Radon is a radioactive gas. 

http://www.ewashtenaw.org/government/departments/environmental_health/indoor_air/radon/eh_radon.html

Timeline

http://www.annarbor.com/news/high-radon-levels-persisted-in-basement-where-ann-arbor-police-officers-worked-records-show/

http://www.annarbor.com/news/washtenaw-county-is-a-natural-radon-hotspot-and-local-experts-suggest-testing-your-home-to-be-safe/

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