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ORIGINAL PAPER
Intrusion of airborne pollen through open windows
and doors
Juha Jantunen ÆKimmo Saarinen
Received: 27 April 2009 / Accepted: 21 May 2009 / Published online: 13 June 2009
ÓSpringer Science+Business Media B.V. 2009
Abstract The importance of the transport of pollen
by air movement into houses was evaluated using six
to eight simultaneously collecting rotorod-type sam-
plers, creating either a sampler line from outdoors to
inside the room, or a sampler grid inside a room. The
number of incoming pollen grains was highly depen-
dent on the outdoor concentration. The highest
concentrations inside (1–2 m distance) and outside
(1 m) the room were 600 and 3,250 grains/m
3
,
respectively, in the Betula pollen season and 1,980
and 5,080 grains/m
3
in the Pinus season. The pollen
concentration and the indoor/outdoor (I/O) ratio
decreased as the distance from the ventilation opening
increased. Inside the room at a distance of 1–2 m
28%, and at a distance of 3–5 m 12%, of the outside
concentration was recorded. In the lower part of the
opening the mean proportion was 63% and in the
upper part of the opening it was 40%. Efficient
ventilation with two open windows increased the I/O
ratio and enabled the pollen to spread throughout the
room. During the Pinus pollen season 3–35% of the
outdoor concentration was simultaneously recorded at
six locations inside the room with two open windows
and only 0.1–3.6% with one open window. At the
same point in the room the I/O ratio varied from\1to
35%, depending on the sampling conditions. Only a
minor effect on the I/O ratio was found between small
and large ventilation windows and the door, although
it was expected that more air and pollen grains would
come indoors through a larger opening.
Keywords Atmospheric transport
Betula Indoor exposure Pinus sylvestris
Pollen Ventilation
1 Introduction
People spend most of their time indoors, which
makes indoor air quality important to human health.
For allergic people indoor exposure to pollen and
fungal spores is of particular concern. Several studies
have evaluated the level of indoor exposure.
Although comparison of the results is difficult,
because of different sampling methods, ventilation
conditions, building designs, and geographical loca-
tions of the studies, the conclusions are very different.
Substantial amounts of pollen have been found
inside houses and public buildings (D’Amato et al.
1996; Enomoto et al. 2004; Takahashi et al. 2008),
but there have also been studies concluding that
pollen is rare in indoor air and occurs only when
pollen production is high (O’Rourke and Lebowitz
1984). For example, during the peak flowering period
for Betula, pollen concentrations indoors remained
mostly at a level barely inducing reactions even in the
most sensitive persons (Hugg and Rantio-Lehtima
¨ki
J. Jantunen (&)K. Saarinen
South Karelia Allergy and Environment Institute,
La
¨a
¨ka
¨ritie 15, 55330 Tiuruniemi, Finland
e-mail: jjantune@nic.fi
123
Aerobiologia (2009) 25:193–201
DOI 10.1007/s10453-009-9124-8
2007). Measurements inside buildings have usually
revealed concentrations 2–6% of the corresponding
outdoor concentrations (Spiegelman et al. 1963;
Yankova 1991; Carin
˜anos et al. 2004; Lee et al.
2006; Hugg and Rantio-Lehtima
¨ki 2007; Ishibashi
et al. 2008) but the average values have occasionally
been as high as 30–53% (Stock and Morandi 1988;
Sterling and Lewis 1998).
Pollen is carried in on the feet and bodies of people
and pets and by airflow through windows, doors and
ventilation holes (Enomoto et al. 2004; Ishibashi
et al. 2008; Vural and Ince 2008). Takahashi et al.
(2008) found that some Japanese cedar pollen could
gain access even through small holes in window sills
and closed ventilation ducts, and another study
indicates that little or no pollen comes in because
of air movement (O’Rourke and Lebowitz 1984).
We evaluated the importance of the transport of
pollen into houses through air movement by studying
indoor pollen concentrations when windows or a door
were opened. We used six to eight simultaneously
collecting rotorod-type samplers, creating either a
sampler line from outdoors to inside the room or a
sampler grid inside a room. The main study period
coincided with the Betula pollen season in early May,
when the daily mean values in southern Finland
between the years 2002 and 2008 peaked at 3,000–
15,000 pollen grains per cubic metre of air (pollen
monitoring data of South Karelia Allergy and Envi-
ronment Institute). The main objectives were to
determine:
1. how much pollen comes inside through ventila-
tion windows and doors;
2. the importance of the airflow and the size of a
ventilation opening to the indoor pollen concen-
tration; and
3. how far inside the room pollen grains are able to
float.
