The global demand to improve the energy performance of buildings has led to greater air tightness and uncertainty in the ability of natural ventilation to maintain adequate indoor environmental quality. A monitoring campaign was carried out to evaluate the long-term indoor environmental quality across a year-long period in energy-efficient Irish dwellings. During the winter months (December, January, February), the mean PM2.5 concentration in kitchens was 19 µg/m³, which is 38% higher compared to the summer months (June, July, August), where the mean concentration was 13 µg/m³. The mean PM2.5 increased 58% in winter (19 µg/m³) than in summer (12 µg/m³). In bedrooms, the mean CO2 concentration during winter was 1116 ppm, reflecting a 19% increase over the summer concentration of 919 ppm. In the living room, CO2 increased 29% in winter from summer. Additionally, radon levels in the dwellings were 52% higher in winter than in summer; however, they remained well below the Irish EPA threshold of 200 Bq/m³. VOC mean values in winter and summer showed similar ranges across the monitored dwellings, referring to the greater impact of occupancy activities than ventilation.
