Pollen
The Danish Aerobiology Group of the Danish Asthma & Allergy
Association started the present pollen project in 1977 with continuous trapping and registration of
pollen and fungal spores. Since 1979 "Today’s Pollen Counts" have been published in the radio
and the daily press.
The first pollen trap, a Burkard-trap, was placed in Copenhagen
on the roof of DMI, app. 15 meters above ground level in 1977. Later similar traps were placed at the
hospitals in Viborg in 1979, and in periods also in Odense (1981-1991), Nykøbing Mors (1979-83) and
Nykøbing Falster (1979-83).
Pollen registration and pollen forecast is today performed by the
Danish Asthma & Allergy Association and DMI.
What is pollen and which pollen are the cause of allergy?
 Pollen
grains are released from the male flowers of flowering plants. The size of the pollen grains varies
from app. 10-100 microns - the largest pollen is just visible to the eye.
Normally only pollen from wind pollinated plants causes allergy.
These pollen, which easily are spread in the wind, are produced in large quantities, a birch tree for
example can produce 100 millions of pollen in a year and a beech tree about 30 millions. It is, however,
not pollen of all these plants, which causes allergy, e.g. allergy to pollen from spruce and pines are
rare, although the concentrations can be very high, as often seen as yellow dust clouds and covers of
pollen on lakes and other surfaces.
The most important allergenic pollen in Denmark listed according
to the time of the year are: alder, hazel, elm, birch, grass and mugwort.
Hayfever is a common complaint in Denmark. A questionnaire performed
in 1978 by the Danish Institute for Clinical Epidemology showed that app. 7% of the random selected
participants had had "hayfever or hayfever-like symptoms" during the last year. In 1987 and
1994 the investigation was repeated, and the percentages were in both cases about 10%.
How are the pollen counts performed?
 The
registrations of pollen is done according to standardised methods. 10 liter air per minute, corresponding
approximately to the human inhalation, is sucked through a narrow slit on the side of the pollen trap.
A wind fan secures that the slit is in the wind direction. The air impact on a sticky tape and pollen
and other particles in the air stick to the tape. The tape is mounted on a drum, which rotate once in
a week at a speed of 2 mm per hour. The exposed part of the tape is taken off the drum and is cut in
a length of 48 mm, corresponding to 24 hours of exposure. After mounting on a slide in a mounting media
containing Gelvatol the counting of the different pollen types are performed in 12 transverses in a
light microscope.
This way the pollen concentrations are determined for every 2 hour.
The concentrations may also be determined at a higher time resolution. In some research projects investigations
using a time resolution of one hour has been done. The practical work with pollen identification and
counting of the pollen is done by biology students from University of Copenhagen and horticulture students
from Royal Veterinary and Agricultural University under the auspices of a biologist from the Danish
Asthma & Allergy Association.
In the pollen season the tape is changed daily for Copenhagen, and
"Today’s Pollen Count" express the average concentration in numbers of pollen grains per m3
of air from 1 p.m. the previous day to 1 p.m. the day in question. Daily pollen counts are also performed
in part of the pollen season for Viborg. The tape is changed at 10 am. after which the slides are send
by plane to Copenhagen for analysis.
The Pollen Calendar
The pollen
calendar shows the distribution of the most important allergenic pollen types though the year. The calendar
is based on pollen monitoring from 1984 to 2001 for Copenhagen.
It shall be emphasised that the pollen calendar shows the normal
season and that the yearly variations can be large both concerning start, length and the strength of
the pollen season. This is especially the case for tree pollen, e.g. in some year the start of the season
for alder and hazel can be in early January, while in other years the start is delayed to March, this
is typical after a hard winter.
The dependence of weather
The
daily pollen concentrations are highly dependent on the meteorological conditions, as in general dry,
warm and sunny weather is favourable for high pollen concentrations. In contrary cloudy and rainy weather
will cause low pollen concentrations as the release of pollen will be minimised and also a wash-out
of pollen in the air. Windy and turbulent conditions will cause large dispersion of the pollen and low
concentrations.
Pollen forecast
Since 1981
daily pollen forecast for birch, grass and mugwort has been performed for Copenhagen and from 1983 also
for Viborg. The forecast is performed by the meteorologist on duty in collaboration with the person
who is doing the pollen count. The pollen forecast gives the average concentration for the following
day and is expressed in three levels:
| Pollen species |
few |
moderate |
high |
|
birch
|
< 30
|
30 - 100
|
> 100
|
|
grass and mugwort
|
< 10
|
10 - 50
|
> 50
|
The threshold levels are determined by medical doctors.
Prediction of the pollen concentrations needs profound knowledge
of the production of pollen, that is how many pollen is potentially available at a given time, the release
and the dispersion and deposition.
The mentioned factors are more or less unknown and will demand comprehensive
model analysis. In stead more simple statistical models are normally used. In Denmark such models has
been done, but the experiences so far have been relative poor, because subjective forecast methods has
shown to be just as successful.
The subjective forecasts are primarily based on the measured pollen
concentration and the weather forecast with special focus on the maximum temperature, sunshine coverage
and precipitation. In a normal season 70-80% of the pollen forecasts are correct and this is satisfactory
when considering all the complex processes determining the concentration of pollen in the air.
See today's pollen (on the Danish site)
See pollen archived data (on the Danish site)
|
