Our study shows that after the introduction of comprehensive smoke-free legislation, there was a reduction in the incidence of asthma among people who did not have occupational exposure to environmental tobacco smoke. Our analysis was limited to data on children; the reduction in the incidence of asthma was observed among both preschool and school-age children.

Previous investigations of the health benefits of comprehensive smoke-free legislation have focused primarily on the incidence of cardiovascular disease. There are now a number of studies showing a reduction in the incidence of coronary events after the introduction of legislation.13,14 The Scottish smoke-free legislation has been successful in reducing exposure to environmental tobacco smoke in public places, such as bars,9,10 resulting in fewer respiratory symptoms among workers in bars.11 A recent study showed that there was a nonsignificant 2% reduction in overall admissions for asthma after citywide smoking restrictions were instituted in Toronto.15 However, restaurants were exempt from the restrictions, and the study included adults whose exposure was reduced as a result of workplace bans on smoking. In Arizona, overall admissions for asthma have also fallen since the implementation of restrictions on smoking in public places, with the greatest reductions observed in counties that had no preexisting partial bans.16 The Arizona study also included adults who were protected from workplace exposure after the legislation.

Before implementation of the Scottish legislation, there was concern that it might result in the transfer of smoking activity to homes, leading paradoxically to an increase in exposure to environmental tobacco smoke among children. Studies of exposure among both adults17 and children4 have shown no evidence of displacement of smoking to the home; rather, the legislation has been followed by an increase in voluntary restrictions in the home.12 Household smoking restrictions reduce the exposure of children to environmental tobacco smoke, irrespective of whether their own parents smoke.18 Hence, the overall exposure of children to environmental tobacco smoke, measured objectively with the use of salivary cotinine concentrations, has fallen since the implementation of the Scottish legislation.4

There is substantial evidence of an association between exposure to environmental tobacco smoke and the risk of asthma. Exposure to environmental tobacco smoke increases the risk of asthma, and among those with asthma, it confers a predisposition to a worse prognosis, including an accelerated decline in lung function, more frequent exacerbations, more severe symptoms, impairment of the quality of life, and a diminished therapeutic response to corticosteroids.6 In a meta-analysis, the pooled estimate of the relative risk of ever having asthma as a result of exposure to environmental tobacco smoke was 1.48 (95% CI, 1.32 to 1.65), the relative risk of current asthma was 1.25 (95% CI, 1.21 to 1.30), and the relative risk of a new diagnosis of asthma was 1.21 (95% CI, 1.08 to 1.36).7 The deleterious effects of exposure to environmental tobacco smoke are greater among children than among adults, since children have smaller bodies, a higher baseline respiratory rate, and smaller airways.7 Preschool children are more likely to be exposed to environmental tobacco smoke in their homes than in public places.19 In contrast, school-age children spend less time with their parents and more time outside their homes and may themselves start smoking. Among children in Scotland, self-reported exposure to environmental tobacco smoke in public places has fallen since implementation of the legislation.18 In the Scottish Schools Adolescent Lifestyle and Substance Use Survey (SALSUS), the prevalence of smoking among 13-year-old boys fell from 5% in February 2004 to 3% in February 2007.3 The corresponding figures for girls were 7% and 4%. We conducted a subgroup analysis with data from preschool and school-age children to determine whether the effect of the legislation differed according to age group.

In our study, we used data from all regions of Scotland, and the results were consistent across the regions. Therefore, the results cannot be attributed to changes in hospital catchment areas. We did not have data on Scottish children who were admitted to hospitals outside Scotland during the study period, but these admissions were probably few in number, and there is no reason to expect that there would have been a systematic bias over time. Our study included only asthma exacerbations that were severe enough to require admission to the hospital. We did not have access to data on less severe exacerbations of asthma that did not require hospitalization, but we are unaware of any systematic change over time in the threshold for hospital admissions for asthma. We cannot determine whether the reduction in hospital admissions was a result of the prevention of asthma exacerbations that would otherwise have occurred or a reduction in the severity of the exacerbations that did occur. The decrease in admissions was not due to an increase in the incidence of deaths before arrival at the hospital. We did not have access to information on smoking status at the individual level or on measures of the level of exposure to environmental tobacco smoke. Therefore, we cannot determine the extent to which the observed reduction in asthma was due to reduced exposure to environmental tobacco smoke in the home, reduced exposure to environmental tobacco smoke in public places, or a reduction in active smoking among school-age children. As with any observational study, there may have been other interventions that occurred during the period studied. However, we are not aware of any national educational campaigns, changes in health care delivery or clinical management, or changes in other exposures, such as air pollution, that coincided with the date on which the legislation was introduced. Awareness of asthma may have increased over time, but it is unlikely that there was a stepwise change in the awareness of asthma at the time the legislation was introduced. Asthma may have been misclassified as infection in the case of some hospital admissions, but again, there is no reason to suspect that there was a systematic error as a result of a change in the percentage of misclassified cases after implementation of the legislation.

In conclusion, our study showed that there was a reduction in the rate of hospitalizations for childhood asthma after the introduction of legislation to make public places smoke-free, suggesting that the benefits of such legislation can extend to populations other than those with occupational exposure to environmental tobacco smoke.