Radon is measured in picocuries per liter (pCi/L) of air. A pico Curie is a measure of the amount of radioactivity of a particular substance. A liter is about equal to a quart. The level of radon in outdoor air is about 0.4 pCi/L. The average indoor radon level is about 1.3 pCi/L. The EPA has established 4.0 pCi/L as the action level for radon in houses, schools and workplaces. This is a technology-based number, not a health-based level. The World Health Organization's latest recommendation is that your home or office be averaging the equivalent to 2.7 picocuries per liter or below!
Radon is element # 86 on the periodic chart of elements. It is a colorless, tasteless and odorless gas with a radioactive half-life of only 3.82 days. Radon is created when radium, element #88, breaks down through radioactive decay. Radium's half-life is 1622 years. The amount of radon in your home will be determined by the amount of radium present in the soil that surrounds your house. It also depends on the ease of entry and amount of negative pressure within the home.
Within everyone's homes are different pressures. You can't feel them but they are there. The upper portion of a home is under a positive pressure with air trying to get out and the lower portion of the home is under a negative pressure with air trying to get in. Somewhere in the middle is the neutral pressure zone where air isn't trying to get in or out. So the higher you go, the stronger the positive pressure. The lower you go, the stronger the negative pressure. It is this negative pressure, that pulls radon into our homes. The greater the temperature differential between the inside of our homes and the temperature outside our homes, the greater the pressures become. So radon levels are usually higher in the winter, then summer with spring and fall being the lowest.
The Environmental Protection Agency’s (EPA) efforts are directed at locating homes with high levels and encouraging mitigation. As a means of prevention, the EPA and the Office of the Surgeon General recommend that all homes below the third floor be tested for radon. Because radon is invisible, a test is the only way to determine high radon levels. The EPA recommends mitigating homes with high radon levels (e.g., above 4 pCi/l) and there are straight-forward reduction techniques that will work in most any home.