A systematic review of threshold values of pollen concentrations for symptoms of allergy

Pollen threshold values used in public warning systems are intended to inform people of the risk of developing allergy symptoms. However, there is no consensus about which pollen concentrations provoke allergy symptoms. The aim of this systematic review was the evaluation of studies investigating the relationship between pollen concentrations (alder, ash, birch, hazel, mugwort and ragweed) and the number of cases in which participants visited a doctor, drug consumption and allergy symptoms. This systematic literature review is registered in PROSPERO (CRD42019112369). A PubMed search was applied and enriched by consultation with experts and a snowball strategy. The included studies were checked for risk of bias (RoB), and extensive data were extracted and compared. Of 511 studies, 22 were eligible according to the previously established inclusion criteria, and 17 from these showed a low RoB. The strongest evidence was reported for ash (Fraxinus) pollen, where an increase of number of doctor’s visits at an interquartile range (IQR) of 18–28 grains/m³ was detected by three studies. Five studies about birch (Betula) pollen showed a threshold value of 45 grains/m³ for increased drug consumption. The evidence of a threshold value was limited for alder (Alnus), hazel (Corylus), mugwort (Artemisia) and ragweed (Ambrosia) pollen. The inconsistent results concerning all types of pollen, except ash pollen, can be the result of multiple factors, e.g., age, gender, allergen content of pollen and individual sensitivity. These influencing factors should be investigated more closely in future research.