2 Materials and methods
Field sampling for airborne pollen was performed in a
two-story building in Joutseno, SE Finland, using
rotorod-type impaction samplers. Most of the data
were collected during the Betula flowering season
but, because of the short peak pollen period,
measurements were also taken during the Alnus and
Pinus sylvestris flowering season. The pollen grains
of Betula and Alnus are of approximately the same
shape and size (22–30 lm, 6.1–6.8 g 910
-9
, set-
tling speed 1.5–1.6 cm/s), whereas the Pinus pollen
grain is considerably larger (65–80 lm) and heavier
(18.4 g 910
-9
) and has a higher settling speed
(3.0 cm/s) (Ertdman 1969; Schwendemann et al.
2007).
Two types of sampling sets were used in the study:
(A) The intrusion of pollen through open windows
and doors was measured in three rooms with
ventilation openings of different sizes using six
simultaneously collecting samplers (Fig. 1a).
The samplers were located (1) 1 m outside the
opening, (2) in the lower and (3) upper parts of
the opening (10–20 cm from the frames), and
(4) 1–2 m and (5) 3–5 m inside the room. In
each set, one sampler was positioned at the
same location in the courtyard at a distance of
Fig. 1 a The intrusion of pollen through open windows and
doors was measured using six simultaneously collecting
samplers (Y). The sixth sampler was located in the courtyard.
bPollen dispersal inside a room was measured using eight
samplers (Out 1 m,Wl, and A–F). The pattern-filled columns
on the right represent tables (3) and storage shelves (2)
194 Aerobiologia (2009) 25:193–201
123
10 m from the building (6). Samplers in the
yard and inside the house were placed at a
height of 1 m from the ground or floor. Indoor
samplers were placed in line with the incoming
air. Ventilation windows in the second-floor
rooms had an area of 0.25 m
2
(0.35 90.72 m)
and 0.55 m
2
(0.42 91.30 m) whereas the
doors in the first floor room were 2.1 m
2
(1 92.1 m) in area. Altogether 26 data sets
(156 single measurements) were collected
between 2.4.2008 and 5.6.2008. The study
rooms in the second floor were vacuum-cleaned
between sampling times and the room in the
first floor every second or third time.
(B) Indoor pollen dispersal was measured using
eight simultaneously collecting samplers in the
second-floor room with small ventilation win-
dows (0.25 m
2
) (Fig. 1b). An outdoor sampler
was located 1 m outside the window and a
sampler was located in the ventilation opening
in the lower part of the window. Six samplers
were systematically placed 2 m from each other
throughout the room (4 98 m). Nine data sets
(70 single measurements) were collected
between 12.5.2008 and 6.6.2008. Four sets
were measured at a height of 1 m, and five sets
at a height of 0.1 m, from the floor. The room
was vacuum-cleaned between sampling times.
The airflow through an opening was monitored by
means of an anemometer placed at a height of 0.3 m
in the middle of the opening. During a constant
sampling time of 1 h 12–15 airflow values were
recorded at intervals of 1 min. The first five mea-
surements were taken 5–10 min after the beginning
of the sampling period, then 5–7 values at the end of
the sampling period. In addition, the minimum and
maximum airflow during the sampling period were
included to calculate the mean airflow. Altogether 19
sampling periods were used with two windows/doors
open on the opposite walls of the room, together with
16 periods with one opening only. Depending on the
wind direction, pollen counts were measured on
different sides of the rooms. Outdoor temperatures
were also measured. Indoor temperatures varied
between 20 and 22°C.
The mean values of the amounts of pollen
collected at the two ends of the U-shaped rods were
used for analysis. Pollen data were converted into
concentration values (grains/m
3
). The multiplication
factor was 0.80 =19(rod width (0.0019 m) 9rod
height (0.019 m) 9head diameter (0.08 m) 9p9
RPM (2,300) 9time rod used (60 min))
-1
. The
concentration of the main pollen was used in the
analysis, this being Alnus sp. from 2.4.2008 to
3.4.2008, Betula sp. from 3.5.2008 to 21.5.2008,
and Pinus sylvestris from 3.6.2008 to 6.6.2008. The
other species comprised only 2.5% of the total
number of pollen grains, on average.
