Pollen nightmare: elevated airborne pollen levels at night

High airborne pollen concentrations are generally associated with daylight hours when it is sunny and warm and plants release pollen into the air (Alcázar et al. 1999; Dahl et al. 2013). In contrast, cooler nighttime periods are usually considered to be the time of low-allergy risk. This opinion is often reflected in pollen allergy avoidance strategies presented by the media, where the most commonly repeated recommendation is to stay indoors during the day and plan outdoor activities for the evening. However, there is evidence to suggest that elevated concentrations of airborne pollen might also occur during the evening (e.g. Norris-Hill and Emberlin 1991). So, is the night really a time of low-allergy risk? We present the results of the comparative analysis of pollen concentrations during daytime and night-time hours for five allergenic pollen types (Burbach et al. 2009), i.e. alder (Alnus sp.), birch (Betula sp.), grasses (Poaceae), mugwort (Artemisia sp.) and ragweed (Ambrosia sp.). Airborne pollen grains were collected by volumetric trap (Hirst 1952) in Poznań, Poland (1996–2013). The trap was sited on the roof at the height of 33 m, approximately 1 km south-west of the city centre (52 240N 16 530E). Two pollen-counting methods have been applied. From 1996 to 1999 pollen data were counted along twelve vertical transects, while from 2000 to 2013 along four horizontal transects. Both counting methods have been shown to produce comparable results and are recommended by the European Aerobiology Society (Galán et al. 2014). The following time intervals were selected to reflect airborne pollen levels during night and day: 08:00–20:00 (equivalent of daytime, 12 h) and 20:00–08:00 (i.e. night-time, 12 h). This division was made to distinguish the time period that is not generally considered hazardous for allergy patients (from late evening to early morning). The rejection of low concentrations of atmospheric pollen guarantees more robust data (Buters et al. 2012), and so only 24-h periods (from 08:00 to 08:00 next day) with mean pollen levels C15 pollen/m were selected for analysis (n = 2177). The bi-hourly pollen concentrations recorded during selected days were averaged for 12-h periods to get mean concentrations for day and night. Daytime and night-time airborne pollen concentrations (mean and maximum values) were compared using the nonparametric Mann–Whitney U test (Real Statistics Add-in to Excel). In addition, the frequency (%) of 24-h periods with mean and maximum pollen levels higher at night has been calculated. Analysis of mean and maximum pollen levels recorded from 08:00 to 08:00 showed that higher atmospheric pollen concentrations were more frequently recorded during daytime hours. It was noted, Ł. Grewling (&) P. Bogawski M. Smith Laboratory of Aeropalynology, Faculty of Biology, Adam Mickiewicz University, Umultowska 89, 61-614 Poznan, Poland e-mail: [email protected]