Why are radon concentrations in homes higher than outdoor concentrations?

• Pressure differences between the inside of the house and the outside can draw soil gas into the home.  This is particularly true in winter, when warm air inside the house tends to rise and escape from the house, thus drawing in air from below.
• Lack of ventilation prevents dilution of indoor air by outdoor air.

What makes you so sure that radon is dangerous at all?

• Studies on highly exposed groups (such as miners) clearly indicate substantially increased risk of lung cancer for cumulative doses equivalent to long-term exposure at more than 20 pCi/L. 
• Experiments on animals also show that exposure to radon leads to higher lung cancer rates.

Why is it so hard to tell if low concentrations of radon are dangerous?

• smoking causes a lot of lung cancer (and second-hand smoke is a major cause of lung cancer even among non-smokers), so the "signal" of radon-related lung cancer deaths is hard to distinguish from the much larger smoking-related deaths.
• Most radon epidemiological studies have been fairly small, with a few hundred to a few thousand cases and controls, and with only slight variation in radon exposures.

Lung cancer risk is thought to depend on lifetime cumulative exposures, but studies are usually only able to measure exposures for a fairly small period of time.

I realize the EPA says everyone should test for radon, and that the authors of this page don't have an official position.  But what about a personal opinion?

• As far as government programs are concerned, we think more emphasis should be placed on finding the homes with really high levels (over 10 pCi/L.  There are even homes with over 30 pCi/L!).
• As far as personal decisions, that depends on what you're comfortable with.  Almost any home can have a high radon concentration in principle, but in some areas of the country it is very, very unlikely.  We recommend using the site http://www.stat.columbia.edu/radon to get advice tailored to your house.