Ambient air pollution is a major global health concern, causing substantial premature mortality, particularly in low- and middle-income countries (LMICs). A recent study estimated that particulate matter air pollution from fossil fuel combustion was responsible for approximately 10.2 million premature deaths in 2012. In Africa, rapid urbanization and industrialization contribute to increasing air pollution levels, resulting in substantial health impacts. Sub-Saharan Africa, in particular, faces challenges from household biomass fuel burning, windblown dust, and agricultural emissions, in addition to rising vehicle traffic. Monitoring data are limited in the region, and epidemiological evidence on health risks remains sparse. The Eastern Africa GEOHealth Hub was established to address this gap by installing BAM-1022 monitors in four capital cities, including Kampala, Uganda. This study focuses on Kampala, using four years of continuous PM_2.5 monitoring data to estimate the associated mortality burden and provide evidence-based estimates of air pollution attributable mortality, focusing on ambient fine particulate matter (PM_2.5). This research aims to provide policy-relevant evidence to advance air pollution control in Uganda, a region where health systems are challenged and exposure to air pollution is not routinely monitored. The specific objectives are to describe the trends of air pollution, to characterize its health impact and to advance its control.

The introduction extensively cites literature establishing air pollution's link to non-communicable diseases and premature death globally, emphasizing the disproportionate impact on LMICs. Studies highlighting the mortality burden attributed to particulate matter air pollution are referenced. Existing literature on air pollution in Africa, particularly Sub-Saharan Africa, is reviewed, noting the limitations in monitoring data and epidemiological evidence. Previous work from the Eastern Africa GEOHealth Hub is mentioned, focusing on the installation of BAM-1022 monitors in four East African capital cities. A previous report on Addis Ababa, utilizing a similar methodology, is also cited, providing context for the current study.

This study employed a time-series design, prospectively collecting data on daily ambient PM_2.5 concentrations using a BAM-1022 reference instrument located on the roof of the Makerere University School of Public Health in Kampala. Data were collected from January 1, 2018, to December 31, 2021. The BAM-1022 provided hourly measurements, which were aggregated to daily averages. Data quality control measures included regular instrument maintenance, background PM_2.5 concentration calibration, and addressing error messages. Missing data, mainly due to power outages and maintenance, were imputed using a 'before-after-mean' method. All-cause mortality data were obtained from the Uganda Bureau of Statistics (UBOS) and the District Health Information Software 2 (DHIS2) from the Ministry of Health. Accidental deaths were excluded from the analysis. The World Health Organization's AirQ+ tool was used to estimate the number of deaths attributable to PM_2.5 pollution, using various WHO interim targets and guidelines for PM_2.5 concentrations as reference values. Descriptive analyses were performed using R statistical software. The study utilized data from various sources, including Kampala's geographical area, population density, GPS coordinates, and the proportion of the population above 30 years.

The four-year average annual PM_2.5 concentration was 39 µg/m³, significantly higher than the WHO annual guideline value of 5 µg/m³. Seasonal variations were observed, with lower concentrations during the rainy seasons (March–June and October–December). Daily variations showed higher concentrations in the mornings (9:00 am) and evenings (9:00 pm), likely due to increased vehicular emissions and weather patterns. Saturday consistently showed the highest daily average PM_2.5 concentration (41.2 µg/m³). The AirQ+ analysis, using the WHO annual guideline of 5 µg/m³ as the reference, estimated that 7257 deaths in Kampala (2018-2021) were attributable to long-term PM_2.5 exposure. Specifically, for the years 2020 and 2021, 17.9% and 19.8% of non-accidental deaths were attributed to this exposure, respectively. The study also provided detailed data on daily and hourly PM_2.5 concentrations showing distinct patterns over the four-year period. Table 1 presents population data, mortality data, and PM2.5 concentrations. Table 2 shows the day-of-week variations in PM2.5 concentrations. Table 3 shows the attributable death rates based on WHO guidelines and interim targets. The figures illustrate daily and seasonal trends in PM_2.5 concentration.

The study's findings reveal significantly elevated PM_2.5 levels in Kampala, exceeding WHO guidelines by a considerable margin and resulting in a substantial mortality burden. Seasonal variations are consistent with other studies, highlighting the role of dry-season factors like dust from unpaved roads and biomass burning. Daily peaks in PM_2.5 concentrations correspond to peak traffic hours, emphasizing the contribution of vehicular emissions. The high mortality burden attributable to PM_2.5 underscores the urgent need for interventions. The study suggests various potential contributors to high pollution levels, including industrial activity at night, household air pollution from indoor cooking practices, and the limited enforcement of regulations. The limitations of using only outdoor PM_2.5 data for the attributable mortality calculation are acknowledged. The study advocates for a multi-pronged approach to air pollution control, involving vehicle emission standards, improvements in public transportation, promoting clean energy for cooking, and encouraging respiratory protection among vulnerable populations.

This study provides the first evidence-based estimates of air pollution-attributable mortality in Kampala, demonstrating a significant public health burden associated with elevated PM_2.5 levels. The consistently high PM_2.5 concentrations, exceeding WHO guidelines by a factor of eight, and the substantial attributable mortality, emphasize the urgent need for comprehensive air quality control measures. Future research should investigate the combined effects of indoor and outdoor air pollution, explore specific population vulnerability, and evaluate the effectiveness of various interventions to reduce PM_2.5 exposure and improve public health outcomes.

The study acknowledges data loss due to technical malfunctions (primarily power outages and pump failures during the COVID-19 lockdown) resulting in a 73.2% data completeness rate. The imputation method used to address missing data might introduce some uncertainty. The study focuses solely on outdoor PM_2.5, not accounting for the contribution of indoor air pollution to the overall health burden. The reliance on a single monitoring site might not fully capture the spatial variability of air pollution across Kampala.