Safe radon limits are the parameters between which radon exposure is deemed not overly harmful to the average person. Because any level of exposure poses a cancer risk, yet radon is ubiquitous in Earth's soil, "safe" radon limits mitigate the risk rather than remove it.
Radon is a radioactive element that is present in soils around the world. The breakdown of radon results in a low level of ambient radiation that poses a cancer risk. At higher radon concentrations, the risk is also higher. Because it is impossible to not be exposed to radon to some degree, the EPA has decided that an acceptable level of exposure is anything below four picocuries per liter of air in a building.
However, even the federally mandated ambient level of radon, set at 0.4 picocuries per liter, presents some cancer risk. The EPA notes that levels below four picocuries per liter are not necessarily safe, and cautions home owners to take action if the radon levels in their home are above the ambient 0.4 picocuries per liter.