Environmental Risks of Nanomaterials - Nanotechnologies for Water Environment Applications - page 605

Health risk evaluations are viewed as a scientific approach in which facts and
assumptions are used to estimate the adverse effects of specific pollutants or other toxic
agents on human health or the environment. For the evaluation, information is required
to understand the current and the potential exposure, consider all possible exposure
pathways, and determine the potential ecological or other environmental effects. The
health risk evaluation process includes four steps: hazard identification, dose-response
assessment or hazard assessment, exposure assessment, and risk characterization (Kwon,
2004). A hazard is defined as a toxic agent or a set of conditions that may cause adverse
effects to human health or the environment. Hazard identification involves an evaluation
of various forms of materials in order to identify the different hazards. Dose-response or
hazard assessment is required in an overall assessment. This step requires that
assumptions be made to correlate experimental data from animals with that of humans.
Exposure assessment will determine the magnitude, frequency, duration, and routes of
exposure to human or ecosystems. Finally, in risk characterizations, toxicology and
exposure data/information are combined to qualitatively or quantitatively express the
risk.
In risk evaluation, all potential sources of emission need to be identified,
including those available during production, handling, and use. Control technologies
should be developed to ensure minimal emissions of NMs. Physicochemical
characteristics (such as the surface area or size, acidity, and metal content) of particles
are particularly important in determining the adverse health effects. In addition to mass
concentrations, particle numbers and surface areas may be more relevant for determining
toxicity. In some cases, NPs have been found to act synergistically with other pollutants,
such as O 3 or NO x . The interactions between NPs and gaseous components could
therefore complicate the risk assessment of the NPs.
All of the available dose-response data must be taken into account in risk
assessment. Because of the uncertainty of dose-response data for humans, arbitrary
safety factors must be applied and stated in quantitative or qualitative terms. This will
allow an assessor to analyze the uncertainties and to set forth in explicit terms to deal
with the uncertainties (Theodore and Reynolds, 1996). In general, the exposure
concentrations are compared with the no observed adverse effect levels (NOAEL) or the
lowest observed adverse effect levels (LOAEL). If the concentration of the NMs in the
environment is higher than the NOAEL, risk management is required. Because it is not
ethical to expose NMs to humans for dose-response data, NOAEL from animal studies
can be used.
Risk calculations for single substance follow:
Risk = concentration ( C ) × Toxicity factor ( T ) (Eq. 17.1)