I was doing laundry one day, and decided to check out the lint that collects in the lint filter in the clothes dryer. It produced counts around 240 CPM, again much higher than the background readings. Many parts of the US, including Maryland, where I live, have relatively high amounts of radon gas. This was never a problem in the past, but many modern homes are very air-tight, to reduce energy costs. This means that the radon gas is more likely to be trapped in the house, where it can decay into the much more dangerous daughter products.
Radon is much more of a problem for smokers.
The radiation dust attaches itself to the smoke particles, and they get trapped to the sticky goo that gets stuck inside your lungs, bombarding your tissue with alpha radiation. Radon daughter products deposit themselves in the container. They then send you a report indicating the estimated radon level in the area, expressed as picocuries per liter of air. A curie is a measure of the activity of radiation. It is equal to 37 billion decays per second.