The percentage proportions of pollen inside the
study rooms and in the opening were all calculated by
comparing the concentrations to the pollen count 1 m
outside the room. The sampling point near the
opening describes the concentration of incoming
pollen better than the sampler in the yard. The
reliability of the results was evaluated by comparing
pollen counts between the samplers in the yard and
outside the opening and also by comparing the
concentrations measured on the roof-top at a 1.5 km
distance from the study site with a Burkard volumet-
ric sampler (Hirst 1952).
Spearman’s correlation test was used to determine
a possible relationship between pollen concentrations
and measurement conditions: airflow through the
opening, number of open windows and doors, size of
ventilation openings, and the distance from the
opening. The difference between different concen-
tration levels, weak and strong airflow, and three
sizes of openings were statistically tested using
analysis of variance (ANOVA). The non-parametric
paired Wilcoxon test was used for a statistical
comparison between indoor/outdoor (I/O) ratios
measured at different heights and the number of
open windows.
3 Results
High pollen concentrations were recorded in both the
window and the door opening and inside the rooms
(Table 1). Compared with the outdoor concentration,
in the lower part of the opening the mean proportion
was 63% and in the upper part of the opening 40%.
Inside the room at a 1–2 m distance it was 28% of the
outdoor concentration, and at a 3–5 m distance 12%
of the outdoor concentration. The peak concentration
inside the room (1–2 m distance) was almost
Aerobiologia (2009) 25:193–201 195
123
600 grains/m
3
in the Betula pollen season and
2,000 grains/m
3
in the Pinus season.
The outdoor samples simultaneously measured
using rotorod-type samplers were highly comparable,
although more than half of the samples outside the
room (1 m from the building) were measured at the
height of the second floor (65%) and on the other side
of the house (54%) compared with the samples in the
yard (10 m from building). More than 20% mean
deviation from the concentrations of the yard and
Table 1 Pollen concentrations (grains/m
3
) in the courtyard, 1 m outside the room, in the lower and upper parts of the open windows
and doors, and at two distances inside the rooms at a height of 1 m
Date (time) Outdoor temp. Air flow Burkard (8–16) Outdoor Window/door Indoor I/O
Yard Out 1 m Lower Upper 1–2 m 3–5 m %
2.4.2008 (12:05)
a,b
11 0.4 275 109 82 63 52 7 2 2
2.4.2008 (13:25)
c,b
11 0.3 275 393 302 185 54 64 12 4
3.4.2008 (10:55)
d,b
9 0.4 223 59 54 33 31 5 3 6
3.4.2008 (12:10)
a,b
10 1.1 223 71 71
e
36 52 22 4 6
3.4.2008 (14:40)
c,b
11 0.4 223 216 239 234 87 102 12 5
3.5.2008 (10:25)
d,b
20 0.2 1,930 849 924 265 460 247 100 11
3.5.2008 (11:35)
a,b
20 0.9 1,930 2,353 3,248 888 508 598 276 8
3.5.2008 (12:50)
c,b
20 0.3 1,930 898 646 252 198 114 9 1
3.5.2008 (14:20)
a
20 0.7 1,930 364 290 140 227 32 1 0.3
4.5.2008 (11:55)
a,b
19 0.2 620 670 576 90 54 32 21 4
4.5.2008 (13:15)
c
19 0.0 620 120 107 143 67 94 4 4
4.5.2008 (14:25)
d
19 0.0 620 66 41 12 15 6 1 2
6.5.2008 (12:10)
d
6 0.4 51 18 48 25 8 2 2 4
6.5.2008 (13:20)
c
6 0.2 51 25 21 28 4 5 0.4 2
7.5.2008 (11:55)
a
9 0.1 17 33 27 6 7 0.4 0.4 1
7.5.2008 (13:35)
d,b
9 1.2 17 1 3 3 3 4 2 67
10.5.2008 (11:30)
d,b
14 1.0 171 84 70 36 20 38 30 43
10.5.2008 (12:40)
c
14 0.3 171 86
e
86 87 0.4 13 2 2
10.5.2008 (13:50)
a
15 0.4 171 155 153 89 56 1 1 1
