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Wildfire smoke was associated with a far greater number of pediatric respiratory care visits than other sources of airborne fine particles, according to a new study, even when wildfires were less severe.

The study, published Tuesday in Pediatrics, examined more than 170,000 emergency and urgent care visits for respiratory concerns from 2011 to 2017 in the Rady Children’s Hospital Network, which cares for around 90% of hospitalized children in San Diego County. The concerns included difficulty breathing, respiratory distress, wheezing, asthma, and cough. Researchers found that a 10-unit increase of airborne particles less than 2.5 micrometers wide — known as PM2.5 — from wildfire smoke was associated with a 30% spike in admissions, compared to 3.7% from non-smoke sources, such as traffic emissions.

“It’s quite a big bit of a difference,” said Rosana Aguilera, the study’s lead author and a postdoctoral researcher in environmental health at the University of California, San Diego. “I don’t know if I was expecting that high number.”

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Scientists have known for decades that airborne fine particles negatively affect children’s respiratory health, in part because their lungs are still developing and they breathe at a faster rate than adults, which means kids can take in more polluted air. As climate change increases the frequency and severity of wildfires, Aguilera and her colleagues set out to find whether PM2.5, which can get deep into the lungs and may even enter the bloodstream, were more harmful to children when they came from wildfire smoke. 

Airborne fine particles have been regulated by the Environmental Protection Agency since the 1970s, and this pollution is generally decreasing across the country, except for wildfire-prone Western states. The health effects of the smoke-derived particles also likely have a disproportionate impact on communities of color, who more often live in areas vulnerable to wildfires. If particulate matter from wildfires is more harmful than that from other sources, it could mean that policies like EPA limits on particulate levels are in need of revision, said Juan Aguilera, a postdoctoral scholar at the Sean N. Parker Center for Allergy and Asthma Research at Stanford University.

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“As we learn more about certain effects and certain consequences, definitely there’s room to improve some of the policy,” said Aguilera, who is of no relation to Rosana Aguilera and was not involved in the study. 

In addition to looking at emergency and urgent care visits in San Diego County, the study authors identified ZIP codes affected by wildfire smoke using satellite data from the National Oceanographic and Atmospheric Administration’s Hazard Mapping System. They also used data from EPA stations monitoring fine particulate matter in these same areas. 

Using environmental data from two different governmental agencies is an innovative approach, said Juan Aguilera, and is “very important when we consider exposure.”

The results support another study, published this month in Nature, from Rosana Aguilera and her colleagues, who documented an increased association between respiratory hospitalizations and wildfire-associated particulates among residents in nearly 700 ZIP codes in Southern California over a 14-year period.  

The wildfires that occurred during the study period of the Pediatrics paper were of relatively modest severity, leading the authors to conclude that wildfires do not have to be extreme to produce impacts in children’s health. However, the 2020 wildfire season was one of the worst in modern history across California. 

“With an increase in wildfires and the intensity and the frequency, there would be an associated increase in health burden,” said Rosana Aguilera. 

That burden will likely fall harder on some populations: A 2018 study found that majority Black, Hispanic, and Native American census tracts were about 50% more vulnerable to wildfires. In San Diego County, a third of the population is Hispanic or Latino.   

Particulates from wildfires may also have long-term impacts, not only the acute ones the study tracked. Juan Aguilera’s colleagues at Stanford recently published a study that shows high levels of fine particulate matter and other types of air pollution are associated with immune effects in children, though the study did not examine individual sources. Studies on exposure to wildfire smoke in Montana have documented reductions in lung capacity and even an increase in influenza cases following severe fire seasons.

In addition to potential policy changes, personal awareness and protective measures might help limit the effects of airborne fine particles from wildfire smoke. People can wear N95 masks, use indoor air filtration devices, and make sure “there’s a proper control of the ventilation, so that smoke doesn’t necessarily get indoors,” said Juan Aguilera.  

Rosana Aguilera noted that the study is not without limitations, such as the fact that children may attend school in — and spend a significant amount of time in — a different ZIP code from where they live. The satellite data the researchers used could, in some cases, misidentify areas with smoke pollution, the paper said. In addition, the findings are based on one California county and may not be generalizable to the whole state or other states.

The higher proportion of smoke-associated respiratory visits was most pronounced among children 5 and younger, who also comprised the majority of emergency visits. This association was not as high among youth ages 6-12 and largely absent among those 13-19. The study authors acknowledged that the comparatively low number of visits for older children makes the data difficult to interpret. Juan Aguilera said parents might be more likely to seek care for respiratory concerns in very young children, leading to an overrepresentation of the youngest age group and underrepresenting older children. 

Even so, Rosana Aguilera said the study shows PM2.5 from wildfire smoke is a pressing children’s health issue worth further study.

“In some regions in the U.S. and in the world, there’s a trend that this type of pollution will increase,” she said.