Atmospheric Environment 40 (2006) 1299–1313 Determinants of indoor air concentrations of PM 2.5 , black smoke and NO 2 in six European cities (EXPOLIS study) H.K. Lai a,Ã , L. Bayer-Oglesby b , R. Colvile a , T. Go¨tschi c , M.J. Jantunen d , N. Ku¨nzli b,c , E. Kulinskaya e , C. Schweizer b,c , M.J. Nieuwenhuijsen a a Department of Environmental Science and Technology, Imperial College, London, SW7 2AZ, UK b Institute of Social and Preventive Medicine, University of Basel, CH-4051 Basel, Switzerland c Department of Preventive Medicine, University of Southern California, Los Angeles, CA90033, USA d KTL, Department of Environmental Hygiene, P.O. Box 95, FIN-70701 Kuopio, Finland e Statistical Advisory Service, Sir Alexander Fleming building, Imperial College, London, SW7 2AZ, UK Received 13 September 2004; received in revised form 5 October 2005; accepted 14 October 2005 Abstract EXPOLIS was a large-scale population-based study of urban adult exposures to multiple pollutants, and was conducted between 1996 and 2000 in six European cities. Measurements made using standardised protocols in Athens (Greece), Basel (Switzerland), Helsinki (Finland), Milan (Italy), Oxford (UK), and Prague (Czech Republic), allow similarities and differences between contrasting European regions, climates and populations to be identified. Two consecutive days of home indoor and home outdoor measurements of fine particulate matter (PM 2.5 ), black smoke (BS), and nitrogen dioxide (NO 2 ) were carried out at the homes of adult participants on different dates and seasons during the sampling period. Regression models with interactions searched by all-possible subset method were used to assess the city effects and the determinants of home indoor PM 2.5 (adj R 2 ¼ 0:60, n ¼ 413), BS (adj R 2 ¼ 0:79, n ¼ 382) and NO 2 (adj R 2 ¼ 0:67, n ¼ 302) levels. Both bi-directional (positive and negative signs of associations) and unidirectional (consistently either positive or negative sign of associations) city effects on different determinants in each indoor model were shown. Smoking, gas-stove usage, outdoor temperature, and wind speed were the common determinants in all three indoor models. Other determinants, including the presence of wooden material, heating, and being located in suburb area, were also identified. They were likely linked to cultural and socio-economic factors. r 2005 Elsevier Ltd. All rights reserved. Keywords: Black smoke; Exposure assessment; Indoor air pollution; Nitrogen dioxide; Particulate matter 1. Introduction Air quality management in European cities is dominated by efforts to control outdoor sources of particulate matter (PM) and oxides of nitrogen. PM is the pollutant usually associated with the greatest effects on human health (WHO, 2000). While European air quality limit values are currently expressed in terms of PM 10 (mass concentration of particles smaller than 10 mm), the finer particulate matter (PM 2.5 ) size fraction is now receiving more attention (CAFE ´ , 2004). With respect to local ARTICLE IN PRESS www.elsevier.com/locate/atmosenv 1352-2310/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2005.10.030 Ã Corresponding author. Fax: +44 20 75949266. E-mail addresses: hklai@graduate.hku.hk, r.colvile@imperial. ac.uk (H.K. Lai).