Increases in Ozone

Ozone is an important air pollutant produced in the outdoor air through chemical reactions of volatile organic compounds and nitrogen oxides in the presence of ultraviolet light from the sun. The chemical reaction rates, hence the ozone production rate, increases with temperature; thus, if all other factors were unchanged outdoor air ozone levels would increase in urban polluted areas as temperatures increase. However, ozone is not stable and the natural ozone destruction rate also increases with temperature. The IPCC projects that temperature increases from climate change will reduce global-average tropospheric ozone but increase ozone within and near urban areas where most people live [1]. Climate change may also modify ozone levels in urban areas as a consequence of changes in air movement, cloud cover, humidity, and the emission rates of reactive volatile organic compounds and oxides of nitrogen. In some urban regions, simultaneous implementation of ozone control measures, as climate change occurs, is likely to outweigh the effects of climate change [2], leading to an overall reduction in ozone levels. However, in these cases climate change would be expected to lessen the reductions in ozone levels.

Increased outdoor air ozone is linked to increases a variety of adverse health effects including asthma, chronic obstructive pulmonary disease, hospitalizations, and mortality [3-6]. Worldwide, approximately 150,000 premature deaths per year are attributable to ozone pollution [7] . While this number is large, it is much smaller than the projected number of deaths from exposures to particulate pollution.

Table 3 below summarizes published estimates of how climate change is anticipated to influence the health effects of outdoor ozone. To isolate the effects of changing climate, many studies have assumed no change in the emission rates of pollutants that are precursors to ozone and no changes in population. Table 3 only includes projections made while making these assumptions. The regions and time periods considered, and the health outcomes, varied among the studies. Also, threshold concentrations above which ozone was assumed to cause health effects varied among analyses from 0 to 40 parts per billion (ppb). The studies predict significant increases in mortality and hospital admission for respiratory health effects, due to climate change if other factors such as emission rates of precursor pollutants are held constant. The predicted magnitudes of increases in health effects vary widely. Predicted percentage increases in daily total mortality range from 0.01% to 0.27%, increases in the portion of total mortality caused by ozone range from 4.5% to 13.7%, increases in total hospital admissions for specific respiratory health outcomes range from 0.24% to 2.1%, and increases in ozone-caused hospital admissions or emergency-room visits for specific respiratory health effects range from 8.2% to 12.4%. Some papers, (e.g., [8, 9], show that changes in emission rates of pollutants that are precursors to ozone as a consequence or air pollution control activities, and changes in population, may strongly modify the extent to which climate change increases the health effects of ozone; however, these results are not included in Table 3 below.

Because people in developed countries are indoors about 90% of the time, the health consequences of changes in ozone will partly be a consequence of changes in ozone levels inside buildings. Ozone reacts chemically with components of buildings and building furnishings [6, 10]. Ozone also reacts with some types of pollutants in indoor air. These reactions reduce indoor ozone levels to below outdoor air levels, unless strong indoor sources of ozone are present. The chemical reactions can produce new air pollutants that may possibly cause adverse health effects [6]; however, the health significance of many of the pollutants resulting from these reactions are poorly understood. Two of the pollutants produced are formaldehyde and ultrafine particles, and for both there is substantial evidence of adverse health effects.

Accounting for the differences between indoor and outdoor air ozone concentrations, Weschler [6] has estimated that indoor exposures to ozone are typically 45% to 75% of total exposures. Because breathing rates are lower when people are indoors, the indoor inhalation intake of ozone is typically 25% to 60% of total ozone intake. These estimates may be low for the population most affected by ozone—infants, the elderly, and those with respiratory and cardiovascular disease. Consequently, if there is no threshold concentration for the health effects of ozone, or if the threshold is low, roughly half of the health effects resulting from increases in ozone with climate change will be a consequence of increased indoor ozone concentrations.

