In the late 21st century, PM2.5 is projected to increase in all regions, with a global mean of 0.28 mg/m3. Populated areas such as East Asia, Eastern USA, Northern India and Africa are some areas projected to face increased PM2.5.27 The source apportionment of PM2.5 pollution is mainly from residential areas (India),59 industrial sectors (India and China),60 buildings (Africa),61 transportation (Africa and the USA).62 The sources, chemical composition, formation, transformation and fate of PM2.5 are markedly different across regions due to the variations in emissions and meteorological conditions.59

In the included studies, PM2.5 was associated with increased CVDs, such as CHD, IHD and stroke. Previous studies showcased the long-term effect of PM2.5 for 12–14 hours, was strongly associated with atherosclerosis, the key to a central mechanism for IHD and stroke.63–65 Cardiovascular parameters such as heart rate variability are strongly associated with PM2.5 exposure.64 66 A population study demonstrated an association between air pollution and respiratory infection severity, causing irritation, cough, phlegm and bronchial hyper-responsiveness.67 PM2.5 is highly associated with increased emergency department (ED) visits for asthma64 68 69 and COPD68 and respiratory infection.70

Apart from PM2.5, O3 also is projected to increase in south China, North India, Northeast USA and Central Africa and decrease in other remote Oceanic regions. Various studies reported that climate change poses a significant O3-related health burden globally and regionally.27 31 71–73 Global and regional health impact assessments, including China, have reported adverse effects of climate change on BoD attributable to O3.31 72 74 75

Future O3-related health burdens in China indicate increased acute excess mortality across the five SSPs under RCP4.5 and RCP8.5, respectively.28 However, the increased premature mortality attributable to O3 exposure is smaller than the premature mortality attributable to PM2.5 exposure as O3 is not as harmful as PM2.5.27

NCDs are projected to increase with the increase in air pollutant concentration. Nevertheless, population size and population ageing increase the projected NCD mortality and morbidity. The population size is projected to increase by up to 3 million people by 2035, leading to population ageing and an increased risk of CHD.41 76 Emerging evidence suggests that near-roadway air pollution is linked to CHD mortality and morbidity.77–81 The proportion of the population living near major roads is also projected to increase, which could increase exposure to traffic-related air pollution and CHD risk.

The increased O3 concentrations under RCP8.5 accompanied by the ageing population will lead to increased acute excess mortality attributable to O3. It was projected that the population size would reduce in 2050, but the steep increase in the elderly population would increase O3-related acute excess mortality.28 Furthermore, RCP8.5 suggests more O3-related deaths in colder months (November to April) due to increased O3 concentrations.28 The population ageing under SSP1 and SSP5 will offset the reduced deaths due to the decreases in age group-specific mortality rates. This will subsequently decrease the acute excess mortality attributable to O3.28

Wang et al were the first in China to estimate the future BoD at the county level.30 Under the scenario of 100% air quality improvement, SSP4 and population growth, premature mortality attributable to PM2.5 was projected to increase in 2020 and 2030.30 Chinese environmental policies can achieve health benefits with an annual PM2.5 concentration of 35 µg/m3. As the population ages and grows, the overall susceptibility to PM2.5 will increase. Thus, IHD and stroke are expected to increase in 2020 and 2030. Apart from that, highly developed regions faced a higher PM2·5-associated BoD compared with less developed regions because of the higher pollution levels and population densities. Mass migration from rural areas to more developed urban areas on the eastern coast82 and the Chinese two-child policy might differentially increase population density in different regions.30

Climate change also affects air pollutant surface concentrations, such as PM2.5 and O3.83 The global mean PM2.5 will increase due to hydrogen peroxide (H2O2). Increased H2O2 is associated with the high moisture and OH concentration in the atmosphere, which are both climate change byproducts.27 H2O2 reacts with SO2 to form sulphate (SO2 + H2O2 −> H2SO4)82 83 the main component of PM2.5.84 Among all PM2.5 components, the largest increases are in sulfate, smaller dust particles and organic matter. The global mean PM2.5 is largely driven by increased water vapour, a reactant in the chemical reactions that form O3, increasing surface O3 concentrations.27 85

