Exposure to Nanoparticles - Environmental and Human Health Impacts of Nanotechnology - page 326

torso of a wearer. The sampling head is typically positioned in the ' breathing zone ' ,
normally on the upper chest, of the wearer and the samples collected referred to
as personal samples. The use of personal sampling is widespread, since aerosol
concentrations in workplaces can have wide spatial variability and a personal
sample represents the closest approximation to actual exposures.
One important exception is the class of fi brous particles such as asbestos or glass
fi bre. Although some of the toxic mechanisms associated with asbestos exposure
remain unclear, it is known that ill health following exposure is associated with
physicochemical properties such as fi bre length and surface chemistry and the
persistence of the fi bres in the lungs. As a result, exposure is not characterised in
terms of averaged mass and composition, but rather by the number (concentration)
of fi bres in the air with a specifi c shape and composition (WHO, 1997). This method
relies on collection of fi bres in air onto a fi lter and manual counting by optical
microscopy to determine exposure and hence concentration.
A key issue in methods to assess exposure to such materials is the defi nition of
a fi bre. The World Health Organisation (WHO) method defi nes a fi bre as an object
with lengths greater than fi ve microns, a width less than three microns and a length
to width to ratio (aspect ratio) greater than 3 : 1 (WHO, 1997). The method is based
on determination of the airborne fi bre number concentrations by phase contrast
optical microscopy. Air samples are collected on a membrane fi lter by means of a
sampling pump, the fi lter is mounted on a microscope slide and is rendered trans-
parent. Fibres on a measured area of the fi lter are counted visually using phase
contrast optical microscopy and the number concentration in the volume of air is
calculated. The analysis relies on manual fi bre counting of a relatively small number
of fi bres and the method is recognised as one of the least precise analytical tech-
niques used in the occupational environment (HSE, 1998). The count is subject to
a number of systematic and random errors as well as individual counter bias includ-
ing limit of detection issues. The minimum visible width depends on the resolving
power of the microscope, the difference in the refracted index between the fi bre
and the medium and visual acuity of the analyst. The WHO method states that with
a good correctly adjusted microscope conforming to the specifi cation of the WHO
method, the limit of visibility is about 0.13 to 0.15

1 0 − 6 m. However, in practice,
the smallest visible fi bres will be about 0.2 to 0.25 microns wide. Since some fi bres
will fall below the limit of visibility, the count represents only a certain proportion
of the total number of fi bres present. Thus, the count represents only an index of
the numerical concentration of fi bres and is not an absolute measure of the number
of fi bres present.
It is clear from the discussion of exposure metrics that for nanometre size aero-
sols measurement of mass is not suffi cient. Particle number would be a more
appropriate metric than mass, though ideally the preferred metric may be particle
specifi c surface area. Hence, an ideal sampler to measure biologically relevant
exposure to nanoparticle aerosols would be a personal sampling device which col-
lects a relevant size fraction and provides either an instantaneous measure of the
sample specifi c surface area or which facilitates the off-line analysis of the sample
to provide a measure of specifi c surface area. Unfortunately, there are no systems
that currently provide a solution of this type.

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