Environmental Engineering Reference
In-Depth Information
macrophages.then.can.migrate.vertically.to.the.bronchioles.where.they.are.entrained.
in. the. mucosal. conveyer. and. rapidly. eliminated.. Alternately,. nanoparticles. may.
be. subject. to. endocytosis. by. macrophages. that. migrate. into. the. lymphatic. system.
where. they. are. cleared. via. the. tracheobronchial. lymph. nodes. or. the. blood.. This.
relatively.slow.process.sometimes.takes.months.to.remove.particulate.material.from.
an.exposed.organism.
The.most.important.considerations.in.assessing.the.risk.from.exposure.to.nano-
materials.in.aerosols.are.the.size.of.the.particles.and.the.rates.of.exposure.relative.to.
the.rates.of.response..From.the.discussion.above,.it.is.apparent.that.dispersed.nano-
materials.will.deposit.all.along.the.airways,.including.the.alveoli..However,.nano-
materials,. particularly. the. current. carbonaceous. materials,. are. rarely. encountered.
in.either.the.occupational.or.general.environment.as.stable.dispersals.(see.Chapter.
6)..The.critical.rate.of.exposure.relates.to.the.rate.and.magnitude.of.injury.relative.
to.the.rates.of.elimination.and.repair..If.injury.resulting.from.exposure.exceeds.the.
airway's.repair.capacity.as.the.result.of.ineficient.removal.capacity,.then.it.can.be.
expected.that.an.adverse.effect.will.ensue.
Inlammation. is. the. most. common. response. to. ibrous. or. particulate. material..
It. results. from. the. activation. of. inherent. defense. mechanisms. mediated. by. mac-
rophages.that,.if.over-stimulated,.will.result.in.localized.cellular.necrosis.and.loss.
of.lung.functions..A.case.study.of.the.potential.risk.associated.with.single-walled.
carbon.nanotubes.follows.
9.4.2  C ase  s tudy : i nhalatiOn OF  C arbOn  n anOtubes
Single-walled.carbon.nanotubes.(SWCNTs).consist.of.a.sheet.of.aryl.carbon.rings.
curved.around.on.themselves.so.as.to.form.a.tube.one.layer.thick..SWCNTs.are.typi-
cally.1.to.4.nm.in.diameter.and.vary.from.as.short.as.50.nm.to.lengths.in.excess.of.
2.µm..Carbon.nanotubes.possess.extremely.low.charge.afinity.compared.to.that.of.
luid.media.such.as.air.and.water..As.such,.they.tend.to.rapidly.form.clumps.by.bind-
ing.to.one.another,.particularly.along.their.long.axes..This.manifests.a.tertiary.struc-
ture.consisting.of.numerous.SWCNTs.in.forms.referred.to.as.nanoropes..Nanoropes.
will.associate.further.into.groups.of.nanoropes.referred.to.as.tangles.and.will.con-
tinue.to.associate.until.the.units.become.so.large.as.to.fall.out.of.luid.suspension.
... 
Pulmonary toxicology
As. of. the. date. of. publication,. no. human. studies. were. available. that. evaluated. the.
pulmonary. toxicity. of. SWCNTs.. Furthermore,. animal. tests. for. direct. inhalation.
were.not.available.due.to.the.practical.dificulties.in.isolating.and.collecting.enough.
SWCNT.particles.to.conduct.these.studies.[22]..As.such,.almost.all.the.current.stud-
ies.are.based.on.either. in vitro .designs.using.tissue.explants.of.cultured.cell.lines,.or.
exposures.of.whole.animals.using.intratracheal.instillation..The.term.“intratracheal.
instillation”.describes.a.technique.where.researchers.inject.a.bolus.dose.of.a.SWCNT.
suspension.into.the.trachea.of.the.test.animal.to.distribute.SWCNTs.throughout.the.
pulmonary. airway. by. aspiration.. While. the. intratracheal. instillation. method. has.
technical.limitations,.it.is.an.accepted.screening.test.for.pulmonary.toxicity.[22-24]..
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