http://cmmgreens.blogspot.com/2006/11/radon-gas.html
I'd read article about houseowners in Morgan's Grant who had paid to do the Radon gas test, found that the radon gas in their house (438 bq/m3) is above the world (a.k.a Europe, WHO, U.S) acceptable limit (150 bq/m3) but below the Canadian limit (800 bq/m3).
You can choose to trust the Canadian government, and believe that 800 bq/m3 won't hurt your health and disregard the fact that most countries in the world believe it's high enough to be hazadous to health.
Radon is known to contribute to lung cancer. From the results of the same study, when a non-smoker is exposed to radon concentrations of 0, 100 and 400 Bq/m3, the risk of lung cancer by age 75 years will be about 4, 5 and 7 in a 1000, respectively. However, for those who smoke, the risk of lung cancer is about 25 times greater, namely 100, 120 and 160 in a 1000, respectively. Most of the radon-induced lung cancer cases occur among smokers.
So, it seems like, as long as you don't smoke, it isn't too bad!
http://www.who.int/phe/radiation/en/2004Radon.pdf