Globally, the estimates suggest that there are 170 new cases of traffic pollution-related asthma per 100,000 children every year, and 13% of childhood asthma cases diagnosed each year are linked to traffic pollution
The country with the highest proportion of traffic pollution-attributable childhood asthma incidence was South Korea (31%), the UK ranked 24th out of 194 countries, the US 25th, China 19th, and India 58th
Of the 125 cities studied, there was a large variation in the proportion of childhood asthma cases attributable to traffic pollution - from 6% in Orlu, Nigeria, to 48% in Shanghai, China. Of the ten cities with the highest proportion of cases, eight were in China, alongside Moscow, Russia, and Seoul, South Korea
Authors suggest that pollution guidelines may need to be re-evaluated, as most children developing traffic pollution-related asthma lived in areas within recommended levels
The first global estimates of their kind suggest that more than one in ten childhood asthma cases could be linked to traffic-related air pollution every year, according to a health impact assessment of children in 194 countries and 125 major cities worldwide, published in The Lancet Planetary Health journal.
With 92% of cases developing in areas that have traffic pollution levels below the WHO guideline level , the authors suggest that this limit may need to be reviewed.
"Nitrogen dioxide pollution appears to be a substantial risk factor for childhood asthma incidence in both developed and developing countries, especially in urban areas," says senior author Dr Susan Anenberg, George Washington University, USA. "Our findings suggest that the World Health Organization guideline for annual average NO2 concentrations might need to be revisited, and that traffic emissions should be a target to mitigate exposure." 
Lead author Ploy Achakulwisut, George Washington University, USA, adds: "Our study indicates that policy initiatives to alleviate traffic-related air pollution can lead to improvements in children's health and also reduce greenhouse gas emissions. Recent examples include Shenzhen's electrification of its entire bus fleet and London's Ultra-Low Emission Zone congestion charges." 
Globally, asthma is the most common non-communicable disease among children, and, according to the WHO, prevalence has increased dramatically since the 1950s. The reasons for this are multiple.
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.
Globally, the estimates suggest that there are 170 new cases of traffic pollution-related asthma per 100,000 children every year, and 13% of childhood asthma cases diagnosed each year are linked to traffic pollution.
The country with the highest rate of traffic pollution-related childhood asthma was Kuwait (550 cases per 100,000 children each year), followed by the United Arab Emirates (460 per 100,000), and Canada (450 per 100,000). Of the 125 cities studied, there was a large variation in the estimated rate of traffic pollution-related childhood asthma - from 83 cases per 100,000 children every year in Orlu, Nigeria, to 690 cases per 100,000 children in Lima, Peru. These rates of traffic pollution-related asthma are influenced by asthma rates overall, as well as pollution levels, and may underestimate true levels in many low- and middle-income countries. This is because asthma cases often go undiagnosed in these regions.
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 24th out of 194 countries, the US 25th, China 19th, and India 58th. The authors explain that India ranks below other countries for this metric because, although levels of other pollutants (particularly PM2.5) 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.
The percentage of new asthma cases attributable to traffic pollution by city ranged from 6% in Orlu, Nigeria, to 48% in Shanghai, China, and largely reflected the variations in NO2 exposures within each region. Of the ten cities with the highest proportion of traffic pollution-related asthma cases, eight were in China (Shanghai, Tianjin, Beijing, Shenyang, Xi'an, Taiyuan, Zhengzhou, and Harbin) alongside Moscow, Russia, and Seoul, South Korea - all of which had high urban NO2 concentrations. Paris ranked 21st (33%), New York ranked 29th (32%), London 35th (29%), and New Delhi 38th (28%).
The authors note some limitations, including that studies linking traffic pollution and asthma, and NO2 monitoring data are largely from North America, Europe and east Asia, and NO2 ground-level monitors are mostly in urban areas, so could overestimate pollution levels in rural areas.
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 NO2 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.
Writing in a linked Comment, Professor Rajen N Naidoo, University of KwaZulu-Natal, South Africa, says: "An important outcome from this study is the further evidence that the existing WHO standards are not protective against childhood asthma. Achakulwisut and colleagues estimated that approximately 92% of the childhood asthma incidence attributable to NO2 exposure was in areas with NO2 concentrations below the values of the WHO annual average guidelines. This strengthens the case for the downward revision of these global standards and for stronger national policy initiatives in countries without air quality standards. Furthermore, these findings not only support the association of NO2 exposure with childhood asthma incidence, but also, because this pollutant serves as an important proxy for broader traffic related air pollutants, highlight that urgent intervention is necessary to protect the health of those most vulnerable in society: children, particularly those with pre-existing respiratory disease."