Ultrafine Particles: Exposure and Source Apportionment in 56 Danish Homes Gabriel Bekö ,* ,† Charles J. Weschler, †,‡ Aneta Wierzbicka, § Dorina Gabriela Karottki, ∥ Jørn Toftum, † Steffen Loft, ∥ and Geo Clausen † † International Centre for Indoor Environment and Energy, Dept. of Civil Engineering, Technical University of Denmark, Nils Koppels Alle ́ 402, 2800-Lyngby, Denmark ‡ Environmental and Occupational Health Sciences Institute, University of Medicine and Dentistry of New Jersey and Rutgers University, 170 Frelinghuysen Road, Piscataway, New Jersey 08854, United States § Division of Ergonomics and Aerosol Technology, Lund University, P.O. Box 118, SE-221 00 Lund, Sweden ∥ Department of Public Health, University of Copenhagen, O ̷ ster Farimagsgade 5, 1014 Copenhagen, Denmark * S Supporting Information ABSTRACT: Particle number (PN) concentrations (10−300 nm in size) were continuously measured over a period of ∼45 h in 56 residences of nonsmokers in Copenhagen, Denmark. The highest concentrations were measured when occupants were present and awake (geometric mean, GM: 22.3 × 10 3 cm −3 ), the lowest when the homes were vacant (GM: 6.1 × 10 3 cm −3 ) or the occupants were asleep (GM: 5.1 × 10 3 cm −3 ). Diary entries regarding occupancy and particle related activities were used to identify source events and apportion the daily integrated exposure among sources. Source events clearly resulted in increased PN concentrations and decreased average particle diameter. For a given event, elevated particle concentrations persisted for several hours after the emission of fresh particles ceased. The residential daily integrated PN exposure in the 56 homes ranged between 37 × 10 3 and 6.0 × 10 6 particles per cm 3 ·h/day (GM: 3.3 × 10 5 cm −3 ·h/day). On average, ∼90% of this exposure occurred outside of the period from midnight to 6 a.m. Source events, especially candle burning, cooking, toasting, and unknown activities, were responsible on average for ∼65% of the residential integrated exposure (51% without the unknown activities). Candle burning occurred in half of the homes where, on average, it was responsible for almost 60% of the integrated exposure. ■ INTRODUCTION A review panel assembled by the U.S. Health Effects Institute recently concluded that experimental and epidemiologic studies provide suggestive, but not consistent, evidence of adverse effects of short-term exposures to ambient ultrafine particles (UFP; particles smaller than 100 nm), while information on long-term exposure is not yet available. 1 The literature indicating that ultrafine particles may adversely affect human health includes reviews by Delfino et al., 2 Weichenthal et al., 3 Mills et al., 4 and Rü ckerl et al. 5 Compared with larger particles, UFP have higher deposition rates in the lower respiratory tract. 6,7 Ultrafine particles enter the indoor environment from outdoors. 8,9 However, given their larger diffusivities, 6 a smaller fraction of UFP penetrate buildings than do 0.1−0.5 μm diameter particles. 10,11 UFP also originate within the indoor environment. Major indoor sources include cooking, tobacco smoking, candle and incense burning, and the use of gas and electric appliances. 12−17 Chemical reactions such as those between ozone and terpenes can also generate UFP in- doors. 18,19 Bhangar et al., 20 Mullen et al., 21,22 and Wallace and Ott 23 present a useful metric to describe indoor exposure to UFP. They report the daily integrated particle number (PN) exposure in units of particles per cm 3 ·h/day. The daily integrated exposure is a normalized form of integrated exposure. Since people spend significant time in their homes, where frequent indoor particle source events result in higher PN concentrations, a large fraction of the total ultrafine particle exposure occurs in the home. 24 Wallace and Ott 23 estimated that for a typical suburban lifetime in the U.S., 47% of the average daily UFP exposure was due to indoor sources, 36% to outdoor sources, and 17% to in-vehicle exposure. Mullen et al. 22 concluded that the daily integrated exposure to PN Received: June 1, 2013 Revised: July 28, 2013 Accepted: August 19, 2013 Published: August 19, 2013 Article pubs.acs.org/est © 2013 American Chemical Society 10240 dx.doi.org/10.1021/es402429h | Environ. Sci. Technol. 2013, 47, 10240−10248