3.1.1. AP of PM_2.5 in Different Cities

3.1.2. Notable Observations and Epidemiological Insights

Among the cities studied, Patna in India consistently registers the highest AP of health outcomes linked to PM_2.5 exposure, including chronic bronchitis and lung cancer. This pronounced burden is likely due to a combination of high population density, a proliferation of unregulated brick kilns, routine agricultural residue burning, and unfavorable winter meteorological conditions that restrict atmospheric dispersion. In China, the city of Suqian shows similarly elevated AP values, particularly for lung-related diseases. This trend is attributed to widespread stubble burning, intensive industrial development, and the relative absence of stringent pollution control mechanisms. In Japan, although overall concentrations of PM_2.5 tend to be low, the city of Hiroshima presents an outlier. This can likely be attributed to localized contributors, including dense vehicular movement in urban corridors, combustion activities on the city’s outskirts, and limited atmospheric dispersion due to topographical and climatic constraints. Such site-specific conditions, when combined with socioeconomic activities, appear to elevate the exposure levels in this region. The case of Hiroshima, alongside other cities with high health burdens, illustrates how urban structure, lifestyle, and environmental features collectively shape the intensity of PM_2.5-related health impacts.

3.1.3. Health Risks Attributable to PM_2.5 Exposure

Fine particulate matter (PM_2.5) is widely recognized as one of the most harmful air pollutants due to its ability to reach deep parts of the lungs and enter the circulatory system. Once inhaled, these particles initiate inflammatory responses in the respiratory tract, leading to aggravated asthma symptoms, bronchial infections, and a decline in overall lung function. For individuals with preexisting conditions, this can result in frequent hospital visits and chronic respiratory distress. Beyond the lungs, PM_2.5 exposure has been strongly linked to cardiovascular complications. It can trigger oxidative stress and systemic inflammation, impairing blood vessels and increasing the risk of heart disease, irregular heartbeat, and stroke. Long-term exposure also elevates the likelihood of lung cancer, due to the deposition of toxic and carcinogenic substances on lung tissue.

More recent studies are beginning to explore the neurological consequences of PM_2.5, especially in aging populations. Preliminary evidence points to potential links with cognitive decline and an increased risk of neurodegenerative diseases, such as Alzheimer’s. In pregnant women, exposure has been associated with poor birth outcomes, such as preterm birth, lower birth weight, and developmental challenges in children. Altogether, long-term PM_2.5 exposure places a disproportionate burden on vulnerable groups including children, the elderly, and individuals with compromised immunity raising the risk of early mortality.

3.1.4. Integrated Strategies to Mitigate PM_2.5 Emissions

Mitigating PM_2.5 emissions demands coordinated action across multiple sectors. In transportation, urban governments should prioritize expanding and modernizing public transit systems, introduce disincentives for private vehicle use (such as congestion fees), and accelerate the transition to electric or hybrid vehicles. Traffic management policies should also focus on reducing idle time and improving fuel efficiency in city centers.

Industrial emissions can be curtailed by mandating compliance with strict pollution norms, offering incentives for cleaner technologies, and shifting towards renewable energy sources. Urban infrastructure, too, has a role to play. Constructing energy-efficient buildings, enhancing natural ventilation systems, and expanding tree cover within cities can help absorb airborne pollutants and cool local microclimates.

Land use policies should aim to limit open burning of waste and biomass, regulate construction in ecologically fragile areas, and ensure green buffer zones near residential communities.

