On the worst day of air quality in Nairobi, particulate concentrations reached 151 micrograms per cubic metre on the highest hour. By standard measures, this level would be considered very unhealthy where people are advised to limit outdoor activity. The United States Environmental Protection Agency (EPA) defines the limit for good quality air as 12 micrograms per cubic metre over 24 hours.

Air quality has become a health concern globally as air pollution from industrial activity and environmental degradation have become more common. It is estimated that long-term exposure to ambient fine particle air pollution (PM2·5) caused 4.2 million deaths in 2015, representing 7·6% of total global mortality and making it the fifth-ranked global risk factor in that year.

The Berkeley Earth air quality page states that PM2.5 (particulate matter of less than 2.5 microns in diameter) is the most damaging form of air pollution likely to be present under typical conditions, contributing to heart disease, stroke, lung cancer, respiratory infections, and other diseases. Experts say PM2.5 can evade our bodies’ defenses, penetrating deep into the lungs and even entering the bloodstream.

Data for other Kenyan cities shows that the highest recorded micrograms per cubic metre were as follows; Thika (about 137), Nakuru (101), Kisumu (94) and Eldoret (about 87). Although the records were for the worst days of air quality, the data shows that most days of the year recorded readings below 50 but above the good quality limit set by EPA.

While city-level data enables city level comparison, the air pollution actually varies within a city, from one neighbourhood to another and from block to block hence the need for careful selection. Major contributors include industrial processes, burning material, motor vehicle emissions and coal emissions.

Data shows that air pollution in New Delhi, India reached 900 micrograms per cubic metre this in 2019 breaching the defined hazardous limit by nearly double.