Experts explain how to determine if air pollution contributes to premature deaths around the world

Ambient, or outdoor, air pollution in both cities and rural areas was estimated to cause 4.2 million premature deaths worldwide per year in 2016.

This mortality is linked to exposure to small particulate matter of 2.5 microns or less in diameter (PM2.5), which cause cardiovascular and respiratory disease and cancers, according to a World Health Organization (WHO) report released on May 2, 2018.

A premature death is a death that occurs before the average age of death in a certain population.

“It [premature death] is a definition that varies between population and group,” said Heather Adair-Rohani, WHO household air pollution and health expert.

“We tend to think about it as a risk, you basically have an increased risk for this disease. And that often leads to a death that is earlier than what would be expected if you had a very healthy and stable environment,” Adair-Rohani said.

People living in low- and middle-income countries disproportionately experience the burden of outdoor air pollution, with 91 percent of the 4.2 million premature deaths occurring in these nations.

WHO estimates that 58 percent of outdoor air pollution-related premature deaths were due to ischaemic heart disease and strokes in 2016. Eighteen percent of deaths were due to chronic obstructive pulmonary disease and acute lower respiratory infections, and 6 percent of deaths were due to lung cancer, according to the WHO report.

Some deaths may be attributed to more than one risk factor at the same time. For example, both smoking and ambient air pollution affect lung cancer. Some lung cancer deaths could have been averted by improving ambient air quality or by reducing tobacco smoking.

The WHO is confident in the publicly provided risk estimates.

“WHO is very particular about what disease outcomes that we actually estimate in terms of attributed deaths or attributed disability,” Adair-Rohani said. “This is why we don't necessarily count for all the diseases that could potentially be linked with air pollution.”

For example, despite the large amount of emerging evidence suggesting that an earlier onset of diabetes is due to exposure to air pollution, WHO does not publicly count the number of diabetes cases. The epidemiological evidence is not yet strong enough for WHO to derive risk estimates.

“We need to have a very strong basis for where we use these numbers and how we performed the calculations,” Adair-Rohani said. “We feel confident in the risk estimates that we provide because we really feel that they are based on a consensus of ample scientific evidence as well as expert input.”

Over the last 40 years, a very large number of epidemiological studies have been conducted that examine the relationship between air pollution exposures and premature death. Many researchers across the scientific community have conducted these studies, including researchers at the U.S. Environmental Protection Agency (EPA).

Studies of air pollution focus on the criteria pollutants, which includes particulate matter, ozone, sulfur dioxide, nitrogen dioxide and carbon monoxide, according to Jason Sacks, Senior Epidemiologist in EPA’s Office of Research and Development.

These studies examine whether daily changes or annual changes in air pollution are associated with premature mortality.

“By evaluating each of the studies that are conducted in different geographic locations and in diverse populations, it is possible to then examine the collective body of evidence across studies, with consideration of data from experimental studies, to draw conclusions on whether there is a relationship between air pollution exposure and premature death,” Sacks said.

The EPA developed a publicly available software program, the environmental Benefits Mapping and Analysis Program – Community Edition (BenMAP – CE) that allows for assessments of the impacts of air quality on health. Other research groups and international organizations have developed similar tools.

These tools use simple algebraic equations that rely on the results of epidemiological studies, as well as population and health incidence data, to calculate the impacts of current air quality or changes in air quality.

The combination of experimental studies and epidemiological studies support that either short- or long-term exposure to an air pollutant could lead to a series of health events within the body, such as changes in heart or lung function.

Exposure could ultimately result in more overt health effects that would require hospital admission or premature death, according to Sacks.

Decades of research has shown that each of the criteria pollutants can provoke different health responses, which are detailed in the EPA’s Integrated Science Assessments (ISAs) for each of the criteria pollutants.

There are also multiple health factors that play a role in the impacts of air pollution on health, which have been examined in the ISAs for each of the criteria pollutants.

While the health factors that potentially increase the risk of a health risk vary depending on the pollutant, life stage, such as children and adults over 65, and pre-existing heart and lung diseases generally have been found to contribute to increased risk.

The relationship between the time exposed to air pollution and increasing health risk will vary depending on the underlying health status of an individual.

“If air pollution concentrations are extremely high, such as during a wildfire event, many people could experience respiratory symptoms, such as respiratory irritation,” Sacks said.

It has also been shown that poor air quality can detrimentally impact lung development in children who grow up in cities with high air pollution concentrations.

However, as air quality improves, studies have demonstrated that there have been measurable changes in health improvements.

For example, children’s lung function improves and overall life expectancy increases as they move to cleaner cities.

Access to healthcare and socioeconomic status are also factors associated with air pollution exposure and disease outcomes, Adair-Rohani said.

“We see very high levels of air pollution in low- and middle-income countries, but many of the studies actually come from high-income countries,” Adair-Rohani said. “WHO is encouraging more research in low- and middle-income countries that have high levels.”

There are fewer regulations in low- and middle-income countries, such as in Southeast Asia. Meanwhile, many western countries have adapted polices and regulations to manage air pollution levels.

There are some measures that can be taken at the individual level to reduce the amount of outdoor air pollutants within the home, such as using air purifiers and filters, according to Sacks.

"The use of central air conditioning or even window air conditioning units versus opening windows can also reduce the amount of outdoor air pollution that is able to penetrate indoors," Sacks said.

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