s of necessity dependant on the number and magnitude of high pollution days during the study period. In addition, limiting the sampling to one school (to increase exposure validity) placed limitations on potential numbers of students willing/able to participate. Retrospective power analysis was therefore undertaken for lung function scores (expressed as Z-scores) between moderate/high (24 hour average> 20 μm/m3 outdoor) pollution days and low pollution days. Assuming a 2:1 split in air pollution days, and given n = 80 students, the study had an 80% power to detect differences in Z-score values of 0.30 in FEV1 and 0.27 in PEFR with α = 0.05 (two-tailed). In addition, with 100 time points a correlation between Z-scores and pollution of >0.28 would have been detected as statistically significant i.e. R2 >10%.
Students' baseline demographics and lung function
A total of 93 students took part in the study, from a total potential boarding house population of 240 students. Eighty-nine students identified themselves as New Zealand European, whilst four (4.3%) came from Asia. There were no students who identified themselves as Maori. In 2001, 6.9% of the population of Christchurch identified themselves as Maori, while 5.5% identified themselves as Asian . Twenty-six (28%) of the students had a doctor diagnosis of asthma. The prevalence of asthma in this age group in New Zealand is 24.4% . Seventeen of these students used short-acting beta agonists on an as-required basis, while eight students were taking regular inhaler corticosteroids. Mean age of the students at study start was 14 years 5 months (range 12 years 9 months – 17 years 6 months). There were no significant age differences between students with asthma and those without asthma. There was no significant difference in % predicted FEV1 between asthmatics and non-asthmatics (97.54% ± 2.481 (mean ± 1 s.e.) n = 26, vs 95.49% ± 1.635 n = 57), though FEV1/FVC ratio was significantly lower in the asthmatic students (FEV1/FVC ratio asthma group 0.803 ± 0.013 n = 26 vs normal group 0.834 ± 0.009 n = 57, p = 0.05).
Asthmatic reliever use
Reliever use in the asthmatic students was generally low. Reliever use was only documented on 3.26% of the days in the diary cards filled in by the asthmatic students. There was no significant difference between the number of puffs of reliever used by the asthmatics on low pollution days when compared to moderate/high days (Mann-Whitney U, p = 0.488). There was however a relationship between days of ANY reliever use and the occurrence of a moderate/high pollution day. For this reason, the daily use of a reliever was used as a covariate in the mixed-model linear regression analysis.
The study was undertaken from the end of March to early September 2004, encompassing autumn and winter. Indoor and outdoor pollution levels are shown in Figure 1 (24 hour average) and Figure 2 (10-min peak). Pollution levels during the winter of 2004 were generally lower than previous years , with the majority of high pollution nights occurring during the school vacation period, when data was not being collected. There were however a number of high pollution events where data was collected. Detailed sampling took place after high pollution nights on June 24 and July 21. The 24-hour average outside PM10 level recorded at the school on those dates was 43 μg/m3 and 72 μg/m3 respectively. Peak outdoor PM10 levels for a 10-minute period reached 105 μg/m3 and 257 μg/m3 respectively. The majority of PM10 pollution was in the PM2.5 range, and indoor pollution levels were similar to outdoor. Occasional very high pollution peaks were identified indoors. These tended to be in the PM10 range, rather than PM2.5, and probably represented resuspension events due to activity in corridors. On average, pollution levels measured at the school were 22.5% lower than those measured at the central Environment Canterbury monitoring station.
Urinary 1-OHP levels
Urine 1-OHP levels are shown in Figure 3. Urine 1-OHP levels were significantly higher on the high pollution days compared to both low