Contribution of human-related sources to indoor volatile organic compounds in a university classroom Abstract Although significant progress has been made in understanding the sources and chemistry of indoor volatile organic compounds (VOCs) during the past decades, much is unknown about the role of humans in indoor air chemistry. In the spring of 2014, we conducted continuous measurements of VOCs using a proton transfer reaction mass spectrometer (PTR-MS) in a university classroom. Positive matrix factorization (PMF) of the measured VOCs revealed a ‘human influence’ component, which likely represented VOCs produced from human breath and ozonolysis of human skin lipids. The concentration of the human influence component increased with the number of occupants and decreased with ventilation rate in a similar way to CO 2 , with an average contribution of 40% to the measured daytime VOC concentration. In addition, the human skin lipid ozonolysis products were observed to correlate with CO 2 and anticorrelate with O 3 , suggesting that reactions on human surfaces may be important sources of indoor VOCs and sinks for indoor O 3 . Our study suggests that humans can substantially affect VOC composition and oxidative capacity in indoor environments. S. Liu 1,2 , R. Li 1,3,4 , R. J. Wild 1,4 , C. Warneke 1,4 , J. A. de Gouw 1,4 , S. S. Brown 2,4 , S. L. Miller 5 , J. C. Luongo 5 , J. L. Jimenez 1,2 , P. J. Ziemann 1,2 1 Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO, USA, 2 Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO, USA, 3 Department of Atmospheric and Oceanic Sciences, University of Colorado, Boulder, CO, USA, 4 Chemical Sciences Division, NOAA Earth System Research Laboratory, Boulder, CO, USA, 5 Department of Mechanical Engineering, University of Colorado, Boulder, CO, USA Key words: Indoor air quality; Volatile organic com- pounds; Human occupants; Skin lipid ozonolysis; Positive matrix factorization; Proton transfer reaction mass spec- trometer. J. L. Jimenez and P. J. Ziemann Cooperative Institute for Research in Environmental Sciences, University of Colorado, 1111 Engineering Drive, 427 UCB, Boulder, CO 80309-0427, USA Tel.: 303-492-3557, 303-492-9654 Fax: 303-492-1149 e-mails: jose.jimenez@colorado.edu, paul.ziemann@ colorado.edu Received for review 30 July 2015. Accepted for publication 18 November 2015. Practical Implications Indoor emissions and production of volatile organic compounds (VOCs) are of concern as many VOCs can have delete- rious effects on human health. Laboratory studies in simulated aircraft cabins and offices suggest that humans can play important roles in indoor air chemistry, mainly through ozone oxidation of human skin lipids. This study complements previous laboratory studies by performing high-resolution, real-time measurements of VOCs and other chemical species in a heavily occupied university classroom. The results indicate that humans contribute substantially to indoor VOCs through respiration and skin lipid reactions, supporting the important role of humans in affecting indoor air quality. Introduction Indoor air quality is closely related to human health as humans are exposed to indoor air for ~85% of their time (Gligorovski and Weschler, 2013). Volatile organic compounds (VOCs) are a major class of pollu- tants ubiquitous in indoor environments, where con- centrations are usually higher than outdoors (Brown, 2002; Brown et al., 1994). Studies have shown that many VOCs have adverse health effects and can con- 1 Indoor Air 2015 © 2015 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd wileyonlinelibrary.com/journal/ina Printed in Singapore. All rights reserved INDOOR AIR doi:10.1111/ina.12272