11.5.2008 (11:20)
a,b
17 1.4 1,630 970 718 326 508 268 73 10
11.5.2008 (12:30)
c
17 0.6 1,630 543 366 306 188 211 149 41
11.5.2008 (13:40)
c,b
18 2.5 1,630 258 294 323 266 207 205 70
11.5.2008 (14:50)
d
18 0.2 1,630 235 181 119 38 35 34 19
3.6.2008 (14:30)
d
16 0.2 772 1,018 352 250 139 178 1 0.3
5.6.2008 (11:10)
c
20 0.6 5,500 1,834 2,156 1,684 9 110 78 4
5.6.2008 (12:20)
a,b
20 0.9 5,500 4,400 5,080 3,368 2,812 1,980 182 4
Means when Out 1 m =
3–86 (n=10) 11 0.5 182 55 51 33 20 10 4 8
107–366 (9) 16 0.6 987 367 254 199 125 85 48 19
576–5,080 (7) 19 0.6 2,720 1,711 1,907 982 650 478 118 6
The ratio of the indoor to the outdoor (Out 1 m) concentrations (%) is calculated from the results at a 3–5 m distance from the
openings. Measurement conditions are expressed by outdoor temperature (°C) and airflow (m/s) through the window/door opening
a
A room with small windows (0.25 m
2
)
b
Two windows or doors open
c
Doors (2.1 m
2
)
d
Large windows (0.55 m
2
)
e
Missing value replaced with the concentration in column ‘‘Yard’’ or ‘‘Out 1 m’’
196 Aerobiologia (2009) 25:193–201
123
outside the opening was found only on 3.6.2008,
with low concentrations on 4.5.2008, 6.5.2008, and
7.5.2008. Both increasing and decreasing trends were
observed during the sampling days (e.g. 3.4.2008 and
4.5.2008; Table 1), which increased the differences
between the results from the rotorod-type sampler
and the Burkard sampler (Fig. 2).
Concentrations outside the opening were divided
into three levels with approximately the same number
of measurements. The threshold values occurred in
large gaps between concentrations of 87–107 and
366–576 (Table 1). The outdoor concentration had a
strong correlation with the amount of incoming
pollen but only a minor effect on the I/O ratio
(Table 2; Fig. 3). For the lowest outdoor concentra-
tion (\10 grains/m
3
) the I/O ratio is probably either
too high to be purely due to chance, or else there was
already pollen present inside the room (Tables 1,3).
An increase in the airflow through the opening had a
positive correlation with the I/O ratio and there was a
significant difference in the I/O ratio between weak and
strong airflows (Fig. 4). Opening the second window
or door increased the airflow and correlated with the
indoor pollen concentration and I/O ratio, especially at
a greater distance (3–5 m) from the opening.
There was no statistical difference in the I/O ratio of
rooms with different ventilation openings of different
sizes, although the one with the smallest windows had
a slightly lower I/O ratio than the other rooms. A low
sampling height (0.2 m from floor) in the lower part of
the door probably resulted in different ratios to those in
the windows (1 m from floor) (Fig. 5). Again, the
difference between the lower and upper parts of the
door (mean =59%; SD =32%) was higher than
the corresponding ratio in the windows (-2%; 21 and
11%; 22%; Anova: P=0.000).
Because of the higher settling speed of Pinus
pollen grains, Betula had a higher ratio of the indoor
to the outdoor pollen count under the same sampling
conditions, such as the same room at a distance of 3–
5 m, the same airflow (±0.1 m/s), and the same
number of ventilation openings. The mean I/O ratios
in five comparative pairs (door on 11.5–5.6.2008,
large window on 11.5–3.6.2008, small window on
3.5–5.6.2008 in Table 1and 12.5–3.6.2008, 12.5–
6.6.2008 in Table 3) were 13% (SD =13%) of
Betula pollen and 2% (SD =3%) of Pinus pollen
(Wilcoxon: P=0.043).