3.2.1. Principal Findings and City-Specific Attributions of PM₁₀ Exposure

The comparative analysis of PM₁₀-attributable health outcomes across major cities in India, China, and Japan reveals a clear spatial disparity in chronic bronchitis prevalence and postneonatal infant mortality. Among all locations, Patna, India, emerges as the most severely impacted, with the highest APs across all three exposure categories low, central, and high for both adult and childhood bronchitis, as well as for infant mortality. These high estimates reflect the city’s chronic exposure to coarse PM, aggravated by persistent winter smog, widespread combustion of biomass, and limited accessibility to healthcare services. Suqian, a city in China, stands out with notably high PM₁₀-related mortality rates, particularly affecting both adults and children with bronchitis. This outcome likely stems from a combination of dense industrial clusters, heavy road traffic, and inadequate local air quality regulations. In Japan, Shizuoka reports the highest PM₁₀-attributable burden nationally, especially in cases of chronic bronchitis and pediatric respiratory deaths. Its geographic setting positioned within a semienclosed terrain combined with industrial activity and steady traffic movement, results in frequent pollution entrapment. By contrast, cities like Kolkata, Shanghai, and Osaka show consistently lower AP values for similar health endpoints. These trends may reflect more advanced pollution control policies, efficient public transport systems, and stronger healthcare networks.

3.2.2. Key Contributors to High PM₁₀-Linked Health Burden in Selected Cities

The substantial health burden linked to PM₁₀ in Patna, Suqian, and Shizuoka arises from multiple overlapping factors. High pollutant levels in these cities are driven primarily by intensive industrial operations, unregulated traffic emissions, and localized practices like the burning of biomass and construction-related dust generation. During winter, weather conditions often worsen air quality due to temperature inversions and low wind movement, causing pollutants to remain concentrated near ground level. Another major concern is access to healthcare; cities with limited hospital capacity or delayed diagnosis tend to experience worse outcomes. Vulnerable populations, such as children and the elderly, are particularly at risk. Poor air ventilation in congested neighborhoods, limited green space, and higher baseline respiratory illness rates in low-income zones further amplify the adverse effects of PM₁₀ exposure.

3.2.3. City-Specific Factors Behind Elevated PM₁₀ Impacts

In Patna, the highest central APs for chronic bronchitis (in both children and adults) and postneonatal deaths are reported. This can be linked to a mix of severe winter smog, heavy dependence on biomass for household energy, brick kilns, and congested traffic corridors. Suqian, located in Jiangsu province, shows a consistent pattern of high AP values, which aligns with its proximity to industrial facilities and seasonal crop residue burning in rural surroundings. Urbanization in this region has progressed rapidly, accompanied by widespread construction and increasing vehicular density both of which intensify PM₁₀ levels.

In Shizuoka, despite the relatively stringent environmental standards in Japan, PM₁₀-related health outcomes are higher than in most Japanese cities. The city’s geographical features, including low-lying basins, likely hinder the dispersion of pollutants. Regular emissions from vehicles and small-scale manufacturing units further add to this localized burden. These findings suggest that even moderate pollution levels, when trapped within limited dispersion zones, can pose serious health risks.

3.2.4. Health Impacts Linked to PM₁₀ Exposure

Although PM₁₀ does not penetrate as deeply into the lungs as PM_2.5, it still contributes significantly to respiratory and cardiovascular illnesses. Prolonged exposure to these coarse particles irritates the airways and reduces lung function, often leading to chronic bronchitis, aggravated asthma, and increased susceptibility to respiratory infections. Studies have also associated PM₁₀ exposure with elevated risks of heart attack, stroke, and hypertension due to inflammatory effects and oxidative damage to blood vessels. In pregnant women, there is growing evidence that PM₁₀ exposure may result in preterm births, lower birth weights, and impaired developmental outcomes. Among elderly individuals and those with preexisting respiratory or cardiac conditions, the risk of premature death increases during high-exposure periods. Short-term symptoms such as sore throat, eye irritation, and persistent coughing are also widely reported in PM₁₀-heavy environments, contributing to productivity loss and higher medical expenses. Reduced lung capacity and prolonged recovery from respiratory illnesses are common outcomes in populations living in persistently polluted areas.

