Research found that pupils who had breathed in dirtier air had a lower mark than those who breathed clean air.
The study of 2,400 students at the London School of Economics adds to a number of papers that suggest air pollution could make people less productive, The Sun reported.
London School of Economics’ Sefi Roth warned that pupils and parents should both think about air quality on and before exam days.
She said: “They can limit their outdoor activity on polluted days or take less polluted routes when they go to school.”
The study analyzed a particulate known as PM10, and when levels of it inside exam halls breached World Health Organization limits students scored more than two percent lower than those below the limit.
The WHO limit is 50 micrograms per cubic metre, but some exam halls were found to have 75 micrograms, enough to reduce scores by 3.4 per cent.