Neither of the above are suitable in a real-estate transaction of course, where the homeowner has incentive to tamper with the test and make sure it produces a nice low result. In such cases, you want a “continuous radon monitor” which also records temperature, humidity, barometric pressure, and movement. If someone carries it outside or opens the windows for 40 of the 48 hours, you’ll see it on the results! CRMs go for a few grand, so just have your home inspector use his.

As for testing your own home, yes, charcoal cans are a great way to start. Hit http://www.epa.gov/radon/ for more info. Do it now! What you don’t know can irradiate you.