 a courtyard within the school grounds. Indoor and outdoor particulate pollution levels (PM10, PM2.5, PM1) were measured continuously using a TEOM Series 1400a Ambient Particulate Monitor, and expressed as 10 minute averages over a 24 hour period, and 24 hour averages, from midday to midday. It has been shown that this TEOM under-reads the true PM10 mass because of the loss of semi-volatile compounds in the heated air stream. Consequently the figures were corrected based on an established factor. Details about this and the more information about the indoor and outdoor pollution monitoring at Christ's College are given in Kingham and co-workers . Weather data including wind speed, temperature, temperature difference and relative humidity were also continuously collected from the outdoor site and expressed as 10 minute averages.
The participants were male secondary school students, aged between 12 and 18 years, attending the boarding houses of Christ's College. All were healthy non-smokers with no significant medical conditions. Smokers were specifically requested not to volunteer for the study, and the non-smoking status of the participants was confirmed by school staff. A sub-group of students with a previous diagnosis of asthma (defined as "doctor diagnosed asthma ever") were also studied. They were being treated with as-required short-acting beta agonist reliever medication, with some taking regular inhaled corticosteroids (see Results section for details). The regular use of oral corticosteroids was an exclusion criterion. Investigators made no alterations to asthma management and gave no advice about reliever inhaler use during the study.
Prior to commencing the study the health status of the students was assessed by taking a brief medical history. Baseline spirometry and skin prick testing for atopic status were also undertaken.
The study protocol and conduct was approved by the Canterbury Regional Ethics committee. All participants provided written informed consent prior to commencement, and parental approval to participate was also sought.
Lung function assessment
All students were issued with personal downloadable electronic spirometers (Piko-1, Ferraris Respiratory Europe Ltd, Hertford, UK) and trained in their use. These spirometers measure Forced Expiratory Volume in 1 second (FEV1) and peak expiratory flow rate (PEFR) to American Thoracic Society standards . Lung function measurement was undertaken twice daily, in the early morning, and around 6.00 pm, supervised by House Matrons, who were all trained nurses given training in spirometry. No specific instructions about withholding reliever medications prior to spirometry were given to the students. Spirometry data were downloaded at least fortnightly by investigators.
All students were issued with daily diary cards to detail respiratory, nasal and eye symptoms. In addition, students with asthma completed diaries detailing asthma symptoms and reliever medication use.
Lung function and diary card data collection occurred during school terms for the duration of the project. When a student was away from the school for more than one night (weekend and school holidays), diaries and lung function measurements were not undertaken.
High pollution peak assessment
Within 24 hours of two separate high pollution days all participating students underwent a more detailed assessment. The investigators were informed by Environment Canterbury the morning after a high pollution night. The school was contacted, and all participating boys were excused their evening study activities, to be assessed by the investigating team. This involved collection of urine samples for analysis of 1-hydroxypyrene and collection of exhaled breath condensate for measurement of pH and hydrogen peroxide. Baseline assessments were also undertaken at the beginning of the study during a period of low pollution (autumn control) and during the winter, after a 1 week period of low pollution levels (winter control).
Urinary 1-hydroxypyrene levels (1-OHP)
Urine samples were stored at -20°C until analysis. Uri