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European birth cohort studies on asthma and atopic diseases: II. Comparison of outcomes and exposures—a GA2LEN initiative. Allergy 61:1104–1111. ESCAPE (European Study of Cohorts for Air Pollution Effects). Kogevinas M, Andersen AM, Olsen J. 2004. Collaboration is 2009. Homepage. Available: http://www.escapeproject.eu/ needed to co-ordinate European birth cohort studies. Int J [accessed 6 July 2011] Epidemiol 33:1172–1173. Eurostat 2011. Homepage. Available: http://epp.eurostat. Koppen G, Den Hond E, Nelen V, Van De ME, Bruckers L, Grandjean P, Bellinger D, Bergman A, Cordier S, Davey-Smith G, Magnus P, Irgens LM, Haug K, Nystad W, Skjaerven R, Vrijheid et al. Future collaboration. The 350,000 mother–child pairs covered by European birth cohorts form an impressive number, but of course this is only a small percentage of all births in Europe [(> 5 million births annually in 25 EU countries (Eurostat 2011)]. For the investigation of any specific exposure–outcome relationship, the number of available subjects will be much smaller than the total number of children followed by the cohorts. The cohorts are in very different stages of follow-up and thus have not all reached the ages at which some outcomes, for example, asthma or puberty onset, can be assessed. Moreover, not all exposure and outcome topics included in ENRIECO are immediately suitable for com- bined analyses of individual data. 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