Outdoor Radon Concentrations in the Vicinity of an Active Home Radon Mitigation System

When compared to indoor radon concentrations, outdoor radon levels are generally low. Within the vicinity of a radon mitigation system, however, exhausted radon may cause unnaturally high levels of outdoor radon. A study was conducted over a period of three weeks on a home with a subslab depressurization mitigation system measuring outdoor radon concentrations correlated with wind speed. The data demonstrated that wind speeds were inversely related to outdoor radon concentrations. Strong wind speeds during the early morning and late afternoon correlated with low radon levels while low wind speeds in the evening resulted in high radon levels. Though the outdoor radon levels did pool during stable atmospheric conditions, the radon levels never exceeded 3 picocuries per liter and averaged 0.345 pCi/1. Further study should be conducted over a longer period of time during different seasons, at various distances and directions from the home, and at different heights above the ground. INTRODUCTION As a class A carcinogen, it is important to study both outdoor and indoor radon concentrations. Residents in Santa Barbara should be particularly concerned as Santa Barbara County has been declared to be a radon hot spot by the California Department of Health Services (Anonymous, 1993). Carlisle and Azzouz determined that the Rincon Shale Formation in Santa Barbara County was responsible for the high radon levels (Carlisle and Azzouz, 1993). The Environmental Protection Agency (EPA) recommends that radon concentrations within residential homes to be less than 4 pCUI in order to minimize health risks. However, it has been calculated that approximately 6% of the homes on the Rincon Shale Formation in Santa Barbara exceed the EPA action level (Hobbs and Maeda, 1996). The adverse health effects of radon and its decay products have been known for many years. Radon decay products are inhaled and stick to the lungs where they decay and release alpha particles. These alpha particles are released at high speeds and may strike the lung cells possibly damaging them. Chronic exposure to radon decay products may eventually lead to lung cancer. Therefore, it is important for persons who have indoor radon levels exceeding the EPA action level to reduce their health risks by implementing radon mitigation systems. A popular, effective, and durable means to reduce radon concentrations have been the subslab depressurization system. The subslab depressurization system diverts soil gas from entering the home by directing the soil gas into a pipe. The system draws on the soil gas from beneath the home with a fan. The soil gas enters a piping system and is exhausted at the roof. The gas is then dispersed and diluted within the atmosphere. However, this outdoor exhausted radon gas may enter the home at the floor level during times of atmospheric stability. To our knowledge, no previous systematic study has been conducted on this problem. It was determined that a diurnal pattern exists in both outdoor and indoor radon levels and a regular pattern was found in wind speeds. Radon levels were inversely related to wind speeds. Radon may pool at night due to stable atmospheric conditions; however, we determined that no serious entrainment exists in the home in our study. Further investigation of this phenomena is required as the home owner wants to lower indoor radon levels as much as possible in order to decrease his health risks. We did not measure the rate of entrainment of radon. 1996 International Radon Symposium 11 4.1