Asthma-like symptoms and the risk for BHR
Current smokers in both our samples had the largest BHR values, with an OR of 1.39 for getting an elevated slope. The tendency to an increased BR for smokers has been observed in previous studies, and it has been supported by pathological evidences of important changes in large and peripheral airways induced by smoke, which lead to different degrees of airway obstruction. Moreover, Mitsunobu and associates reported that the cumulative dose of methacholine causing a significant increase in total respiratory resistance was significantly lower in asthmatics with a smoking history than in those without it. 
We also showed a strong relationship between chronic bronchitis-like or asthma-like symptoms and the risk for BHR (OR, 1.30 and OR, 2.65, respectively), bringing further evidence to a reappraisal of the Dutch hypothesis. Furthermore, our findings underline the association of a positive skin reactivity (OR, 1.32) or of higher total IgE values (OR, 1.61) with an enhanced BR, according to other studies.
We also confirmed the inverse relationship between airway caliber and BHR, which could be ascribed to anatomic, mechanical, and mathematical factors: the flow resistance in a tube is inversely proportional to the radius of the tube to the fourth power Cheap Viagra. This leads to a proportionally greater resistance for a narrow airway. Hence, the influence of baseline lung function must be accounted for when analyzing BHR.
BHR and Urban Residence
In our investigation, after controlling for the independent effects of gender, age, smoking habits, respiratory symptoms/diseases, and atopic status, residence in an urban area appears to be an independent risk factor for BHR (OR, 1.41; 95% confidence interval [CI], 1.13 to 1.76). Interestingly, such an OR is of the same magnitude as the one for active smoking (1.39).
Likely, there is higher outdoor air pollution in urban than in rural areas. Indeed, when we had evaluated pollutants (sulfur dioxide, total suspended particulate) from the two areas, we had found higher mean annual levels in Pisa than in Po Delta, with differences of 40 ^g/m3 for total suspended particulate (94 ^g/m3 and 54 ^g/m3, respectively) and 7 ^g/m3 for sulphur dioxide (15 ^g/m3 and 8 ^g/m3, respectively).