Yes. The EPA calls radon the leading cause of lung cancer among non-smokers and estimates that about 2,900 of the roughly 21,000 US radon-related lung cancer deaths each year occur in people who never smoked.
More detail
The EPA's lifetime table for never smokers: at 4 pCi/L, about 7 of every 1,000 people exposed over a lifetime could get lung cancer; at 8 pCi/L, about 15; at 20 pCi/L, about 36. For smokers the figures are far higher, about 62, 120, and 260 at the same levels, because the two exposures compound. The Ohio Department of Health gives the same message: radon is the number one cause of lung cancer in non-smokers and the second in smokers. In Franklin County, where tested homes had an estimated mean of 9.5 pCi/L from 2008 to 2017, those rows are not abstract. Lowering the level lowers the risk from that point on. See is radon a problem in Columbus.