Editor,—In their study on the effect of various indoor pollutants on peak flow variability, Fielder and colleagues1 should have considered several methodological issues before concluding that environmental tobacco smoke increases airway variability.

Low parental social class is known to be associated with a high prevalence of both parental smoking and respiratory diseases including asthma. Hence, social class is likely to be an important confounding variable, and could easily have accounted for the 5.5% variability in peak flow detected. If parents in the city centre were generally of lower social class than those in the village, it would also have explained higher peak flow variability in children attending school at the …