Nitrogen Dioxide Air Pollution Continues to Play Role in Childhood Asthma

Traffic-related air pollution was responsible for nearly 2 million new asthma cases in children across the globe in 2019, according to a new study.

Of the 1.85 million new pediatric asthma cases that were attributed to nitrogen dioxide (NO₂), two-thirds occurred in urban areas, reported Susan Anenberg, PhD, of George Washington University in Washington, D.C., and colleagues.

“Although the putative agent causing asthma in the traffic-related air pollution mixture is unknown, NO₂ could serve as a surrogate for other pollutants causing observed health effects,” they wrote in Lancet Planetary Health.

In 2019, urban areas had two to four times higher population-weighted NO₂ concentrations compared with rural areas. On average, the NO₂ concentration was 10.6 parts per billion in urban areas and 4.2 parts per billion in rural areas, the group noted. There were 156 asthma cases per 100,000 people attributed to NO₂ in urban areas compared with 40 per 100,000 people in rural areas.

“Air pollution is a modifiable risk factor — it can be reduced — and when that happens, we will see fewer asthma cases, both new and exacerbated, in addition to many other improved health outcomes,” said Haneen Khreis, PhD, MS, of the University of Cambridge in England, who was not involved in the study.

Overall, from 2000 to 2019, annual average NO₂ concentrations in urban areas decreased by 13%. However, these reductions were not consistent across the board.

In high-income cities, NO₂ concentrations declined 38%, from 17.6 parts per billion in 2000 to 11.0 parts per billion in 2019. During that same period, NO₂ concentrations rose by 18% in South Asia to 10.1 parts per billion and by 11% in sub-Saharan Africa to 7.1 parts per billion, Anenberg and team noted.

Subsequently, the proportion of pediatric asthma incidence attributed to NO₂ pollution in urban areas dropped from 19.8% in 2000 to 16.0% in 2019. However, South Asia (23%), sub-Saharan Africa (11%), and North Africa and the Middle East (5%) all saw increases in pediatric asthma incidence attributable to NO₂.

During the first lockdown of the COVID-19 pandemic in the U.S. in 2020, NO₂ concentrations decreased sharply in urban areas, though they remained disproportionately higher in areas with more residents of color, according to a 2021 study.

“Long-term reductions in air pollution are more important than short-term improvements and some chronic health effects, like asthma, are strongly related to long-term air pollution over years,” Khreis told MedPage Today. She suggested continuing vehicle use restrictions, providing alternative transportation methods, and improving affordable public transportation options as long-term solutions.

“My personal opinion is that the best kind of policy changes are the holistic kind, which do not focus on just one environmental stressor (air pollution) and one health outcome (childhood asthma),” Khreis added. “Holistic policy changes would consider the wide range of environmental stressors that humans are exposed to, such as noise, urban heat, lack of green space, and lack of physical activity, in addition to air pollution.”

The study by Anenberg and colleagues analyzed global surface NO₂ concentration data from the NASA Goddard Space Flight Center, and they used an adjusted model of global land use from the 2019 Global Burden of Disease study to determine rural and urban areas.

To estimate NO₂-attributable pediatric asthma incidence, the authors applied a relative risk of 1.26 (95% uncertainty interval 1.1-1.37) per 10 parts per billion annual average NO₂ concentration increase from a previous meta-analysis conducted by Khreis and colleagues. They estimated the global pediatric population from 2000 to 2019 using data from WorldPop, an open access source of spatial demographic data.

Anenberg and colleagues acknowledged that pediatric asthma rates may be underestimated in many low- and middle-income countries. They also noted that many cities and rural areas in these countries do not have ground NO₂ monitors, making concentration estimates more uncertain.