September 24, 2025
5 min read
Key takeaways:
- Researchers evaluated 15 components of PM2.5 plus nitrogen dioxide and ozone as they relate to asthma hospitalizations.
- Rises in the pollutant mixture resulted in rises in asthma hospitalizations.
After breaking down the association between long-term air pollution exposure and asthma hospitalization into singular pollutants, nickel, vanadium, sulfate, nitrate and ammonium emerged as the greatest contributors, according to study data.
These results were published in American Journal of Respiratory and Critical Care Medicine.
“Clinically, this study suggests that physicians should advise their asthmatic patients to use HEPA air filters at home and avoid going outside when the forecast calls for higher levels of particles in the air,” Joel Schwartz, PhD, professor of environmental epidemiology at Harvard T.H. Chan School of Public Health, told Healio. “From a public health perspective, it suggests stronger controls on sources that produce the most important pollutants such as ships, large buildings that burn heavy fuel oil and coal plants.”
Using 2002 to 2016 inpatient records from databases of 11 U.S. states, Schwartz and colleagues evaluated how 15 components of particulate matter with an aerodynamic diameter of 2.5 μm or less (PM2.5) plus nitrogen dioxide (NO2) and ozone (O3) impact asthma hospitalizations among children/adolescents (469,005 hospitalizations) and adults (676,564 hospitalizations) via weighted quantile sum regression models.
“It has long been known that particles in the air exacerbate asthma, resulting in more attacks and even hospitalization,” Schwartz said. “Gases such as nitrogen dioxide and ozone also may play a role. However, there are many types of particles in the air.”
According to the study, the 15 components of PM2.5 selected for analysis based on previous studies and machine learning algorithms included: bromine, calcium, copper, elemental carbon, iron, potassium, ammonium, nickel, nitrate, organic carbon, lead, silicon, sulfate, vanadium and zinc.
“Sulfate particles come primarily from coal burning, elemental carbon particles from traffic, organic particles from wildfires and traffic, etc,” Schwartz told Healio. “What is unknown is how important each type of particle is in producing asthma hospitalizations, and the relative importance of particles vs. gaseous air pollutants.”
Researchers controlled for socioeconomic status in the regression models using temperature and variables aggregated to the annual ZIP code level from the U.S. census.
With each decile rise in the pollutant mixture composed of the 15 PM2.5 components, NO2 and O3, the study reported a 10.6% increase in asthma inpatient hospitalizations per year among children/adolescents and an 8% increase among adults.
When divided into the individual components, researchers found that both age groups had similar particles that contributed the most weight: nickel, vanadium, sulfate, nitrate and ammonium.
Similarly, with each decile rise in the pollutant mixture composed of the five major PM2.5 components by mass (elemental carbon, organic carbon, nitrate, sulfate, ammonium), NO2 and O3, there was a 7% increase in asthma inpatient hospitalizations per year among children/adolescents and a 7% increase among adults, according to the study.
Researchers further observed that sulfate, nitrate and ammonium contributed the most weight in both age groups. In contrast, components that did not contribute much weight in both age groups included elemental carbon, organic carbon and O3.
“We were surprised to find that NO2 and O3 contributed little to the overall association of air pollution with asthma hospitalizations,” Schwartz said. “They have shown up in earlier studies, but they are correlated with particles, and the newer statistical method we used has been shown to do a much better job at figuring out which correlated pollutants are more important.”
Notably, Schwartz told Healio the EPA should tighten its current standards on certain particles because the above results held even while most of the locations had concentrations below these standards.
Looking ahead, research is needed on how these 15 PM2.5 components plus NO2 and O3 impact asthma ED visits among children/adolescents and adults, Schwartz said.
“I think that future studies should continue to use mixture methods to analyze multiple pollutants at once and should look at ED visits, which are more common than hospitalizations,” Schwartz told Healio.
For more information:
Joel Schwartz, PhD, can be reached at joel@hsph.harvard.edu.
Perspective
Understanding the impact of pollution on a disease that affects such a large portion of the population (8% adults and 6.5% children in the U.S.) is definitely significant. These findings allow us to better understand one of the many reasons why asthma related hospitalizations are rising (10.6% increase in children and 8% increase in adults for every decile increase in air pollutants).
This is one of the first studies in the U.S. to look at pollutant mixtures instead of just single pollutants, giving a more realistic picture of real-world exposure. The study also helps us identify specific types of pollutants involved, allowing for an opportunity to be more targeted in mitigation strategies.
Although traffic-related pollutants such as elemental carbon and ozone are a significant cause of disease exacerbation in short-term, long-term impact was found to be more significant from secondary pollutants (sulfates, nitrates, ammonium) and industrial byproducts (nickel, vanadium, plus bromine in adults).
Despite such significant findings, these results are actually not surprising. We know from a wealth of literature over the past few decades that air pollution worsens symptoms of any respiratory disease but particularly asthma, resulting in increased emergency visits and hospitalizations. The findings reinforce what we already know about air pollution and asthma, but they are significant because they highlight new culprits for long-term exposure.
The findings are in line with my clinical experience. We are seeing more and more patients being diagnosed with new onset asthma, as well as patients already diagnosed with asthma having more severe symptoms. Inflammation, oxidative stress and airway irritation are well-known mechanisms by which particles and certain gases exacerbate symptoms of asthma. The study’s emphasis on components that can drive oxidative stress fits those mechanisms.
Clinicians can’t control pollution directly, but by anticipating higher risk in exposed communities, intensifying asthma management and empowering patients with education and tools, they can buffer some of the disparities.
- Screen patients for environmental risk: Ask about where patients live, work or go to school (eg, near highways, ports or industrial zones).
- Schedule high risk patients for more frequent follow-up visits.
- Focus on educating patients about the long-term impact of pollutants. This can serve as a powerful way for our patients to take protective measures.
- Update treatment plans according to the exposure for each patient so that symptoms are controlled before any hospitalization is warranted.
The findings from this study point to very specific pollutant sources that drive asthma hospitalization. Mitigating disparities requires targeted pollution control, focused protections for vulnerable communities and systemic investment in clean energy and housing equity. Here are some potential ways to tackle the situation:
- Strengthen EPA standards for secondary pollutants that were previously thought to be less significant until this study.
- Implement stricter limits on heavy fuel oil combustion (ships, large buildings, refineries, metal processing plants).
- Require more fine-grained monitoring of individual particulate matter and not just total particulate matter mass.
- Create low-emission zones near schools, hospitals and residential neighborhoods.
- Identify “hot spots” in vulnerable communities. Invest in programs to provide HEPA filters, better ventilation and weatherization.
Rehan Mujahid, DO
Disclosures: Mujahid reports no relevant financial disclosures.
Sources/Disclosures
Source:
Vu BN, et al. Am J Respir Crit Care Med. 2025;doi:10.1164/rccm.202409-1853OC.
Disclosures:
Schwarts reports that his institution received support for this manuscript from the NIH, and he received payment for expert testimony from the U.S. Department of Justice. One study author reports receiving support for this manuscript from NIH grants. All other study authors report no relevant financial disclosures.
