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I don't know what an acceptable "clean" level of radon is, and as the article states, the radon levels will change quickly. It depends on how long you are exposed and at what levels. However, radon has a half life of 3.8 days (compared to uranium at 4.4 billion years).
Radon occurs naturally and is almost everywhere. From the following article:
Radon Safe Levels
"Radon gas is a naturally-occurring byproduct of the radioactive decay of Uranium in the soil. Depending on your geographic location, the radon levels of the air you breathe outside of your home may be as high as 0.75 pCi/L. The national average of outside radon levels is 0.4 pCi/L and it is estimated by the National Academy of Sciences that outdoor radon levels cause approximately 800 of the 21,000 radon induced lung cancer deaths in the US each year. Your risk of lung cancer increases substantially with exposure to higher radon levels. Lung cancer risk rises 16% per 2.7 pCi/L increase in radon exposure."
I don't know how those number units correlate to the exposure rate units in the Woolsey fire article.