A nationwide case-control study from Denmark has shown that parental asthma and smoking in pregnancy are risk factors for onset of asthma and persistent wheezing in children. The findings also suggest that exposure to ambient small particles < 2.5 µm (PM_2.5) may be a potential risk factor.

Children born between 1997 and 2014 were identified from a Danish national registry and followed up for onset of asthma and persistent wheezing from the age of 12 months to 15 years.

Overall, 122,842 children were identified as having asthma and persistent wheezing based on records of an asthma diagnosis or the provision of two or more asthma inhalers.

The majority of cases (83%) occurred in children aged under 3 years (average age 1.9 years).

Each child with asthma was matched, by age and sex, with 25 controls who did not have an asthma diagnosis.

Data on parental asthma, maternal smoking patterns, parental education and socioeconomic status were also extracted from the relevant national databases. 

Levels of pollutants were determined from models developed by the Danish air pollution modelling system and related to the children’s home addresses.

Maternal asthma was associated with a higher risk than paternal asthma for asthma in offspring; hazard ratio (HR) 1.72 (95% CI: 1.72-1.77) vs 1.51 (95% CI: 1.48-1.53). Having two parents with asthma was associated with a 2.40-fold (95% CI: 2.34-2.47) increased risk of asthma and persistent wheezing.

Maternal smoking during pregnancy was associated with an increased risk of asthma, and there was a positive dose-response relation between the number of cigarettes smoked daily and an increased risk of asthma and persistent wheezing.

Higher maternal educational level was associated with a reduced risk of asthma and persistent wheezing; HR 0.58 (95% CI: 0.56-0.59) for those with the highest level of education compared with those with the lowest.

The effect of a range of pollutants was examined but only a positive association between PM_2.5 and asthma and persistent wheezing was found across all three models used (HR 1.03, 1.04 and 1.05 respectively) and in sensitivity analyses.

The study authors conclude:

‘The findings from this study strengthen the evidence that parental asthma, parental education, and maternal smoking during pregnancy are risk factors for onset of asthma and persistent wheezing in children.

‘Moreover, we found that children exposed to high levels of PM_2.5 were more likely to develop asthma and to have persistent wheezing. These findings therefore support emerging evidence that exposure to air pollution might influence the development of asthma.’

‘This finding needs to be substantiated in future studies,’ they add.

Dr Peter Saul