3.2.5. Strategies to Reduce PM₁₀ Emissions and Associated Health Risks

Reducing PM₁₀ pollution in urban settings requires coordinated interventions across transport, industry, construction, agriculture, and public behavior. On the roads, promoting electric vehicles, setting up low-emission zones, and expanding affordable public transit are practical steps. In the industrial sector, stricter enforcement of emission norms and adoption of cleaner production systems such as installing particulate filters or scrubbers can yield significant reductions.

Urban development should prioritize greener designs: increasing the number of trees and vegetative buffers, optimizing ventilation in built environments, and encouraging vertical greenery can all help filter dust particles. In construction, mandating dust control practices and regulating demolition activities are key actions. Agricultural emissions should be tackled through policies that discourage open stubble burning, encouraging practices like in situ residue management and composting instead. Finally, awareness campaigns are critical. By informing the public about household contributors to PM₁₀ such as solid fuel use or backyard burning and promoting alternatives, behavioral change can be fostered at the community level.

3.3.1. City-Specific Epidemiological Insights

In Patna, the consistently high central AP for NO₂-related natural mortality points to structural air quality challenges that may include cumulative pollution from vehicular sources, stationary combustion, and atmospheric stagnation. The city’s infrastructure and environmental management capacity appear inadequate to manage nitrogen oxide emissions effectively. In Mumbai, elevated AP under high-exposure scenarios likely stems from a mix of heavy construction activity, intense vehicular movement, and high-energy industrial hubs concentrated within densely inhabited areas. Shanghai’s high central AP can be attributed to its industrial legacy and current urban expansion, which collectively maintain high ambient NO₂ concentrations. Additionally, Shanghai experiences seasonal meteorological conditions (e.g., high humidity) that can trap pollutants close to ground level. Osaka, despite Japan’s overall strong environmental controls, faces unique challenges due to its location in a low-lying basin that limits natural dispersion, especially during temperature inversions, causing NO₂ to accumulate in the urban boundary layer. In contrast, Shizuoka and Tokyo benefit from cleaner transport infrastructure, better urban design, and higher public awareness, all of which contribute to relatively lower NO₂ exposure and related health burdens.

3.3.2. Environmental and Physiological Hazards of NO₂ Exposure

The toxicological impacts of NO₂ are well documented in epidemiological literature. Inhaled NO₂ acts as an airway irritant, triggering inflammatory responses and oxidative stress in the respiratory tract. Short-term exposure can aggravate asthma and bronchitis symptoms, particularly in children, while long-term exposure increases the risk of COPD, myocardial infarction, and stroke. Studies have also linked sustained NO₂ exposure with impaired lung development in children and accelerated atherosclerosis in adults. From an ecological perspective, NO₂ contributes to acid deposition and photochemical smog formation, which can damage crops, forests, and freshwater ecosystems. Additionally, NO₂ reacts with volatile organic compounds (VOCs) under sunlight to form ground-level O₃, compounding its health impacts and further degrading urban air quality.

3.3.3. Key Determinants of High Attributable Risk

Several factors explain the high AP values observed in cities like Mumbai, Patna, Shanghai, and Osaka. First, traffic congestion remains the dominant source of NO₂ in urban cores, particularly from diesel engines and older vehicle fleets. Second, industrial clusters lacking modern emission control technologies emit substantial quantities of nitrogen oxides. Third, geographical constraints, such as valleys or coastal trapping, limit pollutant dispersion, intensifying local exposure. Lastly, population vulnerability, especially among children, elderly, and those with preexisting respiratory conditions, raises the cumulative health burden.

3.3.4. Limitations and Data Considerations

This analysis is based on city-level averages derived from modeled data using AIRQ++ and may not fully capture intraurban variability in NO₂ concentrations or health effects. Additionally, while the AP estimates provide a useful comparative tool, they may be influenced by the availability and accuracy of baseline mortality and morbidity data across countries. Meteorological data, spatial distribution of emissions, and individual-level exposure assessments were not incorporated due to data limitations. Future studies should integrate these variables for more localized health risk profiling and to improve the precision of attributable health outcomes.

