About 4 million children worldwide develop asthma each year because of inhaling nitrogen dioxide (NO²) air pollution, according to an open-access study published in The Lancet Planetary Health by researchers at the George Washington University Milken Institute School of Public Health (Milken Institute SPH). The study, based on data from 2010 to 2015, estimates that 64% of these new cases of asthma occur in urban areas.

The study is the first to quantify the worldwide burden of new pediatric asthma cases linked to traffic-related nitrogen dioxide by using a method that takes into account high exposures to this pollutant that occur near busy roads, said Susan C. Anenberg, PhD, the senior author of the study and an associate professor of environmental and occupational health at Milken Institute SPH.

National annual estimates of number of new asthma cases (A), number of new asthma cases per 100,000 children (B), and percentage of new asthma cases attributable to ambient nitrogen dioxide (NO₂) exposure for children aged 1–18 years. Achakulwisut et al.

Our findings suggest that millions of new cases of pediatric asthma could be prevented in cities around the world by reducing air pollution. Improving access to cleaner forms of transportation, like electrified public transport and active commuting by cycling and walking, would not only bring down NO₂ levels, but would also reduce asthma, enhance physical fitness, and cut greenhouse gas emissions.

—Susan Anenberg

Traffic-related air pollution may result in asthma development as pollutants may cause damage to the airways, leading to inflammation that triggers asthma in genetically predisposed children. Although it is not yet clear which specific pollutant within the traffic-related air pollution mixture is the source of asthma development, reviews by the US Environmental Protection Agency and Health Canada suggest that a causal relationship is likely to exist between long-term nitrogen dioxide (NO₂) exposure and childhood asthma development.

In the new study, the authors used NO₂ as a surrogate for the traffic pollution mixture to focus specifically on the effects of traffic pollution on childhood asthma development. NO₂ is a pollutant formed mainly from fossil fuel combustion, and traffic emissions can contribute up to 80% of ambient NO₂ in cities. NO₂ is just one component of air pollution, which is made up of many pollutants (including particulate matter, ozone, carbon monoxide), which are known to have numerous adverse effects on health.

The authors combined a global dataset of ambient NO₂ (modelled from ground-level monitors, satellite data, and land use variables such as road networks) with data on population distribution and asthma incidence to estimate the number of new traffic pollution-related asthma cases in children aged 1-18 years.

They were then able to estimate the number of new pediatric asthma cases attributable to NO₂ pollution in 194 countries and 125 major cities worldwide.

Findings from the study:

• An estimated 4 million children developed asthma each year from 2010 to 2015 due to exposure to NO₂ pollution, which primarily comes motor vehicle exhaust.

• An estimated 13% of annual pediatric asthma incidence worldwide was linked to NO₂ pollution.

• Among the 125 cities, NO₂ accounted for 6% (Orlu, Nigeria) to 48% (Shanghai, China) of pediatric asthma incidence. NO₂’s contribution exceeded 20% in 92 cities located in both developed and emerging economies.

• The top 10 highest NO₂ contributions were estimated for eight cities in China (37 to 48% of pediatric asthma incidence) and for Moscow, Russia and Seoul, South Korea at 40%.

• The problem affects cities in the United States as well: Los Angeles, New York, Chicago, Las Vegas and Milwaukee were the top five cities in the US with the highest percentage of pediatric asthma cases linked to polluted air.

• The largest number of cases of traffic pollution-related asthma were estimated for China (760,000 cases), which is likely a result of China having the second largest population of children and the third highest concentrations of NO₂.

• Although less than half the size of China’s burden, India had the next largest number of cases (350,000) due to its large population of children. The USA (240,000), Indonesia (160,000) and Brazil (140,000) had the next largest burdens, with the USA having the highest pollution levels of these three countries, while Indonesia had the highest underlying asthma rates.

• The country with the highest percentage of traffic pollution-attributable childhood asthma incidence was South Korea (31%), followed Kuwait (30%), Qatar (30%), United Arab Emirates (30%), and Bahrain (26%). The UK ranked 24^th out of 194 countries, the US 25^th, China 19^th, and India 58^th.

  The authors explain that India ranks below other countries for this metric because, although levels of other pollutants (particularly PM_2.5) in India are among the highest in the world, NO₂ levels from 2010-2012 in Indian cities appear to be lower than or comparable to levels in European and US cities.

• Two-thirds of traffic pollution-related asthma cases occurred in urban centres globally, and when suburbs were included this proportion increased to 90% of cases.

Asthma is a chronic disease that makes it hard to breathe and results when the lung’s airways are inflamed. An estimated 235 million people worldwide currently have asthma, which can cause wheezing as well as life-threatening attacks.

The World Health Organization calls air pollution a major environmental risk to health and has established Air Quality Guidelines for NO₂ and other air pollutants. The researchers estimate that most children lived in areas below the current WHO guideline of 21 parts per billion for annual average NO₂. They also found that about 92% of the new pediatric asthma cases that were attributable to NO₂ occurred in areas that already meet the WHO guideline.

That finding suggests that the WHO guideline for NO₂ may need to be re-evaluated to make sure it is sufficiently protective of children’s health.

—Pattanun Achakulwisut, PhD, lead author of the paper and a postdoc at Milken Institute SPH

The researchers found that in general, cities with high NO₂ concentrations also had high levels of greenhouse gas emissions. Many of the solutions aimed at cleaning up the air would not only prevent new cases of asthma and other serious health problems but they would also attenuate global warming, Anenberg said.

Additional research must be done to more conclusively identify the causative agent within complex traffic emissions, said the researchers. This effort, along with more air pollution monitoring and epidemiological studies conducted in data-limited countries will help to refine the estimates of new asthma cases tied to traffic emissions, Anenberg and Achakulwisut added.

Due to limited data availability, the NO₂ levels used in this study are for 2010-2012, whereas the population and asthma incidence rates are for 2015. Given recent global changes in NONO₂2 levels (decreases in US and European cities, and increases in Asia), the estimates may not be exact and further research with the latest NO₂ levels is needed.

This study was funded by George Washington University. It was conducted by researchers from George Washington University, The University of British Colombia, The Institute for Health Metrics and Evaluation, and Oregon State University.

Resources

• Pattanun Achakulwisut, Michael Brauer, Perry Hystad, Susan C Anenberg (2019) “Global, national, and urban burdens of paediatric asthma incidence attributable to ambient NO₂ pollution: estimates from global datasets” The Lancet Planetary Health doi: 10.1016/S2542-5196(19)30046-4