The authors' full names, academic degrees, and affiliations are listed in the Appendix. Address reprint requests to Dr. Kan at P.O. Box 249, 130 Dong-An Road, Shanghai 200032, China, or at kanh@ fudan . edu . cn.Drs. Liu and R. Chen and Drs. Gasparrini and Kan contributed equally to this article.

Indoor air quality (IAQ) parameters in 73 primary classrooms in Porto were examined for the purpose of assessing levels of volatile organic compounds (VOCs), aldehydes, particulate matter, ventilation rates and bioaerosols within and between schools, and potential sources. Levels of VOCs, aldehydes, PM 2.5 , PM 10 , bacteria and fungi, carbon dioxide (CO 2 ), carbon monoxide, temperature and relative humidity were measured indoors and outdoors and a walkthrough survey was performed concurrently. Ventilation rates were derived from CO 2 and occupancy data. Concentrations of CO 2 exceeding 1000 ppm were often encountered, indicating poor ventilation. Most VOCs had low concentrations (median of individual species <5 lg/m 3 ) and were below the respective WHO guidelines. Concentrations of particulate matter and culturable bacteria were frequently higher than guidelines/reference values. The variability of VOCs, aldehydes, bioaerosol concentrations, and CO 2 levels between schools exceeded the variability within schools. These findings indicate that IAQ problems may persist in classrooms where pollutant sources exist and classrooms are poorly ventilated; source control strategies (related to building location, occupant behavior, maintenance/cleaning activities) are deemed to be the most reliable for the prevention of adverse health consequences in children in schools. Practical ImplicationsThis study provides quantitative assessment for a large set of indoor air quality (IAQ) parameters in public primary classrooms together with information regarding potential building-wide contamination sources. Elevated levels of CO 2 , PM 2.5 , PM 10 and bacteria may be associated with outdoor pollution sources, occupant behavior, maintenance/ cleaning activities and poor ventilation. IAQ investigations often include air sampling, which must be carefully conducted if representative data are to be collected. To better understand sampling results, investigators need to account for the variability of contaminants both within and between schools. These findings are of relevance to public health due to the very large population of exposed school children, especially since attendance in primary schools is compulsory and asthma and allergy are very common diseases in childhood. These data may be useful for assessing health effects of exposure, for understanding the underlying mechanisms and for implementing preventive policies in terms of standards and guidelines.