[RhysU]

Some locales in the US require testing for radon then mitigating during any home sale. These regulations somewhat change the marginal cost/benefit analysis because the money must be spent eventually.

If you own a home in such a locale, you might as well do any mitigation while living in the house. Otherwise you're not getting any cancer risk reduction for your dollars but you'll still pay those same dollars for the mitigation when you sell the house.