2004). This study showed that the five tested heaters did emit NO2 and, in some cases, formaldehyde and carbon monoxide. However, because no old-style heaters were tested using the same protocol, this study did not allow assessment of the extent to which these low- NOx heaters were qualitatively or quantita- tively different, as claimed, from the older-style heaters. Hence, it is difficult to know whether this study validates the application of health data from the clinical studies conducted with the older-style heaters to the low-NOx heaters. In summary, there are real health concerns about indoor exposure to combustion products from unflued gas heaters. Two randomized control trials suggest that benefits may follow from replacement of these unflued gas heat- ers, but the design of these trials imposes some limitations on interpretation. Furthermore, the previous Australian trial included both older- style and low-NOx gas heaters in the control group (Pilotto L, personal communication), and only very limited data are available about potential adverse effects of these newer type of gas heaters. Hence, we conducted a random- ized, double-blind, crossover study comparing the respiratory health effects and air quality consequences of exposure to newer-style low- NOx gas heaters with exposure to non–indoor- air-emitting flued gas heaters. Methods The study was a three-period, cluster- randomized, controlled, crossover study conducted in primary school classes. Two con- ditions were compared: heating with a low- NOx unflued gas heater and heating with a flued gas heater. Classrooms were fitted with both types of heaters. In each classroom the existing low-NOx unflued gas heater was left in situ, and the flued gas heater was installed co-located with it in the classroom. Both heat- ers were surrounded with a screen so that those in the room would not be able to see which one was operating. During the study period either the low-NOx unflued gas heater or the flued gas heater was in active mode for 1-week periods. The 6-week study period was arranged in three consecutive pairs of 2 weeks. For each classroom, the order of operation of the unflued and flued gas heaters was randomized separately for each pair of weeks. Hence, each classroom used a low-NOx unflued gas heater for three 1-week periods and a flued gas heater for three 1-week periods. During this time the heaters were operated at the discretion of the class teacher, in the usual way, accord- ing to heating need. Further details about the installation and control of heaters, blinding of participants, and the randomization procedure are provided in the Supplemental Material (doi:10.1289/ehp.1002186). The study was conducted in schools in the Blue Mountains, Southern Highlands, and Goulburn regions of NSW. These are elevated areas outside the Sydney metropolitan area. They were chosen because of their relatively cold climate and proximity to Sydney. Baseline assessments were conducted in June 2009, a run-in and pilot-testing week was conducted beginning 3 August 2009, and the 6-week study period began on 10 August 2009. The pilot-testing week was 1 week after the 2-week mid-winter break. August is the last month of winter in the southern hemisphere. The trial was approved by the Human Research Ethics Committee of the University of Sydney and the NSW Department of Education and Training State Education Research Approvals Process. Parents gave written informed consent prior to par- ticipation of their child in the research. The trial was registered with the Australian and New Zealand Clinical Trials registry (ACTRN12609000610235; 22 July 2009). Study population. There was a two-stage selection process in which schools and class- rooms were selected and then participants within the selected classrooms were recruited. Eligibility criteria for classrooms were that they were fitted with existing unflued gas heat- ers, that it was feasible to install a flued gas heater, and that the class using the selected room included 20 students in grades 4, 5, or 6. Principals of 38 schools in the Blue Mountains, Southern Highlands, and Goulburn regions were invited to nominate classes to partici- pate. All students in the selected classrooms were eligible for inclusion in the study. Those whose parents gave written informed consent were recruited. Monitoring heater use. Temperature moni- tors (HOBO H8 series data loggers; Onset, Pocasset, MA, USA) were installed in the enclosures to estimate the running times for each heater. Abrupt changes in temperature were used to identify the times when heaters were turned on or off. These temperature log- gers began operating only from the third study week (24 August 2009). In-class temperature was also recorded at 2-min intervals between 0900 and 1500 hours on each study day. Respiratory health assessments. We col- lected baseline information on respiratory symptoms and illnesses and use of gas and other heaters at home using a questionnaire completed by the parents. We also measured spirometric lung function before and after administration of 200 µg albuterol via spacer. Current asthma was defined as reported wheeze in the preceding 12 months and either a reported doctor’s diagnosis of asthma or an increase in forced expiratory volume in 1 sec (FEV1) after albuterol that was ≥ 12% of baseline. Height and weight were meas- ured using a stadiometer and portable scales, respectively. Environmental Health Perspectives • volume 118 | number 10 | October 2010 Health effects of low-NOx unflued gas heaters We measured atopy by using skin prick tests to inhalant allergens Dermatophagoides pteronyssinus (HDM), Dermatophagoides farina (also HDM), cockroach, cat, Alternaria, Aspergillus, rye grass, and a grass mix (Hollister- Stier, Spokane, WA, USA), as previously described (Peat et al. 2004). Glycerol and histamine phosphate 10 mg/mL were used as negative and positive controls, respectively. Wheal sizes were measured 15 min later and those that were ≥ 3 mm and were also greater than negative control were classified as positive. Subjects with positive reactions to any allergen were classified as atopic. The main study outcomes were symptoms and lung function measurements recorded twice daily by participants. Children kept a daily symptom and medication diary for the 6-week study period. Subjects were asked to record each morning the presence of cough and wheeze or chest tightness during sleep. They were asked to record each evening the presence of cough, wheeze or chest tightness, fever, sore throat, runny or blocked nose, eye irritation, sore teeth or gums, and stomach- ache during the day. The last two questions were included to detect possible information bias. For each symptom subjects were asked to indicate whether they had “no symptoms during the day” (scored 0), “symptoms, but did not disturb your daily activities” (score 1), “symptoms that disturbed part of your daily activities” (score 2), or “symptoms that dis- turbed the whole day or most of your daily activities” (scored 3). Subjects also recorded whether they used bronchodilator (reliever) medication twice a day and, if so, whether it was for relief of symptoms or as part of their regular medication regimen. In the evening subjects also recorded if they had taken preven- ter medications for asthma, been exposed to an unflued gas heater or other type of heater at home, and/or been exposed to environmental tobacco smoke (ETS) during that day. We used Mini-Wright Digital (MWD) portable electronic spirometers (Clement Clarke, Harlow, Essex, UK) to record lung function twice daily. Children were instructed to perform three forced expiratory maneuvers into their MWD devices in the morning and in the evening. The device records only the highest FEV1 and peak expiratory flow rate (PEF) from the attempts made during any session. Each recording is made with an asso- ciated date and time stamp. Detailed instruc- tions on how to do this were included on the reverse side of the diary card. Each week the diaries were collected, and MWD data were downloaded during the school visits. The parents were sent a text mes- sage (SMS) the day before the teams were in the schools to remind the children to bring the diaries to the schools the next day. After the diaries were collected they were checked for 1477