A new report says the Macroom bypass has led to a significant improvement in air quality in the town.
Levels of Nitrogen Dioxide were measured at seven locations before and after the bypass was opened.
Air pollution fell by a combined average of 53.8% in the town.
The World Health Organisation says Nitrogen Dioxide is a leading cause of heart attacks and stroke.
Around 1,300 premature deaths are caused by poor air quality in Ireland every year.
