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.