Two opened windows increased both the airflow
and the dispersal of pollen inside the room. The
highest concentration in the middle of the room was
almost 100 grains/m
3
(9–10%) and at the back of
the room (5–6 m from the window) over 70 grains/m
3
(5–7%) (Table 3). The mean I/O ratios were 2.8
times higher when two windows were opened
(means =34%, SD =11 and 12%, SD =5%; Wil-
coxon test: P=0.028) and 5.3 times higher at floor
level compared with a height of 1 m (37%, SD =8
and 7%, SD =5%; Wilcoxon test: P=0.028). The
importance of the ventilation and the sampling height
for indoor pollen counts was distinguishable in the
four comparable data sets during the Pinus pollen
0
1000
2000
3000
4000
5000
29 1 3 5 7 9 1113151719212325272931 2 4 6
Betula (Burkard) Pinus (Burkard)
rotorod-type sampler
JuneMay
grains
/m
3
Fig. 2 Betula and Pinus pollen season measured on a rooftop using a Burkard sampler (grains/m
3
/day) and the concentrations
outside the study house measured by rotorod-type sampler (1-h samples)
Aerobiologia (2009) 25:193–201 197
123
season (Table 4). When only one window was opened
less than 1% of the outdoor pollen concentrations was
recorded throughout the room at a height of 1 m.
4 Discussion
Most of the people allergic to Betula pollen experience
symptoms when concentrations are [100 grains/m
3
(Viander and Koivikko 1978). Our study indicated that
these concentrations are commonly reached inside
houses if the door or windows are kept open during
the peak flowering season of abundant wind-
pollinated trees. High indoor concentrations of pollen
([100 grains/m
3
) were recorded fourteen times at a
distance of 1–2 m and nine times at a distance of
3–5 m. The number of incoming pollen grains was
highly dependent on the outdoor concentration, as
reported in previous studies (Sterling and Lewis 1998;
Lee et al. 2006). The highest concentrations both
outside (5,080 grains/m
3
) and inside the room (1,980
grains/m
3
) were recorded for Pinus pollen. Because of
an abnormally short peak in the Betula pollen period
the daily average exceeded 1,000 grains/m
3
on three -
days only, whereas the annual average from 2002 to
2007 was seven days (pollen monitoring data of South
Karelia Allergy and Environment Institute).
Pollen concentrations both indoors and outdoors
were generally higher than recorded in most earlier
studies. Outdoor concentrations have mainly been less
than 100 grains/m
3
, but the sampling strategy, peri-
ods, and probably also the locations inside the houses
were different (O’Rourke and Lebowitz 1984; Stock
Table 2 Spearman correlation matrix calculated using pollen counts and percentages (pollen counts at a sampling point/1 m outside
the room)
Increasing size
of ventilation
a
Increasing
airflow
Opening of second
window/door
Yard Out 1 m Burkard (8–16)
Air flow -0.191
1–2 openings -0.185 0.453*
Burkard (8–16) -0.012 0.159 0.129
Yard -0.062 0.178 0.206 0.870***
Out 1 m -0.037 0.222 0.267 0.979*** 0.863***
Wl 0.129 0.315 0.216 0.930*** 0.945*** 0.849***
Wu -0.216 0.309 0.381 0.776*** 0.786*** 0.740***
1–2 m 0.160 0.351 0.401* 0.802*** 0.840*** 0.806***
3–5 m 0.086 0.464* 0.484* 0.707*** 0.782*** 0.766***
Wl (%) 0.610*** 0.157 -0.123 -0.265 -0.227 -0.180
Wu (%) -0.197 0.461* 0.350 -0.176 -0.147 0.044
1–2 m (%) 0.351 0.395* 0.401* 0.006 0.054 0.097
3–5 m (%) 0.210 0.531** 0.504** 0.056 0.142 0.205
Sampling points were located in the courtyard, 1 m outside the room, in the lower (Wl) and upper (Wu) parts of the open windows
and door, and at two distances inside the room. The results of the sampling points were also compared to Burkard concentrations
(8 am–4 pm) measured at a 1.5 km distance from the study site. Pollen concentrations are given in Table 1
*** P\0.001; ** P\0.01; * P\0.05
a
Windows: 0.25 and 0.55 m
2
, doors: 2.1 m
2
0 %
10 %
20 %
30 %
40 %
50 %
60 %
70 %
Wl Wu 1-2m 3-5m
3 - 86 (n=10)
107 - 366 (n=9)
576 - 5 080 (n=7)
ANOVA
p= 0.224 p= 0.679 p= 0.655 p= 0.667
Out1m concentrations
(grains/m
3
)
Fig. 3 Pollen proportion in the lower (Wl) and upper (Wu)
parts of the open window and door, and indoors, compared
with the outside concentration in three concentration groups
198 Aerobiologia (2009) 25:193–201
123
and Morandi 1988; Sterling and Lewis 1998; Lee et al.
