summer across Central California.
