Particle-based ambient air pollution causes more than 4 million premature deaths each year globally, according to the World Health Organization. The tiniest particles—2.5 microns or smaller, known as PM_2.5—pose the greatest health risk because they can travel deep into the lungs and may even get into the bloodstream.

Although total PM_2.5 levels have decreased 42 percent in the U.S. since 2000 as a result of clean air regulations, scientists are concerned about the health impacts of even low levels of such pollution. The U.S. Environmental Protection Agency lowered the annual national air quality standard for PM_2.5 from 12 to nine micrograms per cubic meter (µg/m³) this week. EPA administrator Michael Regan said in a press conference that officials estimate the new standard will save up to $46 billion dollars in avoided health care and hospitalization costs by 2032. “Health benefits will include up to 800,000 avoided cases of asthma symptoms, 4,500 avoided premature deaths, and 290,000 avoided lost workdays,” he said. The World Health Organization adopted an even lower 5 µg/m³ standard in 2021, citing the growing evidence of deadly harm.

Beyond investigating their size, scientists are also digging into the chemistry of airborne particles, which, unlike other regulated pollutants such as lead and ozone, encompass a wide array of solid and liquid particles from soot to nitrate. Some airborne particles are directly emitted from car tailpipes or industrial sources; others form in the atmosphere. And the balance of those is shifting. To help states meet the tougher air standards, scientists will need more detailed studies of particle sources.

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In July 2022, for the first time in more than a decade, teams of scientists conducted an intensive campaign to characterize what’s in the summertime soup of particles that New York City residents breathe. The researchers measured the chemical makeup of PM_2.5 over the course of a month.

The team found that the PM_2.5 was 80 to 83 percent organic, or carbon-based—up from roughly 50 percent in 2001, according to the study, which was published January 22 in ACS ES&T Air. “Over the past 20 years, summertime particulate matter has shifted to organic aerosols due largely to the successful reductions of sulfate and other inorganic compounds,” says Tori Hass-Mitchell, the study’s lead author and a doctoral student at Yale University.

Roughly 76 percent of the total organic aerosols measured by the study in New York City were not directly emitted from a source but rather formed in the atmosphere. These so-called secondary organic aerosols are produced when gases, including volatile organic compounds (VOCs), oxidize in the atmosphere. VOCs are produced by a wide range of sources such as cars, vegetation and household chemicals, including cosmetics and cleaners, which complicates efforts to identify the most impactful sources.

Hass-Mitchell and colleagues’ paper is the first to include data from the Atmospheric Science and Chemistry Measurement Network (ASCENT)—a network of 12 sites around the U.S. that is the first long-term monitoring system able to chemically characterize distinct particle types. Sally Ng, who led the design of the $12-million, National Science Foundation–funded network, says Europe has had similar measurement capabilities for more than five years. “It’s time for the U.S. to modernize its air quality measurement infrastructure,” says Ng, an aerosol scientist at the Georgia Institute of Technology and a co-author of the New York City study.

Recent studies have shown that secondary organic aerosols may be linked to serious health problems—especially cardiovascular disease. A study published last September in Environmental Science & Technology found that as organic aerosols oxidize, they produce highly reactive molecules that can break down human cells and cause tissue damage. Oxidized organic aerosols are the most toxic organic component of PM_2.5, Ng says. And her work suggests that secondary organic aerosols become more toxic the longer they oxidize in the atmosphere.

Havala Pye, an EPA research scientist, co-authored a separate 2021 Nature study that found that secondary organic aerosols are strongly associated with county-level heart and lung disease death rates in the U.S. Secondary organic aerosols were associated with a 6.5 times higher mortality rate than PM_2.5.

“There’s a good chance the aerosols are becoming more toxic on a per mass basis, and secondary organic aerosols would be part of the reason why,” says Allen Robinson, an atmospheric scientist at Colorado State University, who was not involved in the new research or Pye’s study. In other words, breathing more oxidized aerosols may be more toxic to humans. But the literature looking at health effects of individual components of PM_2.5 is messy, Robinson notes. More work is needed to unravel the impact of complex combinations of different particle sizes and chemistries in PM_2.5, he explains. Pye also cautions that consistent results from repeated experiments are needed to verify whether secondary organic aerosols carry significantly greater health risks than other particles that make up PM_2.5.

Will a warming climate worsen air pollution health risks?

Previous studies have found that warmer temperatures can lead to greater production of these secondary organic aerosols. Hass-Mitchell and colleagues found in the new study that secondary organic aerosol production increased by 60 percent and 42 percent in Queens and Manhattan, respectively, during a sweltering five-day heat wave in July 2022. “We should expect higher health burdens as temperatures rise in a warming climate, with potentially more frequent extreme heat events in the future,” Hass-Mitchell says.

“Secondary organic aerosols are an increasingly important contributor to particulate matter in the summertime and urban air quality, and [they have] a temperature sensitivity that is really important to keep in mind in the context of future climate scenarios,” says Drew Gentner, a chemical and environmental engineer at Yale University and senior author of the new paper. These compounds “are becoming more oxidized at higher temperatures,” he adds, and increased temperatures can cause greater emissions of reactive volatile organic compounds.

And as temperatures increase amid climate change, more frequent and severe wildfires have already begun to chip away at air quality gains in western states. Although Hass-Mitchell and colleagues didn’t observe smoke from wildfires in the summer of 2022, they expect that organic aerosols from wildfires—such as those in the smoke that choked much of the Northeast and Midwest last summer—will also play a major role as the climate changes.

Many other cities, such as Los Angeles, Atlanta and Seoul, have also documented an increasing proportion of PM_2.5 from secondary organic aerosols. But the exact mix of natural versus human-produced sources varies widely from city to city. To continue reducing PM_2.5, “we need to understand the underlying sources and chemistry contributing to secondary organic aerosol production,” Gentner says.

Until the early 2000s, both the tools to measure secondary organic aerosols and the understanding of their formation were limited, says Benjamin Nault, a co-author of the New York City study and a research scientist at Johns Hopkins University. Currently, most instruments are designed to measure either the size or the chemistry of aerosols but not both, he says. Scientists rely on models to determine how much secondary organic aerosol comes from, for example, live vegetation, asphalt or cooking. But it’s unclear whether some sources are more harmful than others. “There are different signatures for the chemicals that come from taking a shower versus painting [a house],” he says. “Now we’re trying to understand how they come together in an urban environment.”

And that improved understanding is leading to more nuanced pollution research. “As aerosol studies advance, with increasing capabilities to examine the various chemical components of aerosols, we can ask important questions about the relative impact of those components on air quality, human health and the environment,” Gentner says. “It may be less straightforward to address secondary organic aerosol sources compared to primary sources of pollution, but studies [like ours] demonstrate that secondary organic aerosols are the biggest contributor in some urban areas.”

Reporting for this piece was supported by the Nova Institute for Health.