• Taina K Lajunen, Jouni J K Jaakkola, and Maritta S Jaakkola.
• 1 Center for Environmental and Respiratory Health Research.
• Am. J. Respir. Crit. Care Med.. 2013 Oct 1;188(7):776-82.

RationaleIdentification of the subpopulation especially susceptible to the adverse effects of second-hand smoke exposure (SHS) would be useful for preventive actions and interventions.ObjectivesTo investigate whether asthmatic heredity indicates susceptibility to the effects of SHS on the risk of adult-onset asthma.MethodsA population-based incident case-control study of clinically defined adult-onset asthma and randomly drawn control subjects (adults 21-63 yr old) from a geographically defined area in South Finland. After excluding current and ex-smokers there were 226 cases and 450 disease-free control subjects.Measurements And Main ResultsOur outcome measure was new adult-onset asthma. Parental asthma and recent SHS had a synergistic effect on the risk of asthma, the adjusted odds ratio being 1.97 (95% confidence interval, 1.12-3.45) for SHS; 2.64 (1.65-4.24) for parental asthma; and 12.69 (3.44-46.91) for their joint effect (relative excess risk due interaction, 9.08 [-0.22 to 43.18]). Synergistic effect followed a dose-dependent pattern with both recent and cumulative SHS exposures, with relative excess risk due interaction for parental asthma and over 100 SHS cigarette-years of 6.17 (0.57-19.16).ConclusionsThis is the first study showing that individuals with asthmatic heredity have a considerably increased risk of adult-onset asthma when exposed to SHS. SHS exposure has dose-dependent synergism with family history of asthma, the joint effect being stronger with higher exposure levels. Avoiding SHS could be an important preventive measure for reducing the risk of adult-onset asthma among those with asthmatic heredity. Asking about family history of asthma is a useful tool for identifying these susceptible individuals in clinical and preventive settings.

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