Biochemical and haematological data obtained over five consecutive years are presented on the male workforce of a large chemical company. The data have been subjected to statistical analysis and quantify the influence of age and the month of the year in which the samples were taken. The red cell count, white cell count, serum γ-glutamyl transferase, alanine transaminase and creatinine show significant and consistent seasonal variations. The previously published work on the influence of age on several indices is confirmed. Biochemical screening may be a method of detecting early signs of toxicity in man but such factors as age, sex and season must be taken into account when comparing the results obtained from exposed and non-exposed groups.
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