Threshold values of grass pollen (Poaceae) concentrations and increase in emergency department visits, hospital admissions, drug consumption and allergic symptoms in patients with allergic rhinitis: a systematic review

Airborne grass (Poaceae) pollen measurements are used in public warning systems to inform people about the risk of allergic symptoms. However, there is no consensus about which exact thresholds of pollen concentrations provoke the allergic symptoms. The aim of this study was to review the relevant scientific information on the relationship between grass pollen concentrations and the occurrence of emergency department (ED) visits, hospital admissions (HA), drug consumption and allergic symptoms. Literature search was conducted by experts' consultation and snowball strategy. Studies meeting the criteria for inclusion were assessed regarding their risk of bias (RoB). A high RoB resulted in exclusion of the study from data synthesis. Extensive data were extracted and qualitatively compared. The review is registered in PROSPERO. 32 Studies were eligible while 18 showed a low RoB and were qualitatively synthesised. Emergency department visits and hospital admissions were mostly investigated. Threshold values of 10 grains/m(3) and 12 grains/m(3) were reported for ED visits and HA. Evidence exists that an increase of 10 grains/m(3) of air leads to a significant increase in adverse health outcomes. Especially at a three-day lag, adverse health effects were shown. Variations in exposure and outcome measurement make the definition of pollen thresholds difficult. As a consequence, no defined pollen threshold values could be identified. Studies with uniform exposure measures and statistical methods are necessary to gain a better understanding of the impact of grass pollen on human health. Determining personal thresholds could be beneficial for affected people.