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 mortality results due to the fact that premature mortality accounts for the overwhelming majority of total economic damages from air pollution, the study authors also estimated impacts of climate-related air pollution increases on morbidity endpoints (non-fatal heart attacks, respiratory hospital admissions, cardiovascular hospital admissions, and emergency room visits for asthma) and provided these new results for this assessment. Estimated morbidity results were then converted to expected changes in Federal health care costs. For more information on the approach, see Garcia-Menendez et al. (2015) and the Technical Supplement accompanying this report.

Key Limitations and Uncertainties
Health outcomes attributed to climate change are sensitive to assumptions and limitations in underlying global change and atmospheric chemistry models, and the concentration response functions that translate pollution exposure levels to expected health outcomes (USGCRP, 2016). For example, although Garcia-Menendez et al. results show significant increases in PM2.5 concentrations, the strong influence of changes in precipitation and atmospheric mixing on PM2.5 levels—combined with variability in projected changes to those variables—has prevented consensus in the scientific literature with regard to the net effect of meteorological changes on PM2.5 levels in the United States. In addition, the simulation used here does not factor in the possibility of future changes in air quality regulations, population distribution, healthcare or other technology, or human behavior that may impact the extent and pattern of air pollution exposure across the United States and associated morbidity outcomes. For example, Americans may migrate to areas of the country with cleaner air, install air conditioning in greater numbers, or make greater efforts to stay indoors when air quality is poor.

The model also does not capture the effects of climate-related increases in severe wildfire on PM2.5 and ozone formation, morbidity outcomes like acute bronchitis that do not result in hospitalization but may still lead to significant health care costs, the effects of climate-related changes in airborne allergens on allergic disease, or changes in health care costs associated with premature mortality. Changes in wildfire in the western United States in particular could have a significant impact on PM2.5 concentrations (Spracklen et al., 2009). Also not captured are the possible effects of warmer temperatures on human physiological responses to air pollution—for example, increasing the risk of mortality from exposure to a given level of ozone on warmer days. These assumptions and limitations are generally consistent with the existing peer-reviewed climate and health assessment literature.

In addition, estimates of fiscal burden attributed to modeled health outcomes may be sensitive to several economic and policy assumptions such as Medicare enrollment growth rates and health care cost growth. However, even significant changes in these assumptions (e.g., reducing Medicare enrollment growth to match total population growth, or toggling excess cost growth between -1 percent and 2 percent) do not on their own alter the order of magnitude of results. This assessment also does not consider the effect of modeled health outcomes on Federal subsidies to private insurance coverage, or changes to Federal health care policy or economic trends that may impact the Federal share of health care costs—except to the extent that these changes or trends are represented in the health care cost and enrollment growth assumptions used.