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AbstractRecent studies conducted in the USA have shown that adverse health effects of fine particulate matter (PM2.5) persist at levels below the national air quality standards. More recently, particle radioactivity has also been associated with adverse health effects. However, the importance of particle radioactivity at low PM2.5 levels has not been thoroughly explored. The present paper investigates the temporal trends and the relationship of particle gross β-activity (PM-β) and PM2.5 mass in the 48 states of the contiguous USA during the period of 2001–2017. With the implementation of stringent air pollution control policies, national ambient PM2.5 concentrations decreased by 38.5% during this period. However, a smaller decrease of 9.4% was observed for PM-β, while the mean PM-β/PM2.5 ratio increased by 49.1%. PM-β is mostly associated with radon emissions and its progeny, which do not change much with time. The largest PM2.5 and PM-β reductions were observed in the Southeast, while the smallest were found in the West. When the aggregated PM-β to PM2.5 ratio is stratified by PM2.5 levels, the ratio was found to be highest when PM2.5 is <3 μg/m3, with a median PM-β to PM2.5 ratio of 0.77 (0.64–0.88; 25th–75th percentiles). Overall, when not stratified by PM2.5 levels, the greatest state-wide overall PM-β /PM2.5 ratios were found in Wyoming (0.69) and South Dakota (0.51), areas with higher radon, while the lowest (0.17) were in Delaware followed by New Jersey (0.18). These results indicate that the ratio of ambient particle radioactivity to particle mass concentration typically is higher at low PM2.5 levels, and consequently, the toxicity per unit mass is expected to be higher.