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these and other, less frequently studied potential risk factors for their relation to the incidence of asthma among 3- and 4-year-old children. The objective of the study was to estimate the contribution to asthma incidence of chemical, physical, and biologic indoor environmental factors, as well as family history of asthma and past infections, after accounting for personal susceptibility. A case-control study was carried out to meet this objective.

MATERIALS AND METHODS
Case ascertainment

Cases were 3- and 4-year-old children with a first-time diagnosis of asthma made by a pediatrician. We chose this age group to avoid the problem of differential diagnoses for asthma which is more likely at younger ages, and to allow for a plausible but reasonably short time period for risk factor assessment. Cases were recruited between January 1988 and December 1990 at the emergency room of Hôpital Sainte-Justine, the larger of two university-affiliated pediatric centers in Montreal, Quebec, Canada. A computerized roster is kept in the hospital’s emergency room which includes the age of the child, the discharge diagnosis, and the child’s medical record number. From this roster, 3- and 4-year-old children with a diagnosis compatible with any of those listed under International Classification of Diseases, Ninth Revision, code 493 had their hospital medical records checked for previous attendance with a similar diagnosis. Known (previously diagnosed) cases were rejected. A second screening for eligibility took place when the parents were asked whether the child had ever been diagnosed by a physician as having asthma. An additional criterion for eligibility was that the child reside in the greater Montreal region.

Control ascertainment

Controls were children of the same age (± 1 month) and the same census tract (in the urban area) or postal code (in the rural area) as the case at the time of diagnosis. A census tract is defined in the Canadian Census Dictionary (3) as a small geostatistical unit including a mean of about 4,000 persons with maximum economic and social homogeneity. In rural areas surrounding the city, a postal code area indicates a region served by the post office or the postal branch. Controls were chosen from computerized family allowance files for the target region. The family allowance is a government stipend awarded to all families with children. Eligibility for the family allowance program is based on the following: a child must be less than 18 years of age and must reside in Canada. In addition, at least one parent must be a Canadian citizen, a person admitted to Canada as a permanent resident according to the terms of the law, or a person who has been admitted to the country as a visitor or who is holding a visiting permit for at least 1 year, and whose revenue is taxable (4). For reasons of cost, the latest available files from 1987 were used during 1988 and most of 1989. The 1989 files were used until the end of the study in 1990. All children who were eligible on the basis of age and census tract or postal code were enumerated from 1 to n. To choose the first control, we randomly generated a number between 1 and n. If, based on a search of readily accessible sources of information on addresses and telephone numbers, this control was not available, the procedure was repeated.

Data collection

The list of potentially eligible cases and controls was given to a first interviewer, who contacted the parents to confirm that the case was one with a first-time diagnosis by a physician and that the control had had no previous diagnosis of asthma made by a physician. If the parents were willing to participate, an appointment for the interview was made. A telephone interview was conducted by a second interviewer who was blind to the case/control status of the child. The interview had to take place for both cases and controls within 1 month of the case child’s visit to the emergency room.



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