I note the comment by mppm and accept that exact risk levels and associations are problematic while also noting medical students can and have demonstrated increased cancer and tumours in lab animals when exposed to radon.

I once measured broad area environmental levels of background radiation as part of geophysical exploration, I'm not in the medical field (although I did some post grad epidemiology work as part of a mathematics degree).

Medical papers of interest on radon include:

  Indoor radon was declared a human carcinogen in 1987 by the WHO and in 1988 by the United States Environmental Protection Agency (EPA).

* WHO Handbook on Indoor Radon: A Public Health Perspective (2009) - https://pubmed.ncbi.nlm.nih.gov/23762967/

* Radon and Lung Cancer: Current Trends and Future Perspectives (2022) - https://pmc.ncbi.nlm.nih.gov/articles/PMC9264880/

^^ Broad overview handbooks and trends

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Example studies

* Radon in homes and risk of lung cancer: collaborative analysis of individual data from 13 European case-control studies (2005) - https://pmc.ncbi.nlm.nih.gov/articles/PMC546066/

* A 10-year follow-up study of yearly indoor radon measurements in homes, review of other studies and implications on lung cancer risk estimates (2021) - https://www.sciencedirect.com/science/article/pii/S004896972...

From what I can tell, they didn't. The theoretical basis for it is the Linear No-Threshold Model, which is, bluntly put, garbage. The empirical evidence seems to come mostly from so-called case-control studies, which are, bluntly put, garbage.

You conduct a case-control study as follows: 1) Select some number of lung cancer patients and a control group without lung cancer. 2) Try to establish why the cancer patients got cancer and the non-cancer group didn't, by retroactively estimating their smoking habits, radon exposure (from radon maps or retroactive measurements of places they lived at), as well as other factors. 3) Use statistics to try to disentangle suspected radon cases from smoking (90%) and other stuff. 4) Pretend that all this is not totally biased, sampling or otherwise.

The radon-cancer link has been suspected and studied since the mid 20th century, and been suspected in miners before the word cancer even existed.