Effects of contaminated supply air filters on symptoms from the eyes and nose - a double-blind experimental study

In buildings with mechanical ventilation, particles accumulate in the supply air filters. We conducted a field experiment in a school to investigate if such pollutants could affect the health of the pupils. In a school building we changed old and new supply air filters in the air handling units, with a cross over design of the study. Pupils answered a symptom questionnaire, and a subset of pupils was also examined by objective clinical methods. When there was an old supply air filter in the air handling unit, the pupils reported more symptoms from the eyes and throat, and the volume of the nose was smaller, compared to when the filter was new. Substantial microbial growth was found in the filters already after two months usage. Thus, supply air filters should be changed more often than is usually done.