Air pollution is one of the biggest threats to human health, economy, and the world's ecological balance. In the Kenyan capital of Nairobi, studies have shown that air quality is consistently poor, with average annual PM2.5 levels—a measure of air pollution—more than double the World Health Organization's (WHO) guideline levels. Furthermore, the ever-increasing population and the intensification of industries, deforestation, construction work, and vehicular traffic, has caused the air quality to worsen over the years.