Research | Children’s Health Birth Weight following Pregnancy during the 2003 Southern California Wildfires David M. Holstius,1 Colleen E. Reid,1 Bill M. Jesdale,2 and Rachel Morello-Frosch 2,3 1School of Public Health, Environmental Health Sciences Division, 2Department of Environmental Science, Policy and Management, and 3School of Public Health, Community Health and Human Development, University of California, Berkeley, Berkeley, California, USA o Bjectiv term infants. Background : In late October 2003, a series of wildfires exposed urban populations in Southern California to elevated levels of air pollution over several weeks. Previous research suggests that short- term hospital admissions for respiratory outcomes increased specifically as a result of these fires. : We assessed the impact of a wildfire event during pregnancy on birth weight among Methods : Using records for singleton term births delivered to mothers residing in California’s South Coast Air Basin (SoCAB) during 2001–2005 (n = 886,034), we compared birth weights from pregnancies that took place entirely before or after the wildfire event (n = 747,590) with those where wildfires occurred during the first (n = 60,270), second (n = 39,435), or third (n = 38,739) trimester. The trimester-specific effects of wildfire exposure were estimated using a fixed-effects regression model with several maternal characteristics included as covariates. r esult : Compared with pregnancies before and after the wildfires, mean birth weight was esti- mated to be 7.0 g lower [95% confidence interval (CI): –11.8, –2.2] when the wildfire occurred during the third trimester, 9.7 g lower when it occurred during the second trimester (95% CI: –14.5, –4.8), and 3.3 g lower when it occurred during the first trimester (95% CI: –7.2, 0.6). c onclusion : Pregnancy during the 2003 Southern California wildfires was associated with slightly reduced average birth weight among infants exposed in utero. hTe extent and increasing fre - quency of wildfire events may have implications for infant health and development. k ey words : air pollution, birth weight, fetal growth retardation, fires, particulate matter, preg- nancy outcomes. Environ Health Perspect 120:1340–1345 (2012). http://dx.doi.org/10.1289/ ehp.1104515 [Online 29 May 2012] In late October 2003, a series of wildfires burned > 750,000 acres of forest in Southern California (Blackwell and Tuttle 2003). Strong Santa Ana winds carried the resulting plumes of smoke toward Los Angeles and Orange counties, where a large urban pop- ulation was exposed to elevated concentra- tions of air pollutants from the fires (Phuleria et al. 2005). An in-depth exposure assessment study estimated the population-weighted par- ticulate matter (PM10; PM with aerodynamic diameter ≤ 10 µm) and fine particulate mat- ter (PM2.5; PM with aerodynamic diameter ≤ 2.5 µm) concentrations, respectively, at 190 and 90 µg/m3 under heavy smoke con- ditions, and 125 and 75 µg/m3 under light smoke conditions, compared with baseline concentrations of 40 and 20 µg/m3 in the same region (Wu et al. 2006). Using that exposure assessment, a study (Delfino et al. 2009) of cardiorespiratory health effects esti- mated that elevated PM2.5 levels led to a 34% increase in hospital admissions for respiratory conditions 1–2 days later, with the largest associations observed among the very young (0–4 years, 8.3% per 10-µg/m3 increase in PM2.5) and very old (65–99 years, 10.1% per 10-µg/m3 increase in PM2.5); limited evidence supported a small increase in admissions for cardiovascular conditions as well (Delfino et al. 2009). A separate study found that parental recall of the smell of smoke during these fires was associated with increased medi- cation usage, eye and respiratory symptoms, 1340 and physician visits among their children (Künzli et al. 2006). Health effects of wildfires. Particulate matter (PM) is possibly the most important health-related component of wildfire events (Naeher et al. 2007). Wildfire-generated PM may be more toxic, on an equal-mass basis, than ambient PM collected in the same region during non-fire periods (Wegesser et al. 2009), potentially due to the role of atmospheric photochemistry resulting in the formation of secondary pollutants (Wegesser et al. 2010). Wildfires have been shown to enhance PM2.5 levels in many parts of the western United States (Jaffe et al. 2008), and recent studies have linked smoke exposure from wildfire events with spikes in morbidity in Canada (Henderson et al. 2011; Moore et al. 2006), Australia (Cameron et al. 2009; Chen et al. 2006; Johnston et al. 2002, 2007; Morgan et al. 2010; Tham et al. 2009), Southeast Asia (Emmanuel 2000; Mott et al. 2005; Sastry 2002), Finland (Hänninen et al. 2009) and California (Delfino et al. 2009; Künzli et al. 2006; Viswanathan et al. 2006); for a review, see Dennekamp and Abramson (2011). Air pollution may not be the sole mech- anism through which wildfire events affect health. For instance, wildfires threaten per- son and property; and news of an inherently unpredictable force of nature in itself may induce psychosocial stress in the population (Kumagai et al. 2004). Although the main effect of wildfire events on birth weight is likely mediated through their air pollution effects, distinguishing between these potential mechanisms is methodologically challenging. Objectives and study design. Our objec- tive was to estimate the birth weight effects associated with in utero exposure to a wildfire event. To our knowledge, to date one abstract has been published concerning the effects of wildfire smoke exposure on birth outcomes (Breton et al. 2011). Although little is known regarding the health effects of acute maternal exposures to smoke from wildfires, chronic maternal exposures to related hazards, includ- ing ambient particulate matter and indoor biomass smoke, have been linked to adverse birth outcomes, including lower birth weight. Many epidemiological studies have found associations between exposure to ambient PM and preterm birth or birth weight (for reviews, see Bosetti et al. 2010; Glinianaia et al. 2004). A recent meta-analysis of stud- ies examining chronic maternal exposures to indoor air pollution in developing coun- tries, such as that generated by cooking or heating with solid fuels, concluded that such exposures increase the risk of adverse preg- nancy outcomes, including percent low birth weight, stillbirth, and reduced mean birth weight (Pope et al. 2010). The study of natural experiments can be a methodologically useful way to advance research on air pollution and perinatal effects (Parker et al. 2008; Woodruff et al. 2009). Time-series studies can reduce threats to valid- ity posed by exposure misclassification and confounding by variables that are associated with both ambient air pollution and perinatal outcomes, such as social class (Parker et al. 2008); thus they are a useful complement to Address correspondence to R. Morello-Frosch, Department of Environmental Science Policy and Management and School of Public Health, 137 Mulford Hall, University of California, Berkeley, CA 94720-3114 USA. Telephone: (510) 643-6358. Fax: (510) 643-5438. E-mail: rmf@berkeley.edu Supplemental Material is available online (http:// dx.doi.org/10.1289/ehp.1104515). This research is supported in part by the California Air Resources Board, grant agreement 04-308 and by the U.S. Environmental Protection Agency Regional Applied Research Effort (RARE) grant (cooperative agreement X3-83338901-1) and by a cooperative agreement from the Centers for Disease Control and Prevention through the Association of Schools of Public Health grant CD300430. The authors declare they have no actual or potential competing financial interests. Received 20 September 2011; accepted 29 May 2012. ovlume 120 | number 9 | September 2012 • Environmental Health Perspectives