Environmental Engineering Reference
In-Depth Information
...
Risk assessment
Assessing. the. risk. associated. with. SWCNTs.requires. two.separate. considerations:.
(1).the.ability.of.a.material.to.reach.a.sensitive.site.of.action,.and.(2).the.type.and.
magnitude. of. the. resultant. response. at. the. sensitive. site.. Current. studies. based. on.
intratracheal.installation.indicate.that.the.sensitive.site.of.action.for.SWCNTs.is.the.
deep.lung.tissue.—.speciically.the.respiratory.bronchioles.and.alveoli..Indigenous.
macrophages. engulf. SWCNTs. that. reach. this. part. of. the. pulmonary. airway.. This.
phagocytosis.apparently.results.in.an.inlammation.cascade.similar.to.that.seen.in.
silicosis,.which.appears.to.manifest.as.a.long-term.or.chronic.condition.because.the.
macrophages. bearing. SWCNTs. do. not. migrate. into. the. upper. airways. as. is. seen.
with.materials.such.as.particulate.graphite.[25]..Chronic.inlammation.in.the.lower.
airway. will. result. in. damage. to. the. underlying. epithelium. and. the. generation. of.
scar. tissue. often. referred. to. as. ibrosis.. Widespread. damage. throughout. the. lower.
airways. will. reduce. gas. transfer. signiicantly. and. a. condition. akin. to. emphysema.
can.develop..Furthermore,.chronic.inlammations.of.this.type.have.been.associated.
with.the.promotion.of.hyperplasias.that.have.the.potential.to.become.cancerous.[27]..
However,.it.must.be.cautioned.that.this.is.not.necessarily.the.case,.and.there.is.cur-
rently.no.evidence.that.exposure.to.SWCNTs.will.result.in.either.cancer.initiation.
or.promotion.
Exposure. of. the. upper. airways. to. SWCNTs. is. less. toxicologically. signiicant.
for.two.reasons..First,.the.residence.time.of.the.SWCNTs.is.much.shorter.because.
particles.that.impact.within.the.nasopharyngeal,.tracheal,.or.bronchial.regions.of.the.
airway.are.rapidly.removed.via.the.pulmonary.mucous.conveyer..Therefore,.inlam-
mation.appears.to.be.transient.(<2.hr)..Second,.because.the.upper.airway.is.not.the.
site. of. signiicant. gas. transfer,. it. comprises. a. thicker. and. more. robust. epithelium.
with.greater.regenerative.capacity.and.therefore.is.less.likely.to.manifest.signiicant.
ibrosis.[28].
Consequently,.the.greatest.potential.hazard.to.individuals.working.with.SWCNTs.
apparently.would.stem.from.exposure.to.materials.capable.of.depositing.within.the.
deep.lung.tissue..Materials.depositing.within.the.upper.airway.may.be.acutely.toxic.
at.high.concentrations.but.will.not.likely.represent.a.serious.health.issue.at.or.below.
exposure.concentration.limits.established.to.protect.the.deep.lung.
Initial.indications.from.histological.studies.indicate.that.inlammation.does.not.
depend. directly. on. the. size. of. the. SWCNT. iber. impacting. the. pulmonary. tissue.
[24,.25,.29]..Rather,.it.is.the.number.and.distribution.of.the.impacts.that.results.in.
the.overall.toxic.response..As.such,.the.classic.risk.approach.of.quantifying.toxicity.
using.the.mass.dose.per.unit.time.or.unit.body.mass.may.not.be.appropriate..Rather,.
to.capture.the.dose.response,.one.must.quantify.the.exposure.in.terms.of.number.of.
iber.units.per.unit.time,.where.a.iber.unit.is.deined.as.any.independent.SWCNT,.
SWCNT.rope,.or.SWCNT.tangle.
Of.the.current.animal.studies.described.above,.the.study.performed.by.Shevedova.
et.al..[24].provides.the.best.toxicological.characterization.and.quantiication.to.derive.
exposure.guidelines..Using.the.endpoint.of.average.alveolar.thickness.as.a.measure.
of. induced. ibrosis,. Shevedova. et. al.. found. that. a. single. exposure. concentration. of.
3.92.×.10
5
.ibers.per.mouse.had.no.effect.at.either.28.or.60.days.post.exposure.
