Volume 177 - Issue 7
Serial correlation and confounders in time-series air pollution studies
Authors: Bin B Jalaludin, Guy B Marks and Geoffrey G Morgan
Med J Aust 2002; 177 (7): 397. || doi: 10.5694/j.1326-5377.2002.tb04853.x
Published online: 7 October 2002
Published online: 7 October 2002
To the Editor: The recent article by Johnston et al is an important contribution to the small but growing body of literature on the health effects of particulate matter (PM) pollution derived from bush or forest fire.1 The authors studied an important wood smoke PM exposure in Australia and showed consistent associations between higher concentrations of PM and emergency department presentations for asthma. Most research on the effects of PM has focused on motor-vehicle-derived PM pollution.2,3
However, Johnston et al do not appear to have accounted for serial correlation in their data. Measurements connected in time, such as repeated measurements of the same population, are likely to be correlated and not independent.4 Further, school holidays have been shown to influence hospital admission rates.5 The major Northern Territory school holidays in June and July are in the middle of the study period.
Johnston et al adjusted for some important confounders in their analysis (acute respiratory infections and weekdays/weekends).1 However, in time-series data, especially those dealing with asthma, serial correlation, as well as other potentially important confounders such as school holidays and temperature and humidity, should also be assessed. It may be that, even after appropriate adjustments for serial correlation and potential confounders, the rate ratios found by Johnston et al may not alter appreciably. However, it would have been useful for the investigators to have at least discussed any effects that controlling for serial correlation and other potential confounders might have had on their findings.
References
- Johnston FH, Kavanagh AM, Bowman DMJS, Scott RK. Exposure to bushfire smoke and asthma: an ecological study. Med J Aust 2002; 176: 535-538. <eMJA full text>
- Kunzli N, Kaiser R, Medina S, et al. Public-health impact of outdoor and traffic-related air pollution: a European assessment. Lancet 2000; 356: 795-801. BGBCFCCG
- Pope CA, III. Epidemiology of fine particulate air pollution and human health: biologic mechanisms and who's at risk? Environ Health Perspect 2000; 108: 713-723. BGBBBJGA
- Schwartz J, Spix C, Touloumi G, et al. Methodological issues in studies of air pollution and daily counts of deaths or hospital admissions. J Epidemiol Community Health 1996; 50: S3-S11. i1149932
- Storr J, Lenney W. School holidays and admissions with asthma. Arch Dis Child 1989; 64: 103-107. i1149934