Suicide risk in relation to air pollen counts: a study based on data from Danish registers

OBJECTIVES Since the well-observed spring peak of suicide incidents coincides with the peak of seasonal aeroallergens as tree-pollen, we want to document an association between suicide and pollen exposure with empirical data from Denmark. DESIGN Ecological time series study. SETTING Data on suicide incidents, air pollen counts and meteorological status were retrieved from Danish registries. PARTICIPANTS 13 700 suicide incidents over 1304 consecutive weeks were obtained from two large areas covering 2.86 million residents. PRIMARY AND SECONDARY OUTCOME MEASURES Risk of suicide associated with pollen concentration was assessed using a time series Poisson-generalised additive model. RESULTS We noted a significant association between suicide risk and air pollen counts. A change of pollen counts levels from 0 to '10-<30' grains/m(3) air was associated with a relative risk of 1.064, that is, a 6.4% increase in weekly number of suicides in the population, and from 0 to '30-100' grains, a relative risk of 1.132. The observed association remained significant after controlling for effects of region, calendar time, temperature, cloud cover and humidity. Meanwhile, we observed a significant sex difference that suicide risk in men started to rise when there was a small increase of air pollen, while the risk in women started to rise until pollen grains reached a certain level. High levels of pollen had slightly stronger effect on risk of suicide in individuals with mood disorder than those without the disorder. CONCLUSIONS The observed association between suicide risk and air pollen counts supports the hypothesis that aeroallergens, acting as immune triggers, may precipitate suicide.