Indoor Radon Levels and Lung Cancer Incidence on Guam

Abstract

Radon (Rn) is a naturally occurring, radioactive gas that impacts air quality world-wide. It is a known carcinogen and considered by the United States Environmental Protection Agency (U.S. EPA) to be the second leading cause of lung cancer after tobacco smoking. Of several known isotopes of radon, ²²²Rn is the most stable with a half-life of approximately four days. This particular isotope is associated with the uranium (²³⁸U) decay series and accounts for most public ionizing radiation exposures. Most global indoor ²²²Rn emanates from granitic bedrocks located underneath buildings. While such rocks are absent on Guam, the karst limestone formations that overlay the island’s basement volcanics (basalt) are of biogenic origin and are believed to be a significant source of radon. In a recent multi-year survey conducted on Guam by the local EPA, indoor ²²²Rn levels exceeded the U.S. EPA air quality standard of 4 pCi/L in ~40% of all buildings tested. Concentrations were log-normally distributed and exceeded 300 pCi/L in two instances. Weighted average indoor ²²²Rn levels were generally much higher in villages from the northern half of the island where limestone coverage predominates. The relationship between ²²²Rn and lung cancer incidence on Guam was examined in the study reported here. The results were strongly suggestive of a hormetic effect existing between the two variables. Possible confounding effects attributable to smoking and ethnicity were examined and found to be insignificant. In fact, ethnic groups predominantly confined to the northern half of the island (i.e., Filipinos and all other Asians as a collective group) showed considerably lower cancer incidence and mortality rates than the indigenous Chamorro people who are well represented island-wide. The findings of the study lend further weight to numerous other reports that suggest low-level exposures to ²²²Rn have a beneficial health effect. They also support a growing critique of the rationale behind the U.S. EPA adopted linear-no-threshold toxicological model, which assumes that any dose of radiation is harmful, no matter how small. Finally, they also imply that the current U.S. EPA action level of 4 pCi/L for indoor radon is overly conservative and needlessly prompting homeowners to install radon mitigation systems into buildings that really don’t need them.