Los Angeles, California – A dangerous gas could already be building inside homes across Southern California as cooler spring nights and increased air conditioning use keep windows closed, allowing radon—known as the “invisible killer”—to accumulate indoors within hours. The risk can increase quickly when airflow is limited, even during mild seasonal shifts.
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, slab floors, and crawl spaces. Southern California is generally considered a lower to moderate radon-risk region, but elevated levels have been detected in parts of Los Angeles, Riverside, and San Bernardino counties, where some homes exceed the EPA action level of 4 picocuries per liter.
In Los Angeles and surrounding communities including Anaheim, Riverside, and San Bernardino, homes built on slab foundations 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 warns that 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 Southern California.
