Policy-relevant background – This is an estimate based on air pollution models of the natural background ozone concentrations that would exist without sources of air pollution from human activity, and therefore cannot be affected by emissions control efforts (Fiore 2004). We converted the 4-hour, afternoon average policy-relevant background estimate in the Northeast to the policy-relevant back- ground estimate for different metrics used in the ozone studies considered in the health impact assessment by computing the ratio of the 4-hour average to the 8-hour maximum or the 24-hour average, calculated from the hourly monitoring data from sites used in the analysis. Policy-relevant background is approximately 45% of current average ozone concentrations in.
