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.