In Cyprus, eight different cases were simulated to determine how traffic changes by 2030, where all vehicles will adhere to Euro six standards, will affect NOx and PM2.5 pollution. Surprisingly, the premature total mortality and cardiovascular and respiratory risks would still increase across all scenarios despite these changes. The study also investigated different pollution reduction strategies, such as setting a 30 km/hours speed limit, but this worsened matters, releasing 9% more NOx and 3% more PM2.5.39

The US studies projected a linear reduction in premature mortality attributable to PM2.5 concentrations under the AT100% and EV100% scenarios.42 43 Both scenarios present significant implementation and policy challenges.82–86 The AT100% scenario potentially improves health greater than electrifying light-duty PV due to increased physical activity, annual monetised net benefit and modest benefit of PM2.5 reduction. However, the risk of road traffic injuries impedes this benefit.87

Active travel requires a significant financial investment to provide proper facilities for pedestrian, bicycle and transit infrastructure and changes in land use that equilibrate future demand for housing and job growth. Contrarily, EVs require technology and comprehensive policy implementation. Furthermore, EVs require proper development, deployment and financing that addresses battery charging, vehicle range and cost. A few factors stimulate EV implementation; for example, voluntary pledges by vehicle manufacturers to phase out sales of gasoline-fueled cars by 2050, rebates and tax incentives for EV purchases and the banning of selling gasoline-fueled cars by 2035.42 The transition to 100% demands good governmental regulation, commitment and profit to the industry and consumer interest.

Fann et al determined that people gained more years of life as PM2.5 levels decreased over 30 years.43 Lower PM2.5 exposure led to fewer premature deaths and an increased average lifespan. The mean population-weighted annual mean PM2.5 levels decreased from 15.4 mg/m3 in 1980 to 8.8 mg/m3 in 2010 across all counties. Thus, if PM2.5 levels remained at 1980 levels, people born in 2050 could live 1-year longer.43 However, local, state and federal air quality policies are expected to decrease PM2.5 concentrations further.88–94 The estimated PM2.5-related premature mortality in 2000 and 2010 were 140 000 and 120 000, respectively, which was consistent with previous studies.95–98

In a UK study, Williams et al 15 projected decreased all-cause mortality in 2050 due to reduced NO2 and PM2.5. The low-GHG scenario highlighted a greater reduction of mortality attributable to NO2 and PM2.5. Similarly, Panullo et al 6 reported reduced NO2 concentrations under all three RCPs (RCP2.6, RCP6.0, RCP8.0). NO2 concentrations are projected to decrease in 2050, decreasing RHA. A UK study also reported stronger effects on RHA for NO2 compared with PM2.5 and PM10.99

China is the largest GHG emitter globally and the most populated country with a higher risk of climate change impact.28 The PM-related health burden has been the major focus for climate projection research in China, which mainly highlighted reduced premature mortality30 40 and other NCDs such as IHD, stroke,38 40 lung cancer, COPD38 and LRI.30 Improved air quality under the unchanged population scenario projected a decrease in premature deaths attributable to PM2.5 by 2020 and 2030.30 This result was consistent with that of previous studies80 100–102 which estimated 23% reduced mortality under the scenario of 35 µg/m3 PM2.5 (WHO Interim Target-1).

The decrease in age group-specific mortality rates will subsequently decrease the acute excess mortality attributable to O3.28 O3-related mortality was estimated to decrease under RCP4.5. This decline is due to assumed large reductions in emissions of O3 precursors, such as NOx and non-methane VOC. This result was consistent with those for East Asia in previous global studies.74 103

In Nicosia, Republic of Cyprus, the mitigation scenario that involved banning diesel PV and LDV and allowing only vehicles that abide by Euro six standards reduced up to 70% of NOx compared with 2017. Furthermore, premature mortality attributable to PM2.5 was projected to be reduced by up to 30% in 2030 under Case 4 (banning of diesel PVs and LDVs, banning fireplaces, reducing traffic flow).39