Pollen nightmare: elevated airborne pollen levels at night
Aerobiologia
Pollen nightmare: elevated airborne pollen levels at night
Ł. Grewling . P. Bogawski . M. Smith 0
0 . Grewling (&) P. Bogawski M. Smith Laboratory of Aeropalynology, Faculty of Biology, Adam Mickiewicz University , Umultowska 89, 61-614 Poznan , Poland
-
High airborne pollen concentrations are generally
associated with daylight hours when it is sunny and
warm and plants release pollen into the air (Alca´zar
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 Poznan´, 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 (Gala´n 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/m3 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,
however, that the frequency of higher night-time mean
or maximum airborne pollen levels varied depending
on pollen type, ranging from *10.0 % for mugwort to
*35 % for grass, birch and alder, and *60 % for
ragweed (Fig. 1). The magnitude of mean and
maximum concentrations of airborne pollen also varied
depending on pollen type examined (Fig. 2). For the
majority of pollen types investigated (i.e. alder, grass
and mugwort), mean pollen levels were significantly
lower at night. Differences between daytime and
night-time concentrations of airborne grass and alder
pollen were less distinct; the ratio between daytime
and night-time levels varied from 1.3 to 1.6,
respectively. On the other hand, it was found that maximum
atmospheric birch pollen concentrations were almost
the same at night and day (706 and 707 pollen/m3,
respectively). Mugwort recorded very low night-time
pollen concentrations; the mean and maximum
Artemisia pollen levels were significantly lower
during night than during the day (p \ 0.000).
Interestingly, for Ambrosia, another member of the
Asteraceae family, maximum night-time pollen
concentrations were over 30 % higher than recorded
during the day.
Fig. 1 Frequency of 24-h periods with A mean and B maximum
pollen levels higher at night-time (20:00–08:00)
Convective heat transfer, i.e. pollen laden air rising
to the upper atmosphere in convection currents during
daytime, is likely to be responsible for elevated pollen
levels at night. This is because pollen-bearing air
descends at night as it cools, thereby increasing the
concentrations of pollen at ground level (Norris-Hill
1997). Other factors also need to be considered,
however, such as the plant species involved and the
distance that the pollen grains have to travel from the
source. For instance, high concentrations of
atmospheric grass pollen recorded at night might partly be
the result of several grass species releasing their pollen
in late evening (Peel et al. 2 (...truncated)