Changes to buildings could diminish the adverse health effects of increases in ozone. Increased use of air conditioning and associated closing of windows, are likely as the climate warms and could be encouraged when ozone levels are high. While air conditioning appears to pose some health risks [11-13], ozone concentrations are lower in air-conditioned buildings with closed windows. The ratio of indoor to outdoor air concentration in homes is typically 0.2 to 0.4, but closer to 0.1 with air conditioning [6] . There is evidence that ozone less strongly affects health in cities with a higher prevalence of central air conditioning [6, 14], presumably because of less window opening and lower outdoor air ventilation rates during warm weather. In analyses for 18 U.S. cities, the increase in mortality per unit increase in outdoor air ozone concentration was smaller in cities with lower predicted annual average outdoor air ventilation rates in homes [15]. Filters containing activated carbon, through which air is passed using fans, can be effective in removing ozone for an extended period [16-19]. Also, some types of building materials can passively (without fans) remove ozone and remove ozone more effectively if placed where fans increase indoor air motion. Reductions in ozone concentrations were as high as 30% with no fans and as high as 80% with fans used to increase air motion [20]. Modeled reductions in indoor ozone for a range of scenarios were 25 to 70% [21]. Rates of passive ozone removal by activated carbon mats and perlite-based ceiling tiles decreased little over six months, with little production of undesirable pollutants [22]. However, the practicality of these ozone removal systems needs further evaluation. To reduce indoor ozone concentrations by 50% in a typical single family home, it may be necessary to install approximately 100 m² (1100 ft²) of material that passively removes ozone at a high rate [21]. Overall, however, it is clear that actions taken in buildings could substantially reduce the adverse health effects expected from increases in ozone with climate change. These actions would be expected to yield ozone-related health benefits even in the absence of climate change.

Table 3. Projected changes in health effects of ozone from climate change

Reference	Location	Time Period	Changes in ozone	Key Assumptions	Projected Increases in Health Effects
[23]	50 US cities	2050s versus 1990s	In summer: +4.8 ppb, 1 hour max +4.4 ppb, 8-hour max	No changes in ozone precursors and population	Daily total mortality in summer: 0.11% to 0.27% Daily total hospital admissions in summer from: COPD* 0.24% to 1.6%, age ≥ 65 Respiratory effects 0.8% to 2.1%, age ≥ 65 Asthma 2.1%, age ≤ 64
[24]	19 cities in U.S. southeast	2040s versus 2000	0.43 ppb, annual average	No change in ozone precursors and population	Total annual mortality rate: 0.01%
[25]	New York City Metropolitan area	2050s vs. 1990s	0.3 to 4.3 ppb increase in summer average 1-h maximum (varies with location)	No changes in ozone precursors or population	Median 4.5% increase in ozone-related mortality in summer Slightly larger percentage increase in deaths if health models assumes no health effects of ozone less than 20 ppb
[8]	27 countries in Europe	2021-2050 versus 1961– 1990 _____ 2041-2060 versus 1961-1990	Not available (Paper provides total change in product of concentration and time)	1.9 to 2.1 ^oC (3.4 to 3.8 ^oF) increase in temperature No changes in ozone precursors or population; no health effects for ozone < 35 ppb Temperature change for 2041-2060 not provided	8.6% to 13.7% (2402 to 3543) increase in annual ozone-related mortality 8.2% to 12.4% (3135 to 4402) increase in annual ozone-related hospitalizations ______________________________________________ 9.7% (2711) increase in annual ozone-related mortality 9.1% (3467) increase in annual ozone-related hospitalizations
[26]	Sydney, Australia	2051-2060 versus 1996-2005	Daily 1-hour maximum increase is 0.89 to 1.05 ppb	No changes in ozone precursors and population Three climate change scenarios	In Sydney with population of 4.1 million deaths per decade from ozone increase by: 60 with no threshold for health effects of ozone 65 with 25 ppb threshold for health effects of ozone 55 with 40 ppb threshold for health effects of ozone
[27]	New York City Metropolitan area	2020s versus 1990s	Average summer 8-hour maximum increases by 2.7 – 5.3 ppb depending on location	No change in ozone precursors and population (in base case analysis)	Ozone-related emergency room visits in summer of children (age 0-17) for asthma increase by 56 (7.3%)
[28]	United States	2050 versus 2001	-3.8 to 5.4 ppb in annual average ozone depending on location	No change in ozone precursors and population	Mean predicted annual increases in number of cases: Mortality = 279; respiratory hospital admissions = 9699; acute respiratory symptom days = 4.6 million; respiratory emergency room visits = 1618; school loss days = 1.4 million