Quantifying the contribution of ambient and indoor-generated fine particles to indoor air in residential environments Abstract Indoor fine particles (FPs) are a combination of ambient particles that have infiltrated indoors, and particles that have been generated indoors from activities such as cooking. The objective of this paper was to estimate the infiltration factor (F inf ) and the ambient/non-ambient components of indoor FPs. To do this, continuous measurements were collected indoors and outdoors for seven consecutive days in 50 non-smoking homes in Halifax, Nova Scotia in both summer and winter using DustTrak (TSI Inc) photometers. Additionally, indoor and outdoor gravimetric measurements were made for each 24-h period in each home, using Harvard impactors (HI). A computerized algorithm was developed to remove (censor) peaks due to indoor sources. The censored indoor/outdoor ratio was then used to estimate daily F infs and to determine the ambient and non-ambient components of total indoor concentrations. F inf estimates in Halifax (daily summer median = 0.80; daily winter median = 0.55) were higher than have been reported in other parts of Canada. In both winter and summer, the majority of FP was of ambient origin (daily winter median = 59%; daily summer median = 84%). Predictors of the non-ambient component included various cooking variables, combustion sources, relative humidity, and factors influencing ventilation. This work highlights the fact that regional factors can influence the contribution of ambient particles to indoor residential concentrations. M. MacNeill 1 , J. Kearney 1 , L. Wallace 2 , M. Gibson 3 , M. E. H eroux 1† , J. Kuchta 3 , J. R. Guernsey 4 , A. J. Wheeler 1‡ 1 Air Health Science Division, Health Canada, Ottawa, ON, Canada, 2 Consultant, Santa Rosa, CA, USA, 3 Department of Process Engineering and Applied Science, Dalhousie University, Halifax, NS, Canada, 4 Department of Community Health and Epidemiology, Dalhousie University, Halifax, NS, Canada Key words: Infiltration factor (F inf ); Fine particulate mat- ter; Indoor Air Quality; Ambient component; Non-ambi- ent component. Present addresses: † WHO European Centre for Environment and Health, Platz der Vereinten Nationen 1, 53113, Bonn, Germany. ‡ Centre for Ecosystem Management, School of Natural Sciences, Edith Cowan University, 270 Joondalup Drive, Joondalup, Perth, Western Australia, 6027, Australia. M. MacNeill Air Health Science Division Health Canada, 269 Laurier Ave West Ottawa ON K1A 0K9 Canada Tel.: +1 613 954 8580 Fax: +1 613 948 8482 e-mail: Morgan.MacNeill@hc-sc.gc.ca Received for review 30 August 2013. Accepted for publication 2 December 2013. Practical Implications Ambient and non-ambient particles have different risk management approaches, composition, and likely toxicity. Therefore, a better understanding of their contribution to the indoor environment is important to manage the health risks associated with fine particles (FPs) effectively. As well, a better understanding of the factors F inf can help improve exposure assessment and contribute to reduced exposure misclassification in epidemiologic studies. Introduction In North America, people spend approximately 90% of their time indoors (Klepeis et al., 2001; Leech et al., 2002). Here, they are exposed to outdoor-generated particles that have infiltrated indoors, and particles that have been generated indoors from activities such as cooking, cleaning, and other combustion sources. As a result, the majority of exposure to fine particulate matter (FP) of ambient origin often occurs indoors. Because ambient and non-ambient particles have different risk management approaches, composition, 1 Indoor Air 2014 © 2013 Her Majesty the Queen in Right of Canada Indoor Air Ó 2013 John Wiley & Sons Ltd Reproduced with the permission of the Minister of Health Canada. wileyonlinelibrary.com/journal/ina Printed in Singapore. All rights reserved INDOOR AIR doi:10.1111/ina.12084