2006). In the same region in Southeast Finland, in the
hospital entrance hall and in two homes, indoor
concentrations were much lower (means 1.1–
3.4 grains/m
3
) but outdoor concentrations during the
sampling periods were also much lower (7–96 grains/
m
3
) (Hugg and Rantio-Lehtima
¨ki 2007).
The pollen concentration and the I/O ratio
decreased as the distance from the ventilation opening
increased. The highest pollen amounts are usually
found near ventilation windows and holes and in
lobbies (Kiyosawa and Yoshizawa 2001; Hugg and
Rantio-Lehtima
¨ki 2007; Takahashi et al. 2008). The
airflow through the opening and the sampling height
also had a significant effect on the ratio. In the absence
of air movement pollen grains settle (Fahlbusch et al.
2001; Enomoto et al. 2004). At the same point in the
room the I/O ratio varied from \1 to 35%, depending
on the sampling conditions. This explains why it is
very difficult to compare I/O ratios obtained in
Table 3 Pollen concentrations (grains/m
3
) 1 m outside the room, in the lower part of the window opening (Wl), and at six locations
inside the room (A–F) at 1–6 m distance from the window
Date Height Window open Air flow Out 1 m Wl A B C D E F I/O
1m 3m 3m 4m 5m 6m %
12.5.2008 (12:50) 1 1 0.4 85 49
a
2 2 0.8 0.4 0.4 1.3
12.5.2008 (14:10) 1 2 1.1 200 63
a
18 18 18 24 32 11.0
13.5.2008 (11:55) 0.1 1 0.8 5 3 2 2 3 2 0.8 2 39.3
13.5.2008 (13:15) 0.1 2 2.9 5 8 5 3 4 4 5 3 80.0
21.5.2008 (13:30) 0.1 2 1.7 11 6 4 9 2 8 2 2 40.9
3.6.2008 (10:20) 1 1 0.3 1,356 480 5 3 4 2 2 3 0.2
3.6.2008 (11:40) 0.1 1 0.2 1,110 274 40 28 6 30 4 4 1.7
6.6.2008 (12:55) 1 2 1.2 1,520 296 506 280 47 82 72 79 11.7
6.6.2008 (14:20) 0.1 2 1.1 1,006 694 352 260 93 98 40 60 15.0
Means
Two windows open, 1 m: 1.1 860 180 506
a
149 32 50 48 56 20.5
Two windows open, 0.1 m: 1.9 341 236 120 91 33 37 16 22 45.3
One window open, 1 m: 0.5 720 265 5
a
232120.8
One window open, 0.1 m: 0.5 558 138 21 15 5 16 2 3 11.4
The I/O ratio is the mean indoor concentration compared with the pollen count outside the room (Out 1 m). Sampling conditions are
expressed by the height of the samplers (m), the number of open windows, and airflow (m/s) through window
a
Missing values
0 %
10 %
20 %
30 %
40 %
50 %
60 %
70 %
Wl Wu 1-2m 3-5m
0.0-0.7 m/s (n=19)
0.8-2.5 m/s (n=7)
p = 0.017
ANOVA
p= 0.007 p= 0.008 p= 0.942
Fig. 4 Pollen proportion in the lower (Wl) and upper (Wu)
parts of the open window and door and indoors compared with
the outside concentration during a weak and strong airflow
through the opening
0 %
10 %
20 %
30 %
40 %
50 %
60 %
70 %
Wl Wu 1-2m 3-5m
Small window
Big window
Door
p = 0.596
ANOVA
p= 0.270
p= 0.287
p= 0.001
93%
Fig. 5 Pollen proportion in the lower (Wl) and upper (Wu)
parts of the open windows and door, and indoors, compared
with the outside concentration in the three study rooms with
different sizes of ventilation opening (small win-
dow =0.25 m
2
, large window =0.55 m
2
, door =2.1 m
2
)
Aerobiologia (2009) 25:193–201 199
123
different studies. If the sampling conditions and
distances from ventilation openings are not known,
it is virtually impossible to compare the results.
