GSTM1 and asthma: Carriers of the GSTM1 non-null genotype had a modest increased risk of current asthma and significant increased risk of wheeze at 12 years (OR 1.34, per 100 m increase in cumulative lengths of major roads within a 150 m buffer, 95% CI 0.98–1.83, p = 0.06, p interaction = 0.04; and OR 1.42 95% CI 1.05–1.92 p = 0.02, p interaction = 0.02, respectively), but not at 18 years.