Introduction

Air pollution is one of the top five risk factors for disease burden around the world according to the latest estimates from the Global Burden of Diseases, Injuries, and Risk Factors Study.1

• Murray C.J.L.
• Aravkin A.Y.
• Zheng P.
• et al.

Global burden of 87 risk factors in 204 countries and territories, 1990–2019: a systematic analysis for the Global Burden of Disease Study 2019. Reducing disease burden attributed to air pollution is also a key indicator of the United Nations Sustainable Development Goal 3, to ensure healthy lives and promote well-being for all at all ages, as well as Goal 11 for sustainable cities and communities. Long-term exposure to high levels of air pollutions would lead to a variety of non-communicable diseases, particularly for children.1

• Murray C.J.L.
• Aravkin A.Y.
• Zheng P.
• et al.

Global burden of 87 risk factors in 204 countries and territories, 1990–2019: a systematic analysis for the Global Burden of Disease Study 2019. Asthma is the most common chronic disease among children worldwide, with rising incidences in developing economies. While air pollution control measures have been haphazardly effective in controlling particulate matter exposures, anthropogenic sources of NO2 mainly derived from include on-road and non-road transportation tailpipe emissions may not correlate with particulate matter control measures. There are recent advancements in our understanding of NO2 and onset of allergic diseases. Epidemiological studies have provided evidences of the associations between air pollutants and new-onset asthma in children. The evidence for other air pollutants (e.g., fine particulate matter) is mixed except for nitrogen dioxide (NO2).4

• Khreis 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.,5

• Gasana J.
• Dillikar D.
• Mendy A.
• Forno E.
• Ramos Vieira E.

Motor vehicle air pollution and asthma in children: a meta-analysis. Epidemiological studies in Europe, North America, Japan, China, Korea have consistently shown a significant positive association between NO2 and pediatric asthma incidence.4

• Khreis 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. Achakulwisut et al.6

• Achakulwisut P.
• Brauer M.
• Hystad P.
• Anenberg S.C.

Global, national, and urban burdens of paediatric asthma incidence attributable to ambient NO2 pollution: estimates from global datasets. have linked NO2 with about 4·0 million annual number of new pediatric asthma cases in 194 countries. And China has been estimated to have the most considerable national burdens of NO2-attributable pediatric asthma, with 760,000 new cases per year and more than 50% of the cases occurring in urban areas.6

• Achakulwisut P.
• Brauer M.
• Hystad P.
• Anenberg S.C.

Global, national, and urban burdens of paediatric asthma incidence attributable to ambient NO2 pollution: estimates from global datasets.The World Health Organization (WHO) issued new Air Quality Guidelines (AQG) on Sept 22, 2021 (WHO AQG 2021), setting AQG for NO2 of annual averaged concentration of 10 μg/m3 and applied for both indoor and outdoor air.7World Health Organization
WHO Global air Quality Guidelines. Particulate Matter (PM2.5 and PM10), Ozone, Nitrogen Dioxide, Sulfur Dioxide and Carbon Monoxide. NO2 is usually thought of as a proxy of traffic-related air pollution. Whereas there are also evident sources of NO2 indoor, i.e., NO2 produced during combustion processes indoors, such as cooking (i.e., gas combustion) and smoking (i.e., tobacco combustion),8Relationship between indoor and outdoor NO2: a review. even in areas without using solid fuels and kerosene. Urban areas are typical areas with heavy traffic and do not use solid fuels and kerosene, especially in urban China where the population density is extremely high. Our latest study figured out indoor sources contribute ∼33% to human exposure to NO2 in Chinese urban.9Indoor sources strongly contribute to exposure of Chinese urban residents to PM2.5 and NO2. Despite the importance of indoor sources in NO2 exposure, to the best of our knowledge, no studies focus on the disease burden of NO2 from indoor sources,6

• Achakulwisut P.
• Brauer M.
• Hystad P.
• Anenberg S.C.

Global, national, and urban burdens of paediatric asthma incidence attributable to ambient NO2 pollution: estimates from global datasets.,8Relationship between indoor and outdoor NO2: a review. let alone the policies to restrict NO2 emitted indoors. This is unique compared with those previous studies focused on outdoor NO2. The formulation of policies to restrict the NO2 produced indoor, such as implementing a smoking ban and using electric stoves for cooking, may well be an essential initiative to reduce the disease burden attributable to NO2 exposure.

