NO2 increases the risk for childhood asthma: a global concern
Authors: Rajen N Naidoo
Journal: The Lancet Planetary Health (2019)

In the past decade, studies and statements from international agencies on the relationship between ambient air pollution and human health have raised the call for action. An analysis1Cohen AJ Brauer M Burnett R et al.Estimates and 25-year trends of the global burden of disease attributable to ambient air pollution: an analysis of data from the Global Burden of Diseases Study 2015.Lancet. 2017; 389: 1907-1918Summary Full Text Full Text PDF PubMed Scopus (3299) Google Scholar of the data from the Global Burden of Diseases Study 2015 indicated that approximately 4·2 million deaths and 103 million disability-adjusted life-years were attributable to exposure to particulates, with similar data available from WHO.2WHOAmbient air pollution: a global assessment of exposure and burden of disease.https://www.who.int/phe/publications/air-pollution-global-assessment/en/Date: 2016Date accessed: March 26, 2019Google Scholar The Lancet Commission on Air Pollution and Health3Landrigan PJ Fuller R Acosta NJR et al.The Lancet Commission on pollution and health.Lancet. 2018; 391: 462-512Summary Full Text Full Text PDF PubMed Scopus (2009) Google Scholar stated that 16% of all premature deaths in 2015 were attributable to pollution. Among these alarming statistics, adverse respiratory health assumes a priority position, particularly among children. Childhood asthma has reached global epidemic proportions4Asher I Pearce N Global burden of asthma among children.Int J Tuberc Lung Dis. 2014; 18: 1269-1278Crossref PubMed Scopus (421) Google Scholar and, although the evidence for a causal relationship with ambient pollution is still emerging, studies have repeatedly shown associations of pollution with various asthma phenotypes and proxy markers, such as emergency room visits, school absenteeism, and steroid dependency. However, a level of inconsistency exists across various pollutants for these outcomes. For example, a 2017 meta-analysis5Khreis H Kelly C Tate J Parslow R Lucas K Nieuwenhuijsen M Exposure to traffic related air pollution and risk of development of childhood asthma: a systematic review and meta-analysis.Environ Int. 2017; 100: 1-31Crossref PubMed Scopus (411) Google Scholar reported the varying risk for asthma incidence across studies, with odds ratios ranges of 0·93–1·25 for each 4 μg/m3 increase in nitrogen dioxide (NO2) concentrations and 0·84–1·32 for each 1 μg/m3 increase in fine particulate matter (PM2·5) concentrations. Despite these inconsistencies, meta-analyses5Khreis H Kelly C Tate J Parslow R Lucas K Nieuwenhuijsen M Exposure to traffic related air pollution and risk of development of childhood asthma: a systematic review and meta-analysis.Environ Int. 2017; 100: 1-31Crossref PubMed Scopus (411) Google Scholar, 6Bowatte G Lodge C Lowe AJ et al.The influence of childhood traffic-related air pollution exposure on asthma, allergy and sensitization: a systematic review and a meta-analysis of birth cohort studies.Allergy. 2014; 70: 245-256Crossref PubMed Scopus (299) Google Scholar of traffic-related air pollution and childhood asthma incidence provide convincing evidence for increased risk of exposure, particularly for NO2. These effects are seen at concentrations below the ambient air quality guidelines recommended by WHO7WHOWHO air quality guidelines for particulate matter, ozone, nitrogen dioxide and sulfur dioxide: global update 2005—summary of risk assessment.http://www.who.int/iris/handle/10665/69477Date: 2006Date accessed: March 30, 2019Google Scholar of 40 μg/m3 (21 ppb) annual average NO2 concentrations. A better understanding of the pollutant-related effects on childhood asthma incidence at a global level is crucial to addressing interventions and policy initiatives. In The Lancet Planetary Health, Pattanun Achakulwisut and colleagues8Achakulwisut P Brauer M Hystad P Anenberg SC Global, national, and urban burdens of paediatric asthma incidence attributable to ambient NO2 pollution: estimates from global datasets.Lancet Planet Health. 2019; (published online April 10.)http://dx.doi.org/10.1016/S2542-5196(19)30046-4Summary Full Text Full Text PDF PubMed Scopus (191) Google Scholar provide a novel approach to this challenge. By use of well validated global datasets for both exposure and outcome and with estimates of risk derived from the scientific literature, the authors modelled childhood asthma incidence attributable to NO2. Concentration-response functions were derived from a meta-analysis that reported a relative risk of 1·26 (95% uncertainty interval (UI) 1·10–1·37) for every 10 ppb increase in NO2 concentrations. Global NO2 surface concentrations were derived from a land use regression model that extracted data from over 5000 ground-level monitors in 58 countries, as well as satellite-available pollution data.9Larkin A Geddes JA Martin R V et al.Global land use regression model for nitrogen dioxide air pollution.Environ Sci Technol. 2017; 51: 6957-6964Crossref PubMed Scopus (127) Google Scholar The 2015 Global Human Settlement population grid provided population distribution and density information, and country-specific and age-group-specific asthma incidence rates were obtained from the Institute for Health Metrics and Evaluation for 194 countries. The major advance of Achakulwisut and colleagues' study, compared with previous works, was the ability to estimate this incidence at very fine spatial resolution of 250 × 250 m across 21 regions, 194 countries, and 125 major cities globally. The advantage of this resolution allowed for determining within-city variation, differentiating between urban risk zones. On the basis of their analysis, the authors reported that 4·0 million (95% UI 1·8–5·2) new childhood asthma cases could be attributable to NO2 pollution annually, accounting for 13% (6–16) of the incidence worldwide. Attributable burden varied across regions, ranging from 340 cases per year per 100 000 children (95% UI 150–440) in Andean Latin America to 76 cases (33–100) in eastern sub-Saharan Africa. Large-city variation was also evident, ranging from 690 cases per year per 100 000 children (330–870) in Lima, Peru, to 83 cases (36–110) in Orlu, Nigeria, with Chinese cities mostly in the 400–650 range. A substantial percentage of childhood asthma incidence was attributed to NO2 exposure, globally, but this was most striking for the Chinese cities: Shanghai (48%, 95% UI 25–57), Tianjin (43%, 16–42); Beijing (42%, 21–51) and Shenyang (41%, 21–50). Despite providing a novel approach to understanding the relationship between traffic-related air pollution and childhood asthma, Achakulwisut and colleagues' study brings forth important epidemiological challenges. The lower estimates of risk seen in south Asia and sub-Saharan Africa, compared with those in other regions, are largely driven by data availability. Exposure data is sparse for these regions, influencing the development of exposure models from these countries.9Larkin A Geddes JA Martin R V et al.Global land use regression model for nitrogen dioxide air pollution.Environ Sci Technol. 2017; 51: 6957-6964Crossref PubMed Scopus (127) Google Scholar Additionally, asthma incidence data are likely to be very scarce, particularly with the use of so-called doctor-diagnosed asthma outcome definition in countries with poor health-care access, resulting in caregivers and children not being aware that their, or their children's, symptoms might be typical of asthma. These issues are likely to bias the findings from these regions. A better understanding of the risk in such regions is necessary, because research findings might have policy implications for interventions in these low-middle-income countries. 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. I declare no competing interests. Global, national, and urban burdens of paediatric asthma incidence attributable to ambient NO2 pollution: estimates from global datasetsEfforts to reduce NO2 exposure could help prevent a substantial portion of new paediatric asthma cases in both developed and developing countries, and especially in urban areas. Traffic emissions should be a target for exposure-mitigation strategies. The adequacy of the WHO guideline for ambient NO2 concentrations might need to be revisited. Full-Text PDF Open Access