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Air pollution was linked with a higher risk for hospital admission for children with autism spectrum disorder (ASD), a South Korean study found.

Multiple forms of air pollution were associated with increased hospitalization for ASD, according to Yun-Chul Hong, MD, PhD, of the Seoul National University College of Medicine, and colleagues:

• Fine particulate matter (PM_2.5) exposure hospital risk with 1 day of lag: relative risk 1.17 (95% CI 1.10-1.25)
• Nitrogen dioxide (NO₂) exposure hospital risk with 5 days of lag: RR 1.09 (95% CI 1.01-1.18)
• Ozone (O₃) exposure hospital risk with 4 days of lag: RR 1.03 (95% CI 1.00-1.06)

There was also a significant difference in the association of pollution and hospital admission for ASD between boys and girls, with hospitalization being more likely for boys after exposure to PM_2.5 (P=0.03 for interaction) and NO₂ (P<0.01 for interaction), they stated in .

Their study looked at kids ages 5-14 years, and found that the mean count of daily hospital admission for ASD was 8.5, with daily hospitalizations being more frequent among boys (7.0) than girls (1.6). However, when PM_2.5 levels would drop by 10.0 µg/m³, NO₂ by 10 parts per billion, and O₃ by parts per billion, average hospital admissions per day for ASD were reduced to 7.3, 7.8, and 8.3, respectively.

Finally, when all pollutants were considered together in a weighted quantile sum index, pollution levels were more strongly associated with a higher risk of hospital admission for ASD (RR 1.29, 95% CI 1.14-1.46). NO₂ was found to contribute to this effect the most, with a weight of 0.80.

ASD has a prevalence rate of 1.5% in the U.S. population and 2.2% in the population of the Republic of Korea. It can carry a heavy lifetime cost to support affected individuals, the authors explained.

"Air pollution is known to impact the central nervous system by activating microglia and disrupting the blood-brain barrier through systemic inflammation, neuroinflammation, oxidative stress, cerebrovascular injury and neurodegenerative processes," they wrote.

Short-term exposure can activate microglia, which are tied to proinflammatory cytokines. The latter "have been associated with the severity of communication impairment and aberrant behaviours among children with ASD," Hong's group stated. Modulating the inflammatory status of an individual with ASD can improve symptoms, they added.

The authors noted that a larger number of microglia, and more activation due to testosterone plus lower antioxidant levels, may explain the differences they found between boys and girls in the study.

Overall, exposure to air pollution even in the short-term can have an effect on the quality of life of patients with ASD, Hong and colleagues said. The developing nervous system is more susceptible to environmental exposures, which led the authors to focus on hospitalizations as a "proxy for ASD symptom aggravation."

Thy used data from the National Health Insurance Service on hospitalization for ASD from 2011 to 2015. Daily concentrations level of PM_2.5, NO₂, and O₃ where taken in each region and used as a measure of exposure.

ASD cases were defined based on a principal admission diagnosis of the claims data. Causal effects of air pollution were measured for 6 days following exposure.

"These results emphasize that reduction of air pollution exposure needs to be considered for successful ASD symptom management, which is important with regard to quality of life and economic costs," the authors said. "Because this is the first study on this subject, further studies, especially studies directly investigating ASD symptoms in more detail, are warranted to confirm the results and draw policy implications."

Hong's group reported what they called an unexpected finding of "inverse associations between PM_2.5 and NO₂ levels at lag day 0 and hospital admissions for ASD," although they ultimately deemed the results to be "clinically irrelevant ... and biologically implausible (considering the causal pathway via microglia activation and inflammatory status)."

Study limitations included the fact that some findings may be "spurious, occurring by chance due to extensive analyses," Hong's group wrote. Also, the predominantly Korean study sample limits generalizability, and the authors measured regional air pollution levels not individual levels of exposures.

"Hospital admissions for ASD might disproportionately reflect aggravation of symptoms related to hyperactivity, aggression and self-injurious behavior more than those related to deficits in social communication," Hong and colleagues noted. Finally, social stigma for psychiatric treatments in the Republic of Korea may result in patients with mild symptoms not receiving treatment, they said.

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