THE EU funded OFFICAIR project finds seasonal variation in indoor air quality in modern office buildings with mechanical ventilation (higher in summer than in winter). The seasonal difference is important as it indicates that measurements taken at one time may not indicate the on-going state of air quality. Median levels of PM2.5 did not exceed the WHO 24-hour mean guidelines of 25 μg/m3, but the maximum winter concentration found (32 μg/m3) did. The annual mean guideline of 10 μg/m3 was also exceeded.