The number of children treated in hospital for severe asthma attacks declined most in Scotland's most affluent areas after a ban on smoking in cars was introduced.

The Smoking Prohibition (Children in Motor Vehicles) Bill was passed by the Scottish Parliament in 2016, making it illegal to smoke in a vehicle carrying anyone under the age of 18. 

Five years on, new research has found that this change in the law was associated with a marked decrease in hospital admissions in the under-five age group.

The research, which was led by the University of Glasgow, in collaboration with the Universities of Aberdeen and Stirling, looked at data on all asthma emergency hospitalisations in Scotland between 2000 and 2018 amongst children aged 16 and under.

It found that, after the smoke-free vehicle legislation was passed, asthma hospitalisations fell by 1.49% per month among pre-school children, though not in older children.

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The effect was greater amongst children living in the most affluent areas with a 2.27% reduction per month, which equates to 18 less hospital admissions over two years. 

However, the improvement was not seen in the most deprived areas, which the researchers said was likely to be due to differences in car ownership.

Over the 19-year study period (January 1 2000 and December 31 2018), there were 32,342 emergency hospitalisations for asthma among children under 16. Of those 13,954 (43%) affected children under 5 years of age - around 788 per year - and 18,388 (57%) were among children aged 5-15 years.

Of the children hospitalised, 10,179 (31%) lived in areas in the most deprived quintile, 5,775 (18%) in the middle quintile, and 4,046 (13%) in the most affluent quintile.

Globally, exposure to second-hand smoke is responsible for an estimated 169,000 deaths in childhood annually.

Exposure is thought to be mainly due to parental smoking in their homes or family vehicles.

Children exposed to smoke in vehicles are more likely to develop wheeze and have poorer lung function than children exposed to second-hand smoke at home, as well as unexposed children.

England and Wales introduced the car smoking ban ahead of Scotland in October 2015.

Asthma UK Scotland estimates that around 72,000 children are affected by the lung condition.

Estimates of the percentage of cases of childhood asthma attributable to second-hand smoke exposure range from 1.3% to 8.2%.

Second hand smoke exposure also increases the risk of exacerbating asthma symptoms among young children who already have the condition.

Professor Jill Pell, lead author from the University of Glasgow, said: “Our study demonstrates the significant benefit to children that smoke free vehicle legislation has had.

"We found that the reduction in asthma hospitalisations was specific to pre-school children, which is plausible given that exposure to parental smoke is more likely among pre-school children, who spend more time with their parents.

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“Our findings also suggested a possible widening of health inequalities whereby the relative reduction in childhood asthma hospitalisations was greater among those living in the most affluent areas.

"This could be due to different starting points or differences in car ownership or compliance with the legislation; but merits further research.”

Researchers also took into account the fact that in Scotland, childhood asthma hospitalisations had already fallen as a result of legislation in 2006 banning smoking in public places.

David McColgan, Senior Policy and Public Affairs Manager at BHF Scotland, said: “Research has shown for some time now that exposure to second hand smoke is damaging to health and in children can be especially harmful.

"There is also a growing body of evidence that suggests passive smoking is associated with medical risk factors for cardiovascular disease amongst children.

"BHF Scotland campaigned for and welcomed the introduction of legislation banning smoking in cars where children were present, and implementing measures to further reduce exposure to second hand smoke and smoking rates in general will play an integral role in helping improve the health of us all.”

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It follows a separate study by the charity which found that heart attacks have fallen by 74% in Scotland as a direct result of smoking rates more than halving and improvements in blood pressure and cholesterol levels.

Researchers said the figures equated to 42,000 heart attacks being prevented or delayed. The number of strokes fell by 68% over the same period.

Scotland was the first country in the UK to introduce a ban on smoking in public places in 2006 under Labour's Jack McConnell. Only the Tories voted against the ban. 

The study, ‘Impact of smoke-free vehicle legislation on childhood hospitalisations for asthma: an interrupted time-series analysis of Scottish data over 19 years’ is published in The Lancet Public Health.