3.3.5. Mitigation Strategies to Reduce NO₂ Exposure

A comprehensive mitigation strategy to curb NO₂ emissions must address both technological upgrades and behavioral change. At the policy level, transitioning to cleaner fuels (e.g., compressed natural gas, electricity, or hydrogen) in the transportation and industrial sectors is essential. Regulatory enforcement of Euro-VI or Bharat Stage VI vehicle emission norms, along with mandatory catalytic converters, can significantly reduce tailpipe NO₂ emissions. Urban design reforms should promote mass public transit systems including metro rail, electric buses, and bus rapid transit (BRT) to reduce vehicle-kilometers traveled per capita. Anti-idling laws and congestion pricing in dense commercial zones can further limit emissions from traffic bottlenecks. Technological interventions should also focus on optimizing combustion efficiency in industries and power plants, upgrading older boilers and furnaces with low-NO_x burners. Additionally, green infrastructure, such as roadside vegetation and urban tree canopies, can assist in passive pollutant capture and atmospheric filtration.

3.4.1. Factors Contributing to Elevated O₃-Related Health Burden

The formation and impact of ground-level O₃ are governed by multiple interacting variables. High APs in cities like Hyderabad and Xian are likely driven by intense solar radiation that catalyzes photochemical reactions, especially in regions with high vehicular density and limited vertical air mixing. Susceptibility factors such as population age structure, prevalence of asthma or COPD, and smoking habits further compound the respiratory mortality burden. Certain geographic features such as valleys or low-elevation plains often restrict air circulation, allowing O₃ and its precursor gases to remain trapped closer to the ground. This extended residence time can intensify exposure, especially during hot and stagnant weather conditions. Furthermore, differences in the availability and reliability of air quality monitoring stations, along with discrepancies in health data quality across cities, can impact the accuracy of O₃-AP estimates.

3.4.2. City-Level Observations on O₃ Exposure and Health Effects

In Hyderabad, elevated health risks linked to ground-level O₃ can be attributed to multiple overlapping factors. The city’s rapid urbanization, increase in paved surfaces, dense traffic corridors, and high summer temperatures provide ideal conditions for photochemical O₃ formation. These factors, coupled with limited regulatory enforcement, result in sustained exposure, particularly in densely populated pockets. In Xian, the combination of industrial growth and geographic positioning surrounded by hills that limit air movement contributes to the build-up of O₃ precursors like NOₓ and VOCs. Under clear and dry conditions, these compounds react in sunlight, leading to persistent O₃ accumulation. On the other hand, Tokyo, Osaka, and Shizuoka report significantly lower O₃-related mortality. This trend is likely due to well-implemented emission control policies, wide access to public health advisories, and a high level of public compliance with recommended precautions. These cities also benefit from integrated green infrastructure, regulated use of VOCs, and efficient public transportation systems that reduce precursor emissions.

3.4.3. Health Impacts of Ground-Level O₃ Exposure

O₃ at ground level is not emitted directly but forms through reactions involving sunlight, NOₓ, and VOCs. When inhaled, O₃ acts as a strong irritant to the lungs and airways. Even short-term exposure can lead to coughing, throat discomfort, and reduced lung function. For sensitive groups including children, the elderly, and people with asthma or other chronic respiratory conditions O₃ exposure may result in aggravated symptoms and increased hospital admissions. In the cardiovascular system, O₃ has been linked to systemic inflammation, which may heighten the risk of heart-related complications such as arrhythmia or ischemic events. Seasonal spikes in O₃, particularly during summer, coincide with increased outpatient visits and respiratory complaints. Physical exertion outdoors during peak O₃ periods further elevates risk by increasing inhalation rates. Outdoor workers, street vendors, and athletes are especially vulnerable during high-O₃ days. Longer-term exposure can also compromise immune responses and elevate the risk of respiratory infections.