In this study, we estimated the number of pediatric asthma cases and corresponding economic losses attributable to exposure to NO2 from indoor and outdoor sources in Chinese urban areas in 2019. We further projected and compared the health benefits and corresponding economic benefits of policies on restricting the NO2 emissions indoor and outdoor.

Discussion

Our results show that a large number of pediatric asthma cases in Chinese urban areas in 2019 were attributable to NO2, with significant contributions from both indoor and outdoor sources. The cost of each child with asthma is equivalent to about 5% of per capita disposable income in China. The reduction of pediatric asthma cases and its corresponding economic losses by restricting indoor NO2 emissions is comparable to the burden reduction when outdoor air meets WHO AQG 2021 (i.e., the annual NO2 concentration lower than 10 μg/m3). The restrictions on indoor and outdoor NO2 emissions are of the essence in reducing pediatric asthma incidence in urban China. The unique approach of this study is estimating the disease burden attributable to exposure to NO2 from indoor and outdoor sources separately, and comparing the health benefits of restricting indoor and outdoor NO2 emissions.

NO2 is an oxidizing gas associated with oxidative stress in asthma morbidity. Studies have identified associations between NO2 and increased airway responsiveness, airway inflammation, and enhanced responses to allergen.21Outdoor air pollution and asthma. Two latest studies have shown NO2-attributable new pediatric asthma cases at 4·0 million in 20156

• Achakulwisut P.
• Brauer M.
• Hystad P.
• Anenberg S.C.

Global, national, and urban burdens of paediatric asthma incidence attributable to ambient NO2 pollution: estimates from global datasets. and 1·9 million in 201922

• Anenberg S.C.
• Mohegh A.
• Goldberg D.L.
• et al.

Long-term trends in urban NO2 concentrations and associated paediatric asthma incidence: estimates from global datasets. globally. The incidence of new pediatric asthma associated with NO2 exposure in China in both studies (260 (120–340) per 100 000 children in 2015 and about 129 per 100,000 children in 2019, respectively) are lower than that in our study (360 (202-482) per 100 000 children per year), but similar to the scenario when indoor sources of NO2 were completely restricted in our study (194 (109–260) per 100 000 children per year). The biases between our results and those of previous studies reflects the serious impact of NO2 from indoor source on the assessment of NO2-attributable asthma incidence in children.Air Pollution Prevention and Control Action Plan was implemented in China in 2013, aiming to reduce the concentration of ambient air pollutants. But the concentration of ambient NO2 has not decreased significantly in recent years, with the annual average concentration only reduced by 5% from 29·3  g m−3 in 2015 to 27·7 μg m−3 in 2019.23

• Zhou W.
• Chen C.
• Lei L.
• Fu P.
• Sun Y.

Temporal variations and spatial distributions of gaseous and particulate air pollutants and their health risks during 2015–2019 in China. Ambient NO2 is traffic-related air pollution associated with the combustion of gasoline and diesel in vehicles. Transportation contributes up to 80% of ambient NO2 in urban areas. Population growth, extensive urbanization, and wealth creation have combined to create ever-increasing traffic volumes in urban areas in China, making it increasingly difficult to reduce ambient NO2 pollution. Studies have shown that replacing the traditional fossil energy vehicles with new energy vehicles would lead to substantial reductions of ambient NO2 (30–80%) in most of China.24

• Wang L.
• Chen X.
• Zhang Y.
• et al.

Switching to electric vehicles can lead to significant reductions of PM2.5 and NO2 across China. General Office of the State Council of the People’s Republic of China issued the Development Plan for New Energy Vehicle Industry (2021–2035) on November 2, 2020, to increase the new energy vehicles quotas in total vehicle production, aiming at peaking fossil fuels consumption of land transport by 2030.25The State Council of China
Guidelines on the Development of New Energy Vehicle Industry (2021–2035). This policy would bring benefits in terms of reduce ambient NO2 concentrations in China.Despite the policies on developing new energy vehicles, it might be decades before the ambient NO2 meet the WHO AQG 2021. Prior to this time point, restricting NO2 from indoor sources would reap health benefits efficiently, especially in reducing pediatric asthma incidence in Chinese urban areas. Switching from using a gas stove to the electric stove for cooking to reduce the NO2 produced by gas combustion is a vital strategy to restrict NO2 emissions indoors. With the popularity of induction cookers and ceramic cooktops in China in recent years, the acceptability of electric stoves in Chinese families has been increasing year by year. However, using gas stoves for cooking is still mainstream in households in Chinese urban areas.26

• Chan K.H.
• Lam K.B.H.
• Kurmi O.P.
• et al.

