Trans-boundary air pollution is a major effect on the seasonality of air pollution in Sri Lanka. Pollutants from India and China get transported to Sri Lanka during the north-east monsoon. The observed the highest levels of pollution were November-January period and the air pollution levels are lowest during the south-west monsoon since the winds come from the sea where there is no landmass to the south until Antarctica. The air pollution levels from March to October are relatively low in Sri Lanka and the air pollution levels show a noticeable seasonality.

Pollution produced in India due to power plants, industries and motor vehicles travel thousands of kilometres and they reach Sri Lanka where they contribute to increased pollution levels. This phenomenon of trans-boundary air pollution has been made worse this time with the drifting of the affected New Delhi where the air quality index has exceeded 400μg/m³ on average, a situation considered as hazardous. Delhi lies in Northern India which represents high seasonal variation. Looking at the pattern for each month, the seasonality is very apparent; there is much more particulate matter in the air centered November to February period.