Climate change could result in an extra week or more of unhealthy ground-level ozone exposures for a significant percentage of Americans over the next few decades, researchers predict.
The increase could lead to a greater incidence of respiratory illness, such as asthma and COPD, and more frequent exacerbations of these illnesses, the team noted. Children, the elderly, and people who have these respiratory illnesses could be most impacted by the change.
The modeling analysis found that by 2050, residents of California, the Southwest, and Northeast could experience between 3 and 9 additional days each year of dangerously high ozone levels if vehicle emissions remain at present levels.
The model predicted an average national increase in unhealthy ozone days of 2.3 days, while residents of high-ozone areas would see increase of as many as 9 days, said researcher Lu Shen, a graduate student at the Harvard John A. Paulson School of Engineering and Applied Sciences. The study was published online April 21 in the journal Geophysical Research Letters.
Rising temperatures associated with climate change are predicted to increase levels of surface, or tropospheric ozone, which is created when chemicals emitted largely from motor vehicles react with sunlight.
The prediction model developed by Shen, along with Harvard colleague Loretta Mickley, PhD, incorporated observed relationships between temperature and ozone, in contrast to previous modeling studies, which did not rely on existing observations, Shen told MedPage Today.
"Most previous prediction studies have relied on chemical transport modeling [CTM] or chemistry climate models [CCM]. These models have many problems, and they overestimate ozone by as much as 20%."
In the newly published analysis, the researchers used observed ozone-temperature relationships and extreme value theory to predict future ozone levels.
"We apply the point process (PP) model to observed relationships between surface temperature and maximum daily eight-hour average (MDA8) ozone in May to September over 2003-2012," they wrote. "A novel feature of our approach is that it takes into account the possible suppression of ozone concentrations at extreme temperatures."
The researchers obtained 2003-2013 hourly ozone levels from the EPA Air Quality System and converted it to daily MDA8 ozone. Ozone season was defined as May through September, when ozone episodes are most frequent.
At very high temperatures (above the 95th percentile), approximately 20% of U.S. sites exhibited a significant decrease in the ozone-temperature slope.
Increases in surface ozone have typically shown a linear relationship, with rising surface temperatures in most parts of the United States. Previous research from California, however, has documented a phenomenon now known as ozone suppression, in which ozone levels stop rising and stabilize at extremely high temperature -- beginning in the mid-90s Fahrenheit.
The study by Shen, Mickley, and other colleague, Eric Gilleland, is the first to show ozone suppression in other areas of the U.S., including the Northeast, Southwest, and deep South.
Combining the model with future projections analysis showed an increase in high ozone episodes by 3 to 9 days annually in the Northeast, Midwest, and Southwest from 2000 to 2050, and by zero to 2 days in other parts of the country.
"Our method assumes constant anthropogenic emissions at present-day levels, and the climate penalty we report considers only the influence of climate change on ozone episodes," the researchers wrote. "Our results point to the need for ambitious emission controls to offset this penalty, especially in the Northeast and Southwest."
Exposure to ground-level ozone and other forms of outdoor air pollution are known triggers for exacerbations of asthma, COPD, and other respiratory diseases, leading to increased symptoms, emergency department visits, hospitalizations, and even death.
In a study published last September in the European Respiratory Journal, researchers reported that exposure to surface ozone was a highly significant risk factor for COPD hospitalizations and poor outcomes.
"[Our results] indicate the most important risk factor for COPD exacerbations to be associated with ozone, supporting reduction in ambient ozone levels," those researchers wrote.
Funding for this research was provided by NASA and the National Institute of Environmental Health Sciences.
The researchers declared no relevant relationships with industry.
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