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Objective -To examine the importance of parental smoking on passive exposure to tobacco smoke in children and the social and geographical patterns of exposure.Design -Cross sectional study.Setting -Schools in 10 towns in England and Wales; five towns with high adult cardiovascular mortality and five with low rates.Subjects -4,043 children aged 5-7 years of European origin.Main outcome measures -Salivary cotinine concentration and parents self reported smoking habits.Results -1,061 (53.0%) children were exposed to cigarette smoke at home or by an outside carer. Geometric mean cotinine rose from 0.29 (95% confidence interval 0.28 to 0.31) ng/ml in children with no identified exposure to 4.05 (3.71 to 4.42) ng/ml in households where both parents smoked and 9.03 (6.73 to 12.10) ng/ml if both parents smoked more than 20 cigarettes a day. The effect of mothers' smoking was greater than that of fathers', especially at high levels of consumption. After adjustment for known exposures geometric mean cotinine concentrations rose from 0.52 ng/ml in social class I to 1.36 ng/ml in social class V (P<0.0001); and were doubled in high mortality towns compared with the low mortality towns (P=0.002). In children with no identified exposure similar trends by social class and town were observed and the cotinine concentrations correlated with the prevalence of parental smoking, both between towns (r=0.69, P=0.02) and between schools within towns (r=0.50, P<0.001).Conclusions -Mothers' smoking is more important that fathers' despite the lower levels of smoking by mothers. Children not exposed at home had low cotinine concentration, the level depending on the prevalence of smoking in the community.