Trans-generational changes and rural-urban inequality in household fuel use and cookstove ventilation in China: a multi-region study of 0.5 million adults. It is necessary to encourage the use of electric stoves for cooking in Chinese urban areas. The government may need to provide some preferential policies to increase the utilization rate of electric stoves. The increase in the utilization rate of electric stoves is accompanied by the increase in demand for electricity and a decrease in demand for gas in residence. The urban energy system needs to be adjusted accordingly to meet the new demand for energy.The measures for restriction on NO2 emissions from indoor and outdoor reflect the demand to adjust the energy consumption structure of residences and transportations in Chinese urban areas, respectively. Traditional energy consumption is accompanied by greenhouse gas emissions. China pledged to achieve the goal of cutting greenhouse gas emissions to net-zero by 2050 at the 2019 Climate Action Summit.27All hands on deck”: the Paris Agreement and nonstate climate action. Developing new energy and constructing new electrical power systems, including the power systems for transportation and residences, are important measures to achieve this goal.28The State Council of China
Implementation scheme of promoting peak carbon dioxide emissions in China. These actions may be a win-win opportunity for not only China but also other developing countries which are facing both climate change and air pollution challenges.Several limitations exist in our approach. First, we applied the concentration-response function from a meta-analysis of studies in multiple countries around the world4

• Khreis 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. to estimate the NO2-attributable burden of pediatric asthma incidence in urban areas in China. The relative risk from east Asia weighted 22·8% in the meta-analysis. Our sensitivity analysis using the pooled relative risk from studies in east Asia showed a low uncertainty (Figure S3 in appendix). It should be noted that NO2 from indoor cooking is mixed with other air pollutants from burning oil, heating ingredient and stirring ingredient. Applying the coefficient of concentration-response function from ambient NO2 for the indoor part may introduce some uncertainties. However, NO2 is produced from gas combustion during cooking and it can be reduced by changing the type of energy to cook (i.e., switching from gas to electricity), and this is different from those pollutants produced from burning oil, heating ingredient or stirring ingredient. Therefore, other pollutants produced during cooking are not necessarily accompanied by the emission of NO2. As our aim was to analyze the disease burden attributable to NO2 itself, rather than the disease burden associated with all the air pollutants from cooking, the concentration-response function from ambient NO2 is still applicable to this study. Secondly, we estimated exposure to NO2 from two main indoor sources, i.e., cooking and second-hand smoke. Another indoor source of NO2, wall-mounted gas boiler, was not considered in this study. The wall-mounted gas boilers are equipped with enclosed combustors and exhaust ducts, which emit negligible NO2 indoors compared to cooking and smoking. Due to central heating systems in northern China, the wall-mounted gas boilers were mainly used in families in southern China. Studies have shown the low usage of wall-mounted gas boilers in south China.29Contrastive analysis of heating modes of residential buildings in southern Jiangsu. So the contribution of wall-mounted gas boiler to NO2 exposure is too low to affect the incidence of NO2-attributable pediatric asthma incidence in this study. Thirdly, we estimated the incidence of pediatric asthma associated with an annual average exposure of NO2, while the time period of changes in NO2 exposure may also influence the incidence and introduce uncertainties. Zhu et al. studied temporal variations of short-term associations between NO2 concentrations and emergency department visits in Shanghai, China, in 2008–2019. They found the associations of emergency department visits and NO2 remained stable over the study period.30

• Zhu Y.
• Peng L.
• Li H.
• Pan J.
• Kan H.
• Wang W.

Temporal variations of short-term associations between PM10 and NO2 concentrations and emergency department visits in Shanghai, China 2008–2019. This evidence means that effects on pediatric asthma incidence for NO2 may also remain stable over time, and we expect more direct evidence in future studies.

Despite these limitations, our results showed that indoor and outdoor NO2 emissions are associated with the disease burden of pediatric asthma incidence in households without using solid fuels and kerosene. The implementation of the policies on developing clean and alternative energy vehicles has brought hope for reducing ambient NO2 pollution in the future. We call on individuals to use the electricity for cooking instead of gas to reduce exposure to NO2 from indoor sources. The government may need to adjust the energy structure of residences to meet the new demand for energy. These measurements on restrictions of NO2 emissions, i.e., developing new energy vehicles and switching from gas stove to electric stove, support clean energy consumption and aid in achieving climate change mitigation.

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