Accounting for climate variability in vapor intrusion assessments

The distribution of the subslab soil gas attenuation factor α for residential houses was simulated using a mathematical relationship that quantifies the influence of stack and wind effects on vapor intrusion. The simulations presented in this article account for variations in leak distribution on the building envelope, effective leakage area distribution in the U.S. residential housing stock, and temperature and wind speed conditions for 20 U.S. cities in seven temperature-based climate zones. For the most air-tight type of houses (“energy-efficient”) in these climate zones, the 95th percentile of the annual mean α ranges from 0.001 to 0.004 as the annual mean temperature goes from 20°C to 3°C. The less air-tight types of houses (“low-income” and “conventional”) have α distributions with lower values. Utility of the simulations in assessing short-term vapor intrusion exposures is also demonstrated by constructing distributions of the maximum 21-day average for the year. The 95th percentile of these short-term α distributions for energy-efficient houses ranges from 0.002 to 0.005. The results presented in this article support the development of vapor intrusion screening levels that are based on values of α appropriate for a climate zone and housing type.