Measuring RadonThe picocurie (pCi) is used in the US to measure the rate of radioactive decay of radon. One pCi is one trillionth of a Curie. This in turn is used to determine how much radioactivity is in one liter of air (pCi/L).
The United States Environmental Protection Agency (EPA) has set a threshold of 4pCi/L for indoor air. At this level, one liter of air would contain approximately 12,672 radioactive transformations (decay) during a 24-hour period.
The average indoor radon level in the United States is about 1.3pCi/L and approximately 0.4pCi/L is found in the outside air. There is really no "totally safe" level. According to the EPA, any level of radon exposure carries some risk. Even though the EPA threshold is 4pCi/L, action to reduce indoor radon levels is recommended when readings are 2pCi/L and above.