A study published in the journal Nature on July 21, 2025 has highlighted the compound risks of heat and air pollution in cities across sub-Saharan Africa.
Focusing on Kigali, the capital of Rwanda, researchers examined how extreme heat events interact with urban air quality challenges in one of the fastest-growing cities in the region. As a fast urbanising city, Kigali faces increasing threats from rising temperatures, vehicle emissions, biomass burning and limited environmental monitoring capacity.
Heatwaves often involve high-pressure systems that create stagnant air conditions, trapping pollutants near the surface and intensifying exposure risks.
The study, conducted between May 2021 and December 2024, analysed concentrations of fine particulate matter (PM2.5), nitrogen dioxide (NO₂) and ozone (O₃) across 12 air quality monitoring sites in Kigali.
These sites are operated by the Human Environment, Location, Transport and Health Research Lab and the Rwanda Environmental Management Authority. Meteorological data, including daily minimum, mean and maximum temperatures, as well as relative humidity, were sourced from the Rwanda Meteorological Agency.
Real-time data were collected using low-cost, multi-pollutant Real-time Affordable Multi-Pollutant monitors installed across Kigali City.
A heatwave was defined by the researchers as three or more consecutive days where daily maximum temperatures exceed the average by at least 5 degrees Celisus (°C). From 2021 to 2024, the average daily maximum temperature in Kigali was 25.28°C. Researchers identified six heatwave events, with peak temperatures ranging from 32.3°C to 33.5°C.
Findings show that annual averages for nitrogen dioxide were higher in the dry season, while PM2.5 and ozone levels were greater in the wet season. Diurnal analysis revealed that PM2.5 peaked during morning rush hours (6 am-9 am) and again in the evening (5 pm-11 pm), particularly during the wet season. Ozone levels typically peaked between 1 pm and 4 pm in both dry and wet seasons due to photochemical activity. NO₂ concentrations remained consistently higher during the dry season.
Among the six heatwave events, the highest temperature was recorded in March 2022, although this did not coincide with the highest pollution levels. The longest heatwaves lasting five days occurred in January 2022 and June 2023. PM2.5 and ozone concentrations spiked during these longer heatwaves, but NO₂ levels remained relatively stable regardless of intensity or duration.
The study also found that PM2.5 concentrations in Kigali frequently exceeded the World Health Organization’s annual air quality guidelines, even outside heatwave periods. Levels occasionally rose to more than eight times the recommended threshold due to a combination of urbanisation, dust, ageing diesel vehicles and agricultural burning.
“These results highlight the compound risks of heat and air pollution in sub-Saharan African cities,” the paper stated, “underscoring the importance of early-warning systems and robust urban policies that account for both heat and air pollution.”
The combination of extreme heat and poor air quality can exacerbate respiratory and cardiovascular conditions and heighten health risks for vulnerable populations, the authors warned.