What is the estimated number of U.S. homes that have a radon level of 4 pCi/L or higher?

The estimate that about 1 in 15 U.S. homes have radon levels of 4 pCi/L or higher is based on various studies and national surveys conducted by the Environmental Protection Agency (EPA) and other health organizations. This level of radon is considered the action level where mitigation measures are recommended, as extended exposure to radon at or above this concentration is associated with an increased risk of lung cancer.

The figure of 1 in 15 indicates that radon is a common indoor air pollutant, particularly in certain areas of the country where geological conditions are favorable for radon accumulation. Understanding this statistic is crucial for raising awareness among homeowners and encouraging them to conduct radon testing and take appropriate action if high levels are detected.

The other options understate the prevalence of high radon levels in homes across the United States, which does not align with the established findings on radon levels in residential environments.