Exposure to Nanoparticles - Environmental and Human Health Impacts of Nanotechnology - page 323

signifi cantly. For example, if a prune is seen as approximately a sphere then it has
approximately the surface area of a smooth sphere of the same diameter; but if the
surface wrinkles are included then its surface area becomes notably larger. Similarly,
with techniques that use adsorption of gas molecules on to the surface of the par-
ticle to measure the surface, the smaller the gas molecule chosen, the more it can
enter the wrinkles on the particle surface. Nitrogen as the gas molecule for adsorp-
tion and measurement of specifi c surface area may be a good choice, but other
gases (or other surface measurement techniques) are worth considering. ' Biologically
relevant' specifi c surface area is likely to be that accessible to biologically relevant
molecules in the body, although this is not a physically meaningful measurement
and is, therefore, diffi cult to measure and standardise. Likely, this hypothetical
property will vary between organism and between organ/cell type.
While a strong case may be made for using aerosol specifi c surface area as a dose
metric, this may not be universally so. For example, for fi brous aerosols such as
asbestos and glass fi bre and carbon nanotubes, the currently accepted best approach
is to count fi bres (i.e. the metric is particle number concentration). Some types of
nanoparticle could be considered fi bres. Carbon nanotubes, for example, have high
aspect ratios and are highly durable. They can be manufactured with lengths well
within the range conventionally considered to be a fi bre (although their diameters
are much smaller than could be detected with current standard counting methods,
which are based on optical microscopy).
There are also practical issues to consider in selection of a metric. For example,
the concentration of nanoparticles might be very small in terms of mass, quite large
in terms of specifi c surface area and huge in terms of particle number. If nanopar-
ticles were present in the air with larger particles then, depending on the size dis-
tribution, the mass concentration would be totally dominated by the larger particles
and the contribution of the nanoparticles would be almost entirely undetected. In
contrast, the number concentration would be totally dominated by the nanoparticle
component.
Thus, it is apparent that measuring exposures against a mass concentration alone
is unlikely to be suffi cient for nanoparticles. It is probable that there is a need to
consider characterising exposures against specifi c surface area and number concen-
tration until further information and improved methods are available. For each of
these exposure metrics, but particularly in the case of mass concentration, size-
selective sampling will need to be employed to ensure only particles within the
relevant size range are sampled.

8.3.2.2 Dermal Exposure

The importance of dermal exposure to hazardous substances continues to increase.
The UK. Health and Safety Executive (HSE) estimates that there are around 85 000
cases of work-related skin disease at any one time, with people employed as brick-
layers, laboratory technicians and hairdressers being at particular risk (Hodgson
et al. , 1993). Substances that are considered to be potentially harmful via dermal
exposure include pesticides such as methyl parathion and solvents such as carbon
tetrachloride. Harmful effects arising from skin exposure may either occur locally