Based on the epidemiological finding that individuals with schizophrenia tend to be born in winter/spring when compared to the general population, we examined (1) the strength and timing of this effect in Northern Hemisphere sites, and (2) the correlation between the season of birth effect size and latitude. Studies were located via electronic data sources, published citations, and letters to authors. Inclusion criteria were that studies specify the diagnostic criteria used, that studies specify the counts of schizophrenia and general population births for each month, and that subjects and the general population be drawn from the same birth years and catchment area. We extracted data from eight studies based on 126,196 patients with schizophrenia and 86,605,807 general population births and drawn from 27 Northern Hemisphere sites. Comparing winter/spring versus summer/autumn births, we found a significant excess for winter/spring births (pooled odds ratio = 1.07; 95% confidence interval 1.05, 1.08; population attributable risk = 3.3%). There was a small but significant positive correlation between the odds ratios for the season of birth comparison and latitude (r = 0.271,p < 0.005). Furthermore, the shape of the seasonality in schizophrenia births varied by latitude band. These variations may encourage researchers to generate candidate seasonally fluctuating exposures.

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The health effects of employment as a grain handler were studied by examining workers on two occasions, firstly, immediately before or soon after they were hired and again about two and a half months after they were employed. Over this time there was a substantial increase in the prevalence of cough, sputum, and eye irritation, accompanied by small pulmonary function changes suggestive of a restrictive ventilatory defect. No comparable changes in symptoms were observed over a similar number of months in grain handlers employed for an average of nine years or in control workers consisting of newly hired or long term civic outside labourers. The long term grain handlers, however, developed a similar change in their pulmonary function. These findings indicate the occurrence of a change in the health of grain elevator workers after a relatively brief duration of employment.