Background Environmental tobacco smoke (ETS) exposure in children is linked with the development of allergic asthma. However, its influence on allergic sensitisation in children has not been conclusively determined.

Objective To systematically review existing evidence of ETS exposure's impact on markers of allergic sensitisation in children.

Methods CENTRAL, MEDLINE and EMBASE databases were searched. Included studies assessed following markers of atopic sensitisation: total immunoglobulin E (tIgE) concentrations, at least one specific IgE (sIgE+), and positive skin-prick tests (SPTs+) in ETS-exposed and non-exposed children.

Results 8 studies on the influence of ETS on tIgE concentration (2603 patients), 6 studies on ETS and sIgE+ (9230 participants) and 14 papers on ETS and SPT (14 150 patients) met our inclusion criteria. ETS was shown to raise tIgE concentrations by 27.7 IU/mL (95% CI 7.8 to 47.7; I²=58%; results based on 3 studies) and to increase the risk of atopic sensitisation, as assessed by sIgE+ (OR=1.12, 95%CI 1.00 to 1.25; I²=54%; results based on 4 studies) and SPT+ (OR=1.15; 95% CI 1.04 to 1.28; I²=0%; results based on 10 studies). In a subgroup analysis, this effect was most pronounced in children <7 years (preschoolers) by OR=1.20; (95% CI 1.05 to 1.38) and OR=1.30 (95% CI 1.05 to 1.61), (for sIgE+ and SPT+, respectively).

Conclusions Current analysis supports an association between ETS exposure in early childhood and the increased risk of allergic sensitisation. Subgroup meta-analyses demonstrate that younger children suffer the most from detrimental immunomodulating effects of ETS exposure. This study underscores ETS as an important but avoidable risk factor for the development of allergic disease in children.