Spira-Cohen et al. diesel fraction (as indicated by a metric of EC) will be stronger and more consistent than with PM2.5 total mass. We tested this hypoth- esis more definitively than past studies have done by using daily personal PM2.5 measure- ments collected in a group of inner-city chil- dren with asthma. The panel study design we employed in this research allows for each subject to act as his or her own control, whereby low personal exposure days are compared with high-pol- lution days in the same subject. Therefore, factors that vary among individuals remain consistent over time, and the health effects of daily pollutant levels can be quantified and compared. Materials and Methods Our panel of study subjects included 40 fifth- grade elementary school children (10–12 years of age) who were either doctor-diagnosed with asthma or had experienced asthma attacks while at school for which the child visited the school’s nurse. Subjects were referred to the study by the school’s nurse as suffering from asthma. On enrollment, baseline asthma questionnaires adapted from the ISAAC (International Study of Asthma and Allergies in Childhood) (Asher et al. 1995) were com- pleted by the child and his or her parent or guardian to determine the asthma history of each subject and to confirm the child’s asthma status. All questionnaires were available in Spanish, so all Hispanic parents who were not comfortable in English were given a Spanish version. These questionnaires were adminis- tered by a bilingual staff person from NYU. No participants came from homes where par- ents were not proficient in either English or Spanish. Participants were volunteers, and parents/guardians gave written informed con- sent for their child. The study protocol was approved by the NYU Institutional Review Board and meets all U.S. requirements for human subjects research. During approximately a 1-month sam- pling period at each school, personal and outdoor school-site monitoring data were col- lected at four elementary schools: two schools adjacent to and two schools farther away from a highway. The school with the highest traffic impact was located 173 feet from the Major Deegan Expressway [annual average daily traffic (AADT), 112,051 vehicles/day], and the school with the lowest traffic impact was located 2,419 feet from the Bruckner Expressway (AADT, 100,230 vehicles/day) (New York State Department of Transportation 2010). Local street traffic was minimal at the two schools farther away from highways; how- ever, all schools sampled experienced some amount of local traffic from the various truck routes in the area (e.g., leading to and from the Hunts Point produce markets). 560 Daily health outcomes and personal PM2.5 exposures were collected simultaneously for 10 subjects from each of the four South Bronx schools. One school was sampled during each sampling session: spring 2002, spring 2004, fall 2004, and spring 2005, for a total of 4 schools sampled. Sampling took place during the spring or fall seasons to minimize effects of summer haze air pollutants such as ozone (O3) and acidic aerosols, and because the schools were in session at those times. Personal expo- sures were collected using a rolling backpack with air pollution monitoring equipment attached to the upper handle as close as pos- sible to the breathing zone. The subjects took the backpack along with them 24 hr/day dur- ing the sampling period. Daily time–activity diary data were recorded every 15 min, includ- ing nearby cooking and smoking. Any missing diary data from the previous day were filled in retrospectively with NYU staff at the school. In the case of school absences, NYU staff vis- ited children in their homes to change filters and check diary entries whenever possible. The diligent follow-up by NYU staff led to a data collection rate of close to 100% for weekday diary data. Data from motion sensors and the activity diaries were reviewed daily to confirm that the child took the backpack along during all recorded activities. The subjects’ backpack samplers collected 24-hr integrated weekday samples of PM2.5 and the EC portion. Simultaneously collected school-site continuous PM2.5 (TEOM 1400a; Thermo Electron Corp., East Greenbush, NY, USA) and BC (aethalometer model AE-21; Magee Scientific, Berkeley, CA, USA) were averaged for 0900 hours to 0900 hours week- days to match the personal measurements. Twice daily (0700 hours to 1400 hours, 1400 hours to 0700 hours) weekday PM2.5 filter samples (ACCU sampler; R&P, Thermo Electron Corp.) were collected and subjected to X-ray fluorescence analysis for sulfur deter- mination. Details on sampling instruments and methods have been published previously (see Spira-Cohen et al. 2010, online supple- ment). Hourly concentrations of nitrogen oxides (Columbia Scientific, Austin, TX, USA), O3 (Thermoenviron, Hopkinton, MA, USA), and sulfur dioxide (SO2; Monitor Labs, Englewood, CO, USA) were also monitored at the schools. Background measurements of PM2.5 from a New York State Department of Environmental Conservation rooftop monitor on one of the four schools (PS154) were also available for comparison. Subjects recorded respiratory symptoms three times daily by rating their symptoms on a scale of 0 to 5, with 0 indicating no symptoms present and 5 indicating a hospital or asthma clinic visit, following the Inner City Asthma Study (ICAS) protocol (Mortimer et al. 2002). Daily symptom severity indices were created for wheeze, cough, and shortness of breath by adding morning and afternoon daily symptom scores, and a composite of symptom severity score (wheeze + cough + shortness of breath) was also computed to account for the fact that types of symptoms may differ for different chil- dren. Weekday morning and afternoon lung function measurements were administered under the supervision of the NYU research- ers at the school following American Thoracic Society guidelines for spirometry (Miller et al. 2005) using portable Asthma Monitor 1 spirometers (Jaeger Instruments, Hoechberg, Germany); thus, only acceptable spirom- etry maneuvers were included in the data set. Measurements were downloaded daily, and anomalous data points from unsupervised tests were flagged. Afternoon (1500 hours) peak expiratory flow (PEF) and forced expiratory volume in 1 sec (FEV1) were investigated in this analysis because NYU researchers were present during these maneuvers, and because these measures of airway obstruction are tra- ditionally used for diagnosis of asthma severity (Sawyer et al. 1998). Mixed-effects models have been used suc- cessfully for longitudinal studies of exposure– health effects (Buckeridge et al. 2002; Delfino et al. 2004, 2006; Trenga et al. 2006) and were applied here using an a priori subject- specific random intercept term, as used in similar panel studies, which effectively con- trols for the nonindependent nature of obser- vations within the same subject. A generalized form of the mixed model (see Bailey and Alimadhi 2007) was applied to the symp- tom severity indices, which followed a Poisson distribution. A standard linear mixed model (Pinhiero and Bates 2000) was applied to the lung function metrics. The models were run in S-Plus (version 6.1; Insightful Corporation, Seattle, WA, USA), using the “MASS” and “nlme” modeling libraries for symptom and lung function models, respectively. School was included as a covariate in final symp- tom models. Height, sex, and temperature were included in final lung function mod- els. The addition of school as a covariate also accounted for any potential seasonally related confounding. Because school was not a signif- icant covariate in the lung function models, this term was not included in the final regres- sion models. All models included an AR-1 autocorrelation term to address the time-se- ries nature of the data. Health associations with personal and school-site measurements were evaluated with same-day concentrations, 1-day lag concentrations, and an average of the same day and previous day’s pollution concentrations (2-day average). Longer lags were not investigated because of the availabil- ity of weekday personal data only, because it was not feasible to follow up with all children in their homes on weekends. All results are ovlume 119 | number 4 | April 2011 • Environmental Health Perspectives