Exposure to Nanoparticles - Environmental and Human Health Impacts of Nanotechnology - page 330

not been validated. Despite the diffi culties, this is currently the only practicable
approach for assessing HARN number concentration.
Measurement of mass concentration. An assessment of mass could be based on a
size selective personal sampler with a pre-determined cut-off point, either 100 nm,
or at some other point in the nanometre size range considered to be more appro-
priate. The sample could be analysed by weighing or by chemical analysis. There
are no commercial devices of this type currently available, but there is no reason
in principle why they could not be developed. Particle size selective stages based
on diffusion would make appropriate pre-selectors for devices of this type. However,
at fl ow rates typical for current personal samplers (up to fi ve litres per minute) one
limitation could be the small mass that would be collected. Given a typical limit of
detection for a gravimetric sample collected on a fi lter of 0.01 mg, the lowest mea-
surable concentration based on a full shift collection would be 0.02 mg/m 3 .
An important issue here relates to what amount of nanoparticle material it would
be necessary to measure by this or any other method. To be useful, a measurement
method needs to have a limit of detection that is lower than that at which it is
considered that health effects might occur based on either toxicological or epide-
miological studies. For all new nanoparticles and for many existing nanoparticles,
a health effects level has not yet been established. Health effect levels for any
nanoparticle are likely to be highly dependent on the specifi c particle, its morphol-
ogy, surface, composition and size.
Provided that a satisfactory judgement about an effective level could be made,
the limitation of detection issue could be overcome by using a high volume static
device with an appropriate cut-off point. However, at this point no devices of this
type are in common use nor have they been used to assess concentration levels
associated with nanoparticle production.
A more recent device is the Electronic Low Pressure Impactor (ELPI) sold in
commercial form by Dekati (www.dekati.com). It is claimed that this device can
measure particle size distribution and concentration in the size range 7- 10 nm. In
this device, sampled particles are charged and then passed into a low pressure
impactor with electrically isolated collection stages. The electrical current carried
by the charged particles onto each impactor stage is measured in real time by a
sensitive multi-channel electrometer. The particle collection into each impactor
stage is dependent on the aerodynamic size of the particles. After the collection
period, collected particles can be removed from the impactor stages for further
analysis. Other low pressure impactors such as the nanoMoudie (http://applied-
physicsusa.com/moudi.asp) are also available.
A major limitation of all of these measurement methods is that they cannot
discriminate agglomerates of nanoparticles from single larger particles. As evidence
does indicate that agglomerated nanoparticles can have increased toxicity com-
pared to larger particles (Oberdorster et al. , 1994), this limits the usefulness of these
methods to assess exposure to aerosols of this type. (It is very probable that most
nanoparticle aerosols will be agglomerated to some extent.)
Measurement of specifi c surface area concentration. This issue of agglomeration
further indicates that measurement of specifi c surface area is preferable, although