Environmental Risks of Nanomaterials - Nanotechnologies for Water Environment Applications - page 606

where C is concentration of the substance, T is from the slope of a dose-response curve.
The T used to be estimated by HQ (Hazard Quotient), the ratio of the exposure dose or
concentration to the reference dose or concentration. The HQ is defined as:
HQ = E/RfD (Eq. 17.2)
where E is exposure or intake dose, and RfD is the reference dose, defined as an estimate
of the daily exposure for humans that will result in no significant adverse health effects
in most populations. The problem of the lack of the measurement of environmental
concentrations remains. Since we do not have measurements of NMs to estimate
reference doses for humans, no NOAEL data are given for humans incidentally exposed
to NPs.
Standard toxicity tests include mammalian toxicity (acute and chronic tests, oral
toxicity, dermal toxicity, and skin irritation tests), mutagenicity tests and ecotoxicity
tests in risk assessment. They should be used with caution in toxicity tests of NMs. This
is because the list of methods does not consider particle size that may significantly
change the physico-chemical properties of NMs. One of the assumptions in standard
toxicity tests is that the concentration of the toxicant at the target is dose dependent. NPs
do not form simple solutions. They may aggregate at high ionic strength (e.g. in salty
solutions) or adsorb onto surfaces. NMs may be trapped in the mucous layer on
epithelial surfaces rather than being absorbed into the cells in a predictable dose-
response manner. In order to accurately determine the toxicity of NPs, it is necessary to
improve the existing test methods to account for the size dependent physicochemical
properties of NMs, or add an extra uncertainty factor to risk calculations.
Though toxicological studies are not adequate to fully assess the lifecycle
impacts of these NMs, Robichaud et al. (2005) reported a relative risk assessment for
five NMs (SWCNTs,C 60 , quantum dot, alumoxane NPs, and nano-TiO 2 ) focused on the
activities surrounding the fabrication of NMs, exclusive of any impacts or risks with the
NMs themselves. Results suggests that the in-plant processes and materials used in
fabrication of these five NMs pose moderate to relatively low risks from an insurer's
perspective. By decoupling risks associated with handling inputs and wastes in the NM
production process, NM fabrication was brought into the risk assessment framework
used to qualify industrial risks. The relative risk assessment can be used as a baseline of
information concerning hazards posed by manufacturing NMs, and the industry can
focus on how to develop safely and responsibly.
17.4.3 In Vitro Study for the Toxicity of NMs
Developing testing strategies to characterize the toxicity of emerging NMs is one
of the most significant challenges faced by the producer and regulatory agencies. LD 50