Is there a short-term relationship between air pollution and severity of ADHD symptoms?

Certain air pollutants can produce free radicals and inflammatory cytokines that can penetrate the central nervous system and affect behavior. Long-term exposure to air pollution has been associated with a higher risk of developing ADHD.

There has, however, been little focus on the short-term effects of exposure. Might there be any correlation between levels of air contaminants and subsequent healthcare visits of adolescents for severe spikes in ADHD symptoms (frequently but not always associated with comorbid conduct disorder, oppositional defiance disorder, or mood disorder), such as extreme hyperactivity, serious rule violations, theft, or aggression to people or animals?

A South Korean (Republic of Korea) research team explored this question through a nationwide cohort study using the database of the National Health Insurance Service, a single-payer system, that covers the entire population.

Using a time-series approach, they compared measured levels of three airborne pollutants – particulate matter with a diameter ≤ 10 μm (PM10), nitrogen oxide (NO2), produced by vehicular traffic, and sulfur dioxide (SO2), produced by manufacturing industries– with healthcare visits with a principal diagnosis of ADHD. They chose these three contaminants because they have been associated with ADHD in long-term studies. What made this approach feasible is that healthcare visits are typically unscheduled in Korea, making it possible to get quick medical attention.

The team divided the country into sixteen regions, looked at boys and girls separately, and also split adolescents into two age groups (10 to 14 years and 15 to 19 years). They estimated region-specific daily concentrations of the three pollutants from 318 government-run monitoring sites, located according to population density and distribution.

The researchers next calculated zero(same day) to five-day lag figures for ADHD-related healthcare visits in each region and ran meta-analyses on the time-series data.

There were 7,200 ADHD-related healthcare visits in the 2013-2015 study period. Major increases in PM10 levels were associated with increased ADHD-related healthcare visits from the day of the spike to three days later, peaking the day after the upturn. Major increases in SO2 levels were associated with increased ADHD-related healthcare visits from one to four days later, peaking the day following the upturn. Major increases in NO2 levels were associated with increased ADHD-related healthcare visits from one to four days later, peaking three days after the spike.

There were no significant differences between male and female adolescents, and between younger and older adolescents.

The strongest increased risk for ADHD-related healthcare visits was for NO2 spikes (up 47 percent), followed by SO2 spikes (up 27 percent), with PM10 spikes coming in last (up 12 percent).

Among the limitations, the authors were unable to evaluate the most hazardous types of particulate emissions, because the smaller-diameter PM2.5 particles (≤2.5μm) have only been measured partially in South Korea since 2015. On the other hand, they pointed out that this was the first study to investigate associations between short-term air pollution exposure and ADHD-related healthcare visits, and that it included all ADHD-related healthcare visits in South Korea, making the possibility of selection bias negligible. They recommended conducting similar studies on other national populations.

Jiyoon Park, JiHoon Sohn, Sung Joon Cho, Hwa Yeon Seo, Il-Ung Hwang, Yun-Chul Hong, Kyoung-Nam Kim, “Association between short-term air pollution exposure and attention-deficit/hyperactivity disorder-related healthcare visits among adolescents: A nationwide time-series study,” Environmental Pollution (2020) 226, https://doi.org/10.1016/j.envpol.2020.115369.

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