Chemistry Reference
In-Depth Information
resuspension of particles by human movement and particle formation from organic
components with ozone may have been significant indoor sources. Regression
analyses of indoor and outdoor concentrations and indoor source use data within
the EXPOLIS and RIOPA studies also documented that a large fraction of the
indoor PM
2.5
source contribution was unidentified [
8
,
15
]. Another approach is to
limit the calculation of I/O ratio to hours with little indoor source activity, typically
the nighttime [
16
,
17
,
19
]. The influence of this approach was demonstrated in a
study of hourly ultrafine particle concentrations in four European cities [
13
]. When
all 24 h were considered, the correlation between simultaneously measured hourly
indoor and residential outdoor concentrations was weak (range 0.25-0.62). When
only the nighttime period was included, a strong correlation was found in the four
cities between indoor and outdoor ultrafine particle concentrations (range
0.64-0.87).
Many studies have used the
infiltration factor
(
F
inf
), defined as the fraction of
outdoor particles that penetrate indoors and remain suspended [
11
]. Compared to
the I/O ratio, a major advantage of the use of the infiltration factor is that it is not
affected by indoor sources. Equation (
3
) describes the factors that influence it,
which includes both penetration and decay rates.
Finally studies have reported the
penetration factor
which describes penetration
and not decay. Penetration factors are not directly observable and they are usually
calculated from infiltration rates and decay rates [
11
].
4.2
Indoor-Outdoor Relationships of PM
2.5
H
¨
nninen and coworkers [
9
] have recently evaluated the original data of European
studies of indoor-outdoor relationships for PM
2.5
. The overall average infiltration
factor was 0.55, illustrating significant infiltration of outdoor fine particles. A recent
review including European and North American studies reported infiltration factors
from 0.3 to 0.82 for PM
2.5
[
11
]. Since western people spend on average about 90%
of their time indoors, human exposure to fine particles of outdoor origin largely
occurs indoors. Infiltration factors were consistently higher in the summer season
than in the winter season (Fig.
6
). The implication is that for the same outdoor
concentration, actual human exposure of subjects in the summer season is higher
than in the winter season [
9
]. This could be one of the explanations for the often
higher health effects reported in the summer months compared to the winter months
in epidemiological studies that are based upon outdoor concentrations. Higher
infiltration factors in the summer are explained by higher air exchange rates in
the summer season compared to the winter. The air exchange rate (AER) was
measured in the EXPOLIS and RUPIOH study in different seasons. Mean AER
(h
1
) was 1.38 (summer) versus 0.56 (winter) in EXPOLIS and 1.07 (summer)
versus 0.36 (winter) in RUPIOH [
9
]. Differences in infiltration factors across
Europe were less consistent, with, e.g., little difference between Northern and
southern European cities (Fig.
6
). Methodological differences across studies may
have complicated assessment of
regional differences; however,
two studies
