Table 1.

Summary of gaps and future directions in air pollution research and mitigation.

Gaps/limitations identified	Current status	Suggestions/future directions
Air pollution monitoring
- PM2.5 mass concentration determines standards. - Central fixed monitor-based air pollution estimates.	Current standards are based on only mass concentration, not particle characteristics. Air pollution exposure of individuals are calculated/modeled based on fixed location estimates assuming the population within a particular radius is exposed to the calculated concentrations; central monitors are limited and primarily located in urban areas. Air monitoring coverage in LMICs are needed, but very limited.	Monitoring, policy analysis, and regulation needed by particle composition. Source-sector specific health effect estimates are needed, as well as submicron, PM1, mass monitoring. More extensive personal monitoring using low-cost sensors and methods, use of satellite data in combination with sensors for better personal estimates, better air pollution exposure models that integrate data from various sources, improved software and prediction capabilities. Open source databases of air pollution data, including data on LMICs, affordable technology, and collaborations between scientific resource rich and resource deficient countries.
Exposure assessment uncertainties
Examples of interest:  - Exposure during visits to polluted cities.  - Exposures in underground transit systems.	Disease risks are often based on exposures estimated from central monitors, not incorporating variations in microenvironments and personal exposure variations. Despite over a billion individuals traveling to international destinations annually, where air pollution levels vary from their home cities, only limited research has explored the health implications of exposure to such varying level and compositions. Although limited, emerging studies have found very high levels of PM in underground systems, particularly high levels of metals; and few studies have explored the health effects/risks.	Personal monitoring with low-cost devices; factoring in study participant mobility; consider the total exposures (Exposome), and apply data for precision medicine to identify and mitigate health outcomes. Further studies and understanding of the physiological changes associated with exposure to varying air pollution during travel, studies on travel health - future travel and migration is expected to increase; early warnings for vulnerable populations. Studies on the health risks commuters and workers face due to being exposed to high levels of subway air pollution, which may be high in iron and black carbon, are warranted.
Regulatory standards and policies
	Stringent standards and policies in HICs; resources for research and policy implementation have yielded significant improvement in HICs. In-contrast – although exposed to high levels, data, research and standard implementation is very limited in LMICs; increased risk of air pollution related cardiovascular mortality. Current air quality indices (AQIs) are based on considering PM in all regions as equally toxic, despite research to the contrary.	More interest and resource sharing with LMICs needed, as well with underrepresented and socioeconomically disadvantaged communities in HICs; local solutions needed for local problems and increase education and awareness. Considering PM composition and source when setting of locally appropriate AQIs could maximize the health benefits of pollution mitigation measures.

HICs, High-Income Countries; LMICs, Low- and Middle-Income Countries; PM, particulate matter; AQIs, Air Quality Indices.