asthma (doctor diagnosis, age of the first/ last attack), the history of exposure to active smoking and social class. The questions used in the first and second surveys have been published elsewhere \[[@B11]\].

After the second postal wave, the life and residence status of all non responders to the mail survey were obtained from vital statistics records. Then all subjects who had neither moved nor died before or during the survey were sent a third postal wave and were finally interviewed by phone.

At the end of follow-up, 748 (12.4%) subjects had moved and 47 (0.8%) died before or during the study; 352 (5.8%) could not be traced through anagraphic records, 1004 (16.6%) did not reply or explicitly refused to answer the questionnaire and 3880 (64.3%) replied.

The protocol of the(having had an attack of asthma in the last 12 months or currently taking any medicine for asthma) as well as those who, in the second study, reported having had the first asthma attack more than 3 years before the first survey. This was done considering that a discrepancy of less than three years could be due to recall bias when reporting the age of onset (Pattaro C. Long-term repeatability of a questionnaire for lifelong asthma assessment.*Personal communication*). New cases of asthma were considered those, among the population at risk, who gave a positive answer to the question \"Have you ever had asthma?\" in the second survey.

Incidence rates were computed by dividing the number of new cases by the total number of person-years. PY was computed as the