The often-referenced value of 4.0 pCi/L is defined as the EPA Action Limit for mitigation (reduction) of radon in residential buildings. This value seeks to protect the general public which includes the most vulnerable members of our population including young children and the elderly. Workplace action limits are set to protect healthy working adults and as such are higher than the EPA Action Limit. There are several other values which are often cited including 100 pCi/L, 25 pCi/L, 30 pCi/L, 7.5 pCi/L, and 8.1 pCi/L.
100 pCi/L – The Occupational Health and Safety Act of 1970 specifies that no employer shall allow for an employee to be exposed to airborne radioactive material in excess limits set forth by 10 CFR Part 20 Appendix B Table 1. However, it should be noted that the enforceable regulations is the original version of the referenced federal regulation which is the 1969 version of 10 CFR Part 20 Appendix B Table 1 which lists the Maximum Permissible Concentration of Radon-222 as 1E-7 μCi/L, or 100 pCi/L.
25 pCi/L – This number is often referenced as it is 25% of the 100 pCi/L value above. OSHA defines an airborne radioactive area as one in which material exists in concentrations exceed 25% of the 10 CFR Part 20 Appendix B Table 1 values (again noted that this is the 1969 version).
30 pCi/L – This is the current valued listed in 10 CFR Part 20 Appendix B Table 1 as of March 25, 2021. This is not the value intended as reference by OSHA.
7.5 pCi/L – This is 25% of the current value from 10 CFR Part 20 Appendix B Table 1, but consistent with OSHA interpretation it is not the correct value for the airborne radioactive area limit for Radon-222.
8.1 pCi/L – This is the value of 300 Bq/m3 after the unit conversion. This value was set forth by the International Commission on Radiation Protection (ICRP) as the upper limit of a range of reference values from 100 Bq/m3 to 300 Bq/m3 (2.7 pCi/L to 8.1 pCi/L). Their recommendation was that authorities should set national reference levels as low as is reasonably achievable within that range.
As true for any carcinogen, a safe level of radon exposure cannot be identified because damage to a single cell could cause a cancer. The above listed guidelines were all developed to reduce risk of developing cancer from a lifetime of radon exposure.