Large longitudinal cohort study finds association between nitric oxide in air pollution and ADHD (hyperactivity)

Taiwan’s National Birth Registration database tracks every birth. Using all entries from 2005, random sampling was used to create the Taiwan Birth Cohort Study of 24,200 mother-infant pairs, which were geographically distributed and represented 12 percent of all births.

As this was an eight-year longitudinal study, there were dropouts, and 17,256 pairs completed the study. After excluding cases with fetal distress, smoking or alcohol use during pregnancy, and missing information on covariates, the final study population was 16,376.

Participants’ addresses during gestation were geocoded to the township level, and local air pollution data was retrieved for the years 2004 to 2006 from air quality monitoring stations administered by Taiwan’s Environmental Protection Administration. Every hour, each station records levels of nitric oxide (NO), nitrogen dioxide (NO2), nitrogen oxides (NOx), sulfur dioxide (SO2), carbon monoxide (CO), and particulate matter of diameter 10 μm or less (PM10).

The Taiwanese research team addressed several potential confounding factors: sex, maternal age, delivery method (cesarean section or not), birth in summer (June–August), urban or rural residence, and annual household income.

Because ADHD is primarily observable as hyperactivity rather than inattention among young children, the team focused on relating symptoms of hyperactivity to levels of air pollutants.

After adjusting for covariates, the team found no significant association between PM10 and SO2 levels and hyperactivity disorder. Nitrogen oxide levels, on the other hand, were associated with more than 25 percent higher odds of hyperactivity.

Breaking that down further, they found no significant association with NO2, and that the 25 percent higher odds were exclusive with nitric oxide (NO), a pollutant emitted by internal combustion engines in cars, trucks, and busses. NO is a free radical, meaning it has an unpaired electron that can damage the body because of its strong chemical reactivity.

The authors noted, “Our study has some limitations. First, hyperactivity was diagnosed based on a parent-reported physician or specialist diagnosis. Although we did not inquire which specialists made these diagnoses, they must have been a pediatrician, psychiatrist, or special education teacher. … Second, the calculated exposure to ambient air pollutants was not an accurate personal exposure because it did not take indoor air pollutant levels and time-activity patterns into consideration. Applying personal environmental monitoring in such a large sample is impractical, especially when the subjects are pregnant.”

Nevertheless, they emphasized, “this is the first study attempting to discover which component of NOx is more critical to the development of hyperactivity in offspring.”

Ping Shih, Ching-Chun Huang, Shih-Chun Pan, Tung-Luang Chiang Yue Leon Guo, “Hyperactivity disorder in children related to traffic-based air pollution during pregnancy,” Environmental Research(2020),188: 109588, https://doi.org/10.1016/j.envres.2020.109588.

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