Vol.:(0123456789) 1 3 European Journal of Epidemiology https://doi.org/10.1007/s10654-019-00491-9 RESPIRATORY EPIDEMIOLOGY Asthma and selective migration from farming environments in a three‑generation cohort study Signe Timm 1 · Morten Frydenberg 1 · Michael J. Abramson 2 · Randi J. Bertelsen 3 · Lennart Bråbäck 4 · Bryndis Benediktsdottir 5,6 · Thorarinn Gislason 5,7 · Mathias Holm 8 · Christer Janson 9 · Rain Jogi 10 · Ane Johannessen 11,12 · Jeong‑Lim Kim 13 · Andrei Malinovschi 14 · Gita Mishra 15 · Jesús Moratalla 16 · Torben Sigsgaard 1 · Cecilie Svanes 11,12 · Vivi Schlünssen 1,17 Received: 26 October 2018 / Accepted: 29 January 2019 © Springer Nature B.V. 2019 Abstract Individuals raised on a farm appear to have less asthma than individual raised elsewhere. However, selective migration might contribute to this as may also the suggested protection from farm environment. This study investigated if parents with asthma are less likely to raise their children on a farm. This study involved three generations: 6045 participants in ECRHS/ RHINE cohorts (born 1945–1973, denoted G1), their 10,121 parents (denoted G0) and their 8260 oﬀspring participating in RHINESSA (born 1963–1998, denoted G2). G2-oﬀspring provided information on parents not participating in ECRHS/ RHINE. Asthma status and place of upbringing for all three generations were reported in questionnaires by G1 in 2010–2012 and by G2 in 2013–2016. Binary regressions with farm upbringing as outcome were performed to explore associations between parental asthma and oﬀspring farm upbringing in G0–G1 and G1–G2. Having at least one parent with asthma was not associated with oﬀspring farm upbringing, either in G1–G2 (RR 1.11, 95% CI 0.81–1.52) or in G0–G1 (RR 0.99, 0.85–1.15). G1 parents with asthma born in a city tended to move and raise their G2 oﬀspring on a farm (RR 2.00, 1.12–3.55), while G1 parents with asthma born on a farm were less likely to raise their G2 oﬀspring on a farm (RR 0.34, 0.11–1.06). This pattern was not observed in analyses of G0–G1. This study suggests that the protective eﬀect from farm upbringing on subsequent asthma development could not be explained by selective migration. Intriguingly, asthmatic parents appeared to change environment when having children. Keywords Asthma · Farming · Selective migration · ECRHS · RHINE · RHINESSA Background Numerous studies suggest that being born and raised on a farm reduces the risk of asthma [1–4]. The protective eﬀect from farm upbringing has been ascribed to a greater or more diverse microbial exposure in the farm environment, complementing the hygiene hypothesis which proposes that immunological competence is impaired after low micro- bial stimulation in early life [2, 5, 6]. However, ﬁndings in this ﬁeld have been inconsistent [7, 8], and one important concern has been the possibility for selective migration over generations. One could therefore question if the apparently protective eﬀect from farm upbringing is a result of asth- matic parents avoiding the farm environment rich in airway irritants such as endotoxin, allergens and organic dust [9] and raising their children in the cities. This would leave the farm eﬀect as the mere product of selective migration rather than a biological eﬀect of the farm environment. One thing is to claim that farm upbringing could explain less asthma in adulthood; another thing is if a healthy selection among farm dwellers contributes to curb heredity of asthma among their children. Evidence of potential healthy selection patterns in farm- ing is scarce, inconsistent and does not take both parents into account. One study found that 35–39-year-old men were less likely to take over the family farm if they had asthma at conscript examination, and one study found that asthmatics Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10654-019-00491-9) contains supplementary material, which is available to authorized users. * Signe Timm signe.timm@ph.au.dk Extended author information available on the last page of the article