Atmospheric Nanoparticles - Environmental and Human Health Impacts of Nanotechnology - page 170

opening the outside door in the absence of a strong outdoor source increased the
indoor particle number concentrations by about 180%, while it decreased PM 2.5
concentrations (He et al. , 2004 ).
In the absence of signifi cant indoor sources or under unoccupied conditions,
indoor particle number concentrations are smaller than those outdoors and the
geometric mean diameters of the particle number size distributions are larger
indoors than outdoors (Hussein et al. , 2005a). In this case, an appreciable fraction
of the indoor NPs is of outdoor origin and indoor and outdoor concentrations are
well correlated showing that outdoor emissions infl uenced greatly the indoor con-
centration levels (Harrison et al. , 2004 ; Diapouli et al. , 2007); in particular emissions
from motorized vehicles are signifi cant (Zhu et al. , 2005). Filtration and other pen-
etration processes reduce particle number concentrations and reduce the number
concentrations of small particles more than those of coarser particles. Also, deposi-
tion of aerosol particles onto indoor surfaces and volatilization (see below) reduce
nanoparticle concentrations in indoor air (Riley et al. , 2002 ). Various indoor - to -
outdoor ratios (I/O) ranging from 0.1 to 0.8 have been found for ultrafi ne particles
(Koponen et al. , 2001 ; Harrison et al. 2004 ; Hussein et al. , 2005a, 2006; Diapouli et
al. 2007). Differences in building construction and in ventilation conditions or air
exchange ratios contribute to the different indoor-to-outdoor relationships observed
(Zhu et al. , 2005 ; Guo et al. , 2007). Different ranges of particle sizes measured may
also contribute to the variability, since indoor-to-outdoor ratios strongly depend on
the particle size (Koponen et al. , 2001 ; Hussein et al. , 2005a, 2006; Zhu et al. , 2005 ).
The ratio is larger for particles of 70-100 nm (from 0.6 to 0.9) than for particles of
10-20 nm (from 0.1 to 0.4). An I/O ratio as low as 0.1 was found for particles ranging
from 8 to 25 nm in diameter (Koponen et al. , 2001). In the presence of indoor activi-
ties or under occupied conditions, indoor particle number concentration levels
cannot be directly estimated from outdoor number concentration measurements
(Hoek et al. , 2008 ).
Infi ltration of particles in indoor atmospheres may result in volatilization of the
most volatile particles. It has been demonstrated that ammonium nitrate is trans-
formed into ammonia and nitric acid when it penetrates into homes (Lunden et al. ,
2003). The equilibrium between ammonium nitrate and the gaseous components
ammonia and nitric acid is shifted towards the gaseous components because of
change of temperature, relative humidity and/or concentrations in indoor environ-
ments. Since available studies have shown that outdoor NPs are mainly comprised
of volatile species (particularly those from vehicular emissions), this suggests that
NPs of outdoor origin may decrease in size or evaporate entirely as they penetrate
into indoor environments during periods when the indoor environment is warmer
than the outdoors or because of different concentrations of volatile species. The
resulting changes of outdoor particles will likely affect their toxicity and has impli-
cations for human exposure.
People are also exposed to NPs in occupational indoor settings or during a daily
commute. Ultrafi ne particle concentrations (

100 nm) may reach 10 8 particles cm − 3
in photocopy centres with a peak concentration at a size of about 50 nm (Lee and
Hsu, 2007). Some printers also generate NPs with various emission rates (from zero
to the equivalent of smoking a cigarette) and various mean sizes (35- 94 nm) (He

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