Background Asthma is the most common chronic childhood medical condition globally. After sharp rises in prevalence over the second half of the twentieth century, falling prevalence has been found in some countries including the United Kingdom during the first decade of the twenty-first century. In order to gain insight into the hitherto unconfirmed factors underlying changing susceptibility to asthma, we used data from one of the longest-running paediatric asthma epidemiology studies in the world: the Aberdeen Schools Asthma Survey (ASAS). We hypothesised that the relationship between asthma and associated risk factors had changed between 1964 and 2014.

Methods An ecological study design was used. Parents of children aged 8–13 in state schools in the City of Aberdeen, North East Scotland, were invited to participate in a questionnaire survey in May 1964, May 1989 and then at five-year intervals to 2014. Child history of asthma and eczema, parental smoking, parental asthma, sex and socioeconomic status (SES) were determined. 2 knot structural change models, with knots after 1964 and 1999, were constructed to assess changes in the relationship between child history of asthma and these risk factors over time.

Results Data for analysis were available for 15 108 children aged 8–13 (75% response rate). Asthma prevalence rose from 4% in 1964 to 28% in 2004 before falling to 22% in 2009 and 19% in 2014. Parental smoking prevalence fell in a near-linear fashion from 58% in 1989 to 28% in 2014. The odds ratio (OR) for a child with asthma to have eczema increased between 1989 and 1999 by 1.031 (95 confidence interval 1.028, 1.035) and by 1.042 (1.038, 1.047) between 2004 and 2014. The OR for a child with asthma to have a parent who smoked rose by 1.032 (1.028, 1.036) between 1989 and 1999 and by 1.043 (1.038, 1.047) between 2004 and 2014. The OR for a child with asthma to have a parent with asthma, to be male and to be from the most deprived communities also rose over the study period.

Conclusion As hypothesised, we found that the relationship between asthma and associated risk factors such as child eczema, sex, parental smoking, parental asthma and deprivation changed over the period 1964 to 2014. Limitations in our study include regulatory changes and falling response rates over time. The changing nature of relationships with asthma suggests that modification of environmental exposures, e.g. to second-hand smoke, can reduce population asthma susceptibility.