3.4.4. Strategies for Reducing O₃ Levels and Exposure Risks

Reducing ambient O₃ levels requires a focus on limiting both NOₓ and VOC emissions, the two primary ingredients in O₃ formation. Urban transportation plays a major role here. Key measures include the promotion of electric mobility, investment in metro and BRT systems, and implementation of odd–even vehicle rules during high pollution days. Cities may also adopt congestion charges or restrict access to central areas during peak hours to reduce vehicular load. From the industrial standpoint, technologies such as selective catalytic reduction (SCR), thermal oxidation systems, and vapor recovery units should be encouraged to treat emissions at the source. Regulations should also target domestic sources by promoting low-VOC consumer products. Public awareness on the use of low-emission paints, aerosols, and cleaning agents can be enhanced through labeling and education campaigns.

In urban design, strategies like increasing green canopy coverage, planting O₃-absorbing tree species, and maintaining airflow corridors can aid in natural pollutant dispersion. Reducing energy consumption in buildings especially during peak hours further contributes to lowering precursor emissions from power generation. Combining these strategies creates a more comprehensive framework to manage O₃ pollution and reduce its health burden across vulnerable city populations.

3.4.4.1. Analysis of APs of PM_2.5 and PM₁₀ Air Pollution Among Cities (Figure 5)

This study investigates the relationship between fine particulate matter (PM_2.5) and coarse particulate matter (PM₁₀) air pollution and various health outcomes in 15 cities in India, China, and Japan (see Figure 5). The AP represents the estimated percentage of deaths caused by a certain air pollutant.

3.4.5. Key Findings

The graph (A) Figure 5 suggests that the average AP% among the 15 cities is highest in Mumbai.

3.4.5.1. Air Pollution and Mortality Rates: A Comparison of Cities (Figures 5 and 6)

This study investigates the relationship between air pollution and mortality rates from various causes in 15 cities in India, China, and Japan (see Figures 5 and 6). The AP represents the estimated percentage of deaths caused by a certain air pollutant.

3.4.6. Key Findings

Mumbai, India, appears to have a high AP for NO₂-related mortality, emphasizing the importance of air quality improvement measures in the metropolis. In Shizuoka, Japan, notably low AP values for both NO₂ and O₃ suggest the presence of effective air pollution management practices and well-regulated urban environments. These outcomes may also reflect public compliance with air quality advisories, cleaner transportation systems, and reduced reliance on fossil fuels in both industrial and domestic sectors. By contrast, Xian, China, records a concerning AP related to O₃-induced respiratory disease mortality, indicating a possible combination of emission-intensive activities and geographical constraints that trap pollutants during specific seasons. These findings call for further focused investigation into the interaction between precursor emissions, topography, and meteorological conditions in the region. [1].

By emphasizing the spatial variability of health impacts, the research provides valuable evidence for implementing localized air quality management strategies. The insights into urban-specific health burdens will aid policymakers in tailoring interventions to the unique needs of each city. The study uniquely assesses how episodic events such as dust storms, agricultural burning, and seasonal biomass burning contribute to annual average PM concentrations. This focus provides a more accurate understanding of pollutant contributions in each city, especially in regions where such episodic events significantly impact air quality. Evaluating the contribution of episodic events to annual average pollutant levels is an original aspect of this work, as it integrates short-term, high-impact episodes into long-term exposure assessments, adding a critical layer to understanding the dynamics of air pollution. This comparative assessment presents valuable insights into how air pollution affects public health across urban centers in three major Asian countries. The study makes it evident that not all cities face the same level of health risk from air pollutants cities like Tokyo and Osaka demonstrate significantly lower AP values, offering practical models for more polluted urban centers in India and China. Their success in managing air quality through cleaner public transport, stricter emission controls, and better public health outreach could inform strategic improvements elsewhere.