Biomedical Engineering Reference
In-Depth Information
of pulmonary inflammation and metal fume fever that does not progress to the pulmonary disease
[57]. In contrast, greater toxicity is expected for more reactive NPs such as fullerenes and CNTs.
Although most toxicological studies with nanomaterials have been
in vitro
, or short-term
in vivo
studies involving unnatural delivery (e.g., intratracheal instillation) in limited species and types of
NPs, the National Toxicology Program is planning short- and long-term studies, including oral,
dermal, and inhalation exposures for some NPs (http://ntp-server.niehs.nih.gov/files/nanoscale05.
pdf). Nanomaterial research and risk assessments will ultimately need to address multiple potential
health effects including cardiovascular, carcinogenicity, reproductive/developmental, immunologi-
cal, and neurological.
Screening assessments of exposures to the more studied metal oxides could be conducted by
developing toxicity benchmarks using the weight of evidence from studies of (1) nanoscale metal
oxides in the toxicological and pharmacological literature, (2) fine-scale forms corrected for the
proportionally greater surface area of nanoscale particles, (3) more toxic particles such as UFPs,
and (4) the toxicology and epidemiology of metal fumes. Uncertainties in such assessments will
have to be considered given data limitations; however, collectively, the available studies are begin-
ning to reveal important features necessary for initial risk assessments of specific NPs.
19.6 ISSUES RELEVANT TO RISK ASSESSMENT
Critical steps in the risk assessment of nanomaterials remain, so far, the same as those used for
the risk assessment of other types of chemicals, notably (1) hazard identification meaning identi-
fication of nanomaterial properties that may cause hazards to health; (2) hazard characterization,
which requires defining of dose responses for critical target organ(s), cell(s), and mechanisms of
toxicity [58]. Also, the potential of different engineered nanomaterials to react with constituents of
cells at the port of entry and beyond, that is, lipids and proteins, should also be assessed [59]. To
evaluate the translocation or distribution of these materials in the body, one should also assess their
capability to cross internal barriers such as blood-brain barrier, blood-placental barrier, blood-tes-
ticular barrier, and many others [60].
The next and third step in risk assessment is the assessment of exposure. Nanomaterials are pro-
duced from many substances, in many forms and sizes, and with a variety of surface coatings. The
health risk assessment of such diverse materials requires validated analytical methods both for their
characterization in bulk samples, and for their detection and measurement in workplace air. This is
because nanomaterial levels may be higher in occupational than in other environments, at least during
certain operations; nanomaterials are handled in large quantities in workplaces and, hence, occupa-
tional settings carry the greatest potential for human exposure [61]. Risk assessment also requires
an understanding of the transport processes between the source and human receptor and how they
modify the characteristics of the nanomaterials [62]. Airborne nanomaterials (“aerosols”) can be char-
acterized by measuring several metrics, especially number concentration, surface area, and mass [61].
These four steps in risk assessment—hazard identification, hazard characterization, exposure
assessment, and risk characterization are ultimately combined in the risk assessment process. Risk
assessment integrates the results of the four steps of the risk assessment process and aims at assess-
ing the likelihood of occurrence of a given hazard in a certain exposure situation. The ultimate goal
of the current risk assessment paradigms is to be able to provide quantitative predictions of the given
risks, enabling evidence-based risk management that is based on quantitative assessment of a given
risk in a population.
19.7 STAGES OF RISK ASSESSMENT
Many of the most important health and safety concerns regarding nanomaterials and nanotechnolo-
gies are due to the lack of knowledge of levels of occupational and other types of exposure to nano-
materials during their production and use. Many nanomaterials are known to be much more reactive