Occurrence of Occupational Asthma: Material and Methods

All of these hospitals provide care to a significant indigent population. All patients with the primary diagnosis of asthma (ICD-493) between the ages 20 to 65 years discharged from these three hospitals in 1990 were contacted in the summer of 1991. The patients physician was first sent a letter asking if he had any objection to our contacting his patient. None of the physicians objected. Each patient was sent an introductory letter and a consent form. Each patient was interviewed over the phone by the same interviewer. The interviewer administered a standardized medical questionnaire that included questions on symptoms, medication, and a lifetime occupational history. The same question used in the study of Social Security disability recipients regarding whether the patient thought their asthma was work related was included in the questionnaire. This question is: “Was your asthma caused by bad working conditions such as smoke or chemicals?”
By reviewing the completed questionnaires, each patient was classified by one of the authors, a board-certified internist and occupational medicine physician, according to criteria of the National Institute for Occupational Safety and Health (NIOSH) (see Table 1).
All patients met criteria A because their primary hospital discharge diagnosis was asthma. If the patient only met criterion A, he was classified as not having occupational asthma. To meet criterion B, a patient must have reported having been bothered at work by shortness of breath, chest tightness, or wheezing. In addition, these symptoms must have begun after the patient started a new occupation and at least initially improved when they were away from work either on the weekends or on vacation. If the patient did not know if the symptoms improved at these times, then in order to meet criterion B, he must have stated that the symptoms got worse during the day when he worked and that they got worse throughout the work week. For a patient with reactive airways dysfunction syndrome (RADS) to meet criterion B, he had to have evidence that his asthma developed after a short-term exposure at work. However, an evaluation regarding whether the patient had nonspecific airway reactivity was not performed. Patients met criterion С if their work history indicated that their symptoms developed at a job previously associated with occupational asthma according to the medical literature. If a patient had a physician diagnosis of asthma (criterion A), an association between symptoms of asthma and work (criterion B), and workplace exposure to an agent or process previously associated with occupational asthma (criterion Cl), he was considered to have probable occupational asthma. Charts were not reviewed, and thus we did not have access to pulmonary function tests that would be necessary to determine the presence of criteria C2-C4. If the patient met only criteria A and B, then he was classified as having possible occupational asthma.
Results were analyzed by x* analysis. Fishers exact test analysis was performed where the expected was less than 5. To account for differences in the percentage of patients interviewed in each hospital and differences in the percentage of patients with possible occupational asthma in each hospital, rates were adjusted in two ways. First, rates were calculated assuming none of the noninterviewed patients had probable or possible occupational asthma. Second, the rates were calculated assuming the noninterviewed patients had the same rates of probable and possible occupational asthma as the interviewed patients.

Table 1—NlOSH Criteria

A. A physician diagnosis of asthma and
B. An association between symptoms of asthma and work and
C. Any one of the following criteria:
1. Workplace exposure to an agent or process
2. Significant work-related changes in forced expiratory volume in one second (FEV,) or peak expiratory flow rate (PEFR) or
3. Significant work-related changes in airway responsiveness as measured by nonspecific inhalation challenge or
4. Positive response to inhalation provocation testing with an agent to which the patient is exposed at work.
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