MAH_2019

MAH 2019 Companion Guidance Section D Understanding RDP Measurements 8 MAH Companion Guidance Section D UNDERSTANDING RADON DECAY PRODUCT MEASUREMENT Introduction The unit of measurement provided by devices that measure radon decay products is Working Level (WL). The devices measure the activities of short-lived radioactive elements that result from the radioactive decay of radon gas. Often termed “radon progeny,” these decay product atoms are solid elements rather than gaseous and are left suspended in the air we breathe as radon gas decays. It is the total Alpha energy from the short-lived decay products of radon (Po-218 and Po-214) that is measured when conducting radon decay product measurements. Historically, action levels have been cited at 0.020 WL in the U.S. The 0.020 WL is mathematically determined based upon an assumption that, when considering a national residential average, about 50% of the radon decay products remain suspended in the air and available for inhalation. More recently, some publications 3 and authorities have indicated that 40%may be a more correct assumption. Measurements While these assumed equilibrium factors of 50% or 40% may be reasonable for national risk calculations, the normal operating conditions for each home, building or room are affected by a wide range of parameters that can cause significant deviations compared to any assumed equilibrium factor. It is important for the measurement professional to understand and identify the presence of those parameters that can either increase or decrease the suspended radon decay products to determine if those effects are temporary or long-term-either of which can affect an interpretation of the results. • One benefit of obtaining a radon decay product measurement is to identify situations where ventilation or other conditions have resulted in higher percentages of radon decay products suspended in the air compared to traditional assumptions. This situation would result in greater risk than indicated by a radon gas measurement. • Another benefit of obtaining a radon decay product measurement is to identify situations where conditions cause lower percentages of radon decay products suspended in the air compared to traditional assumptions. If it can be verified that conditions that cause equilibrium factors to be lower than traditionally assumed are reliably stable, the risk may be lower than indicated by a radon gas measurement. Regardless, the radon decay product measurement alone cannot pinpoint the equilibrium factor that existed during a test. Therefore, it alone cannot corroborate observations related to building systems or environmental conditions that may or may not be the normal, reliably stable condition. However, by simultaneously conducting a measurement of both radon gas and radon decay products, the measurement professional can better gauge the stability of conditions that may affect an occupant’s risk. Factors Influencing Equilibrium Factors The following table provides examples of operating parameters that can increase or decrease the equilibrium factor (EF). This list is not comprehensive, and the radon professional conducting radon decay product measurements should view any situation with the following two rules of thumb: 1. Any situation where air circulation is high, more decay products will attach to physical surfaces rather than remain suspended in the air and, hence, the EF will decrease. Conversely low air circulation will cause an increase in EF. 2. Anything that will serve to increase other indoor particles to which RDPs can attach before they contact physical surfaces will increase the suspended RDPs and, hence, the equilibrium factor. Conversely, where the air is very clean, fewer RDPs will attach to suspended particles (with more plating out on physical surfaces) causing the suspended RDPs and the EF to decrease. 3 US EPA, EPA Assessment of Risks From Radon in Homes, page 3, June 2003