Particulates, ozone and smog coupled with extreme temperatures form a more dangerous health threat than these problems would be individually, researchers have found.
The combined threat is apparent in both hot and cold ends of the temperature spectrum.
Scientists have long understood that air pollution is a serious health threat. The World Health Organization reported that in 2012, 1 in 8 deaths worldwide stemmed from air pollution.
These concerns were the foundation of sweeping legislation like the Clean Air Act, the same law under which the Obama administration is seeking to limit greenhouse gas emissions (ClimateWire, June 3).
President Obama argued that air pollution and carbon dioxide often come from the same sources, so cutting carbon yields health benefits in the near term.
Extreme temperatures also have a known risk profile, leading to more hospital visits from asthma and heat attacks. As the climate changes, heat waves are poised to grow more severe and frequent. These risks will increase as people move to urban areas, which generally warm faster than their surroundings.
However, researchers are still mapping out the combined effects of air quality and climate.
"Most studies focus on one or the other," said Alexandra Schneider, a senior scientist and team leader at Helmholtz Zentrum München, a federal health research institution in Germany.
Heart failure linked to temperature spikes
Schneider and her collaborators published two studies this year, one in the journal Heart examining short-term temperature changes and heart health, and another paper published in Science of the Total Environment looking at how pollution and temperature influence health risk.
In both reports, the researchers looked at public health data from three cities in southern Germany -- Nuremberg, Munich and Augsburg -- between 1990 and 2006. These cities have a temperate climate year-round, and few homes in the region have air conditioning, though most have heating.
The team focused on varieties of cardiovascular disease among 188,000 deaths during this time. It then overlaid hourly temperature, weather and air quality data to figure out what variables were linked.
The Heart paper showed that when temperatures rose from 20 degrees Celsius to 25 C, deaths from cardiovascular disease rose by 9.5 percent. Surging heat created a residual impact that lasted a day or two.
However, when temperatures fell from minus 1 to minus 8 C, heart-related fatalities rose by 7.9 percent. These effects lingered up to two weeks after a cold snap.
"We were a bit surprised at how big the effects had been for heart failure," Schneider said.
The study on pollution showed that ozone, which tends to form on hotter days, changes the risk profile of heat stress. However, particulate matter didn't seem to have any impacts on the links between temperature and deaths.
Reasons, though, remain unclear
The mechanisms linking temperature, pollution and death remain murky. "There are several hypotheses, and nothing is really 100 percent clear," Schneider said.
One idea is that warmer weather may increase the risk of blood clots, an important factor in many heart problems. The fact that temperature surges over one or two days have such a strong effect indicates that variability, not just extremes, can drive up risk.
The elderly and people with existing heart conditions are especially vulnerable.
But warmer winters under climate change may not offer any refuge, according to Philip Staddon, a research fellow at the University of Exeter Medical School who co-authored a study on this topic earlier this year (ClimateWire, Feb. 24).
"This looks like a nice paper," he said of Schneider's findings. "And I'm pleased that their result confirms our hypothesis about short-term changes in temperature."
Health effects from low temperatures tend to increase more slowly and linger for weeks rather than days, Staddon observed. This makes it more difficult to figure out what is harming people at a physiological level.
With these findings, researchers expect to tailor warnings to specific vulnerable populations. Schneider suggested issuing combined air quality and temperature alerts for regions at risk for surges in heart attacks and heart failures.
The next step for her team is to start looking at individuals instead of just populations, identifying blood markers and other physiological signals that illustrate how a person is responding to his or her environment, even before needing to go to the hospital.
"We are trying to look at subclinical markers that change with temperature," Schneider said. "That is actually our goal: to find the real mechanism behind these effects."
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