Bidirectional relation between asthma and smoking habits

Smoking in Italy has been following a declining trend but still a primary cause of preventable death in the country. The primary purpose of the thesis was to investigate the bidirectional relationship between smoking habits and asthma using the data from a longitudinal study on responders to a cross–sectional prevalence study. It was aimed to study the predictors of smoking cessation as a function of asthma and the incidence of asthma as a function of smoking habits. The secondary aim of the study was to study predictors of smoking cessation and determinants of incidence of asthma. Indeed, it was not possible to prospectively investigate smoking initiation when the population under study is older than 20 years because in Italy, the age of start smoking is around 17 years old. The thesis was based on the analysis of three study cohorts, two from Verona (North of Italy) and one from Sassari (in Sardinia, one of the two main islands of Italy). The longitudinal data were collected at two periods of time, 1998–2000 (baseline study) and 2007–2009 (follow–up study). To build each cohort, 3000 subjects (1500 males:1500 females) were randomly selected from the general population aged 20–44 years to answer a screening questionnaire. Each subject was asked to respond to a postal screening questionnaire that contained questions on respiratory symptoms and smoking habits. Follow–up time was 9.1 ± 0.8 years. Of 5, 933 subjects who had participated in the baseline study, 3, 597 (60.6%) took part in the follow–up. The Cox proportional hazards regression was used to assess the association between study outcomes and baseline characteristics of participants. Smoking prevalence decreased in the population from 31.5% at baseline to 22.2% at follow–up (crude prevalence). At baseline, high smoking percentage was found in young people (20–30 years old), workmen and unemployed. During the follow–up, around 30% of smokers had stopped smoking for at least one month. Smoking cessation incidence rate was 42.1/1, 000 person–years. We found that having current asthma at baseline was one of factors that stimulate smoking cessation (Hazard ratio (HR) of quitting smoking = 1.57, 95% Confidence Interval (CI) 0.92–2.68: Heavy smokers had a tendency to quit smoking during the follow–up almost 40% lower than light smokers (HR= 0.58, 95%CI 0.45–0.74). The results suggest that the intensity of smoking significantly affected smoking cessation in the study population. The observed incidence rate of asthma was 5.2/1, 000 person–years. In terms of the relationship between incident asthma and smoking, we observed that16 abstract the HR of asthma onset was higher in ex-smokers compared with current and never smokers, although not significantly. Interestingly, there was an interaction between smoking habits and having asthma–like symptoms: the incidence of asthma was four times higher in ex–smokers with asthma–like symptoms than never smokers without asthma–like symptoms (HR = 4.12, 95%CI 2.12–7.99). The results were probably affected by a reverse causation which happened at baseline: when having asthma–like symptoms, smokers were liable to stop smoking. Besides, allergic rhinitis showed to be a risk factor for developing asthma: individuals with allergic rhinitis were prone to develop asthma with an adjusted HR of 3.85 (95%CI 2.71-5.46) compared with individuals without allergic rhinitis. In conclusion, smoking in Italy presents a declining trend. The rate of smoking cessation is high in people having current asthma at baseline. Between smoking and asthma there is a reverse causation that influences smoking habits: asthma is most likely a factor that causes smoking cessation while it is not possible to confirm the causal liaison of smoking to asthma. The fact that smoking intensity strongly affects smoking cessation requires that smoking cessation program should be tailored for light and heavy smokers separately. And lastly, individuals having allergic rhinitis and asthma-like symptoms should receive proper treatments and health consultants to control the diseases