Unexpectedly, the size of the ventilation opening
had only a minor effect on the I/O ratio, although
more air and pollen grains should automatically flow
indoors through a larger opening. Pollen concentra-
tions in the lower and the upper parts of the opening
differed between the windows and the door. The low
sampling height at the door, which was 20 cm from
the floor and 70 cm from ground level, probably
increased the amount of pollen. The difference
between concentrations in the lower and upper parts
of the opening was especially high for three mea-
surements, when only 4–14% of the concentrations in
the lower part of the door was found in the upper part
(6.5.2008, 10.5.2008, and 5.6.2008 in Table 1). In
each case measurements were taken only when one
ventilation door was open, probably forcing the air to
rotate vertically inside the room. Cool air (6, 14, and
20°C) flows in at the bottom and warmer indoor air
(20–22°C) flows out at the top of the door opening.
Guidelines for good indoor air quality tend towards
a preference for short but efficient ventilation now and
then. However, ventilation achieved with two open
windows also enables pollen to spread throughout the
room. During the Pinus pollen season 3–35% of the
outdoor concentration was simultaneously recorded in
the room with two open windows and only 0.1–3.6%
in that with one open window. The locations of the
ventilation openings and furniture determine the
floating and settling of the incoming particles
(Richmond-Bryant et al. 2006). Because of the
position of the window the pattern of the decreasing
trend in the amount of pollen is a clockwise rotation in
the study room. Near the window the pollen concen-
tration in the outgoing air tends to be slightly higher
than at the opposite side of the room.
It can be concluded that large amounts of pollen can
float inside the house through open windows and
doors, if the outdoor pollen concentration is high. The
number of airborne pollen grains decreased rapidly
with increasing distance from ventilation openings.
With increased airflow through an opening more
pollen grains could come inside and penetrate deeper
into a room. Allergic people can reduce indoor
exposure to pollen by keeping the windows and doors
closed during the peak flowering season, or by using
appropriate air filters in ventilation windows and
ducts. The time when ventilation occurs has an effect
on the intrusion of pollen. The highest pollen concen-
trations are recorded during a dry and sunny afternoon
and evening, but pollen grains can be found through-
out the day during the peak pollination period (Rantio-
Lehtima
¨ki et al. 1991; Clot 2001). Rainy weather
usually reduces the pollen concentration even during
the peak flowering period. Filters and closed doors are
unable to keep all pollen grains outdoors, because
pollen can be carried in by the house’s occupants and
pets (Ishibashi et al. 2008; Takahashi et al. 2008).
Acknowledgments The study was financially supported by
the South Karelia Regional Fund of the Finnish Cultural
Foundation. We thank Auli Rantio-Lehtima
¨ki for her valuable
comments on the manuscript.
Table 4 Outdoor pollen concentrations and the ratio of the indoor to the outdoor pollen counts at six locations inside the room (A–F
in Fig. 1b)
2 windows, 1 m/1window, 1 m Indoor
2 windows, 0.1 m / 1 window, 0.1 m
—a) 33.3% / 0.4% b) 18.4% / 0.2%
Outdoor concentration Airflow (m/s) — 35.0% / 3.6% 25.8% / 2.5%
1006 / 1110 grains/m31.2 / 0.3
1520 / 1356 grains/m31.1 / 0.2 c) 3.1% / 0.3% d) 5.4% / 0.1%
%7.2/%7.9%5.0/%2.9
e) 4.7% / 0.1% f) 5.2% / 0.2% —
—%4.0/%0.6%4.0/%0.4
At each sampling point four values are given. The first column indicates the results measured when two windows were opened
(incoming air near point a and outgoing air near point f) and the second corresponding numbers when one window was opened (near
point a). The upper rows were measured inside the room at a height of 1 m, and the lower rows at a height of 0.1 m, during the Pinus
pollen season on 3.6 and 6.6
200 Aerobiologia (2009) 25:193–201
123
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