Exploring inequalities in experiencing the double burden of thermal discomfort and ultrafine particle exposure across urban communities

The combined impact of the urban heat island (UHI) and the urban pollutant island can increase the risk of respiratory and cardiovascular illness, in addition to heat stress, thereby elevating morbidity and mortality by intensifying vulnerability, particularly among population groups already facing social disadvantage.

This study develops a method to investigate co-exposure to summer heat and ultrafine particles (UFP, < 0.1 um) in Toronto using high-resolution datasets for temperature and air quality derived from spatially extensive monitoring campaigns conducted in 2022 and 2023. The temperature data is used to derive three indicators of thermal discomfort (Apparent Temperature, Discomfort Index, and a composite Hotspot Index), which are subsequently used to generate exposure surfaces using a Machine-Learning based land-use regression model. Similarly, the UFP data is used to generate a model and an exposure surface.  

Heat and UFP exposure surfaces are combined and analyzed using a multivariate Local Indicators of Spatial Association (LISA) approach, to identify clusters where both burdens are simultaneously elevated. These co-exposure hotspots were then linked with four dimensions of the Ontario Marginalization Index to evaluate disparities across socioeconomic and ethnocultural population groups.

Results show that combined heat–pollution hotspots are concentrated in Toronto’s central and southern neighbourhoods, particularly along major traffic corridors, and exposure levels rise consistently from the least to the most marginalized areas. In the most marginalized areas, up to nearly 90% of residents live in High–High co-exposure zones. These findings show that areas with higher levels of social marginalization consistently have higher combined heat and air pollution exposure, meaning that residents of these neighbourhoods are more likely to experience multiple environmental stresses at the same time. The results provide actionable evidence to support climate and air-quality policy aimed at reducing environmental health inequities.