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Background: Continuous ambient air quality monitoring in Kenya has been limited, resulting in a sparse data base on the health impacts of air pollution for the country. We have operated a centrally located monitor in Nairobi for measuring fine particulate matter (PM 2.5 ), the pollutant that has demonstrated impact on health. Here, we describe the temporal levels and trends in PM 2.5 data for Nairobi and evaluate associated health implications., Methods: We used a centrally located reference sensor, the beta attenuation monitor (BAM-1022), to measure hourly PM 2.5 concentrations over a 3-year period (21 August 2019 to 20 August 2022). We used, at minimum, 75% of the daily hourly concentration to represent the 24-hour concentrations for a given calendar day. To estimate the deaths attributable to air pollution, we used the World Health Organization (WHO) AirQ+ tool with input as PM 2.5 concentration data, local mortality statistics, and population sizes., Results: The daily (24-hour) mean (±SEM) PM 2.5 concentration was 19. 2 ± 0.6 (µg/m 3 ). Pollutant levels were lowest at 03:00 and, peaked at 20:00. Sundays had the lowest daily concentrations, which increased on Mondays and remained high through Saturdays. By season, the pollutant concentrations were lowest in April and highest in August. The mean annual concentration was 18.4 ± 7.1 (µg/m 3 ), which was estimated to lead to between 400 and 1,400 premature deaths of the city's population in 2021 hence contributing 5%-8% of the 17,432 adult deaths excluding accidents when referenced to WHO recommended 2021 air quality guideline for annual thresholds of 5 µg/m 3 ., Conclusion: Fine particulate matter air pollution in Nairobi showed daily, day-of-week, and seasonal fluctuations consistent with the anthropogenic source mix, particularly from motor vehicles. The long-term population exposure to PM 2.5 was 3.7 times higher than the WHO annual guideline of 5 µg/m 3 and estimated to lead to a substantial burden of attributable deaths. An updated regulation targeting measures to reduce vehicular emissions is recommended., Competing Interests: The authors declare that they have no conflicts of interest with regard to the content of this report., (Copyright © 2024 The Authors. Published by Wolters Kluwer Health, Inc. on behalf of The Environmental Epidemiology. All rights reserved.)