Environmental Engineering Reference
In-Depth Information
therefore.not.a.scalable.value..Using.this.
k
β
,.an.allowable.daily.exposure.rate.[
C
]
β
can.
be.determined.as.follows:
(
)
−
k t
[
C
]
=
[
C
] ·
1
−
e
β
β
o
.
.
(9.15)
5
where t
= 1
day
, [
C
]
= 9.83
×
10
fibers per day
β
Based. on. study. results. reported. by. the. U.S.. EPA. [32],. the. ventilation. rate. for.
an. adult. undertaking. medium. activity. is. 1.02. m
3
/hr.. This. equates. to. an. exposure.
volume.of.8.16.m
3
.per.8-hr.day..Therefore,.to.ensure.that.the.total.lung.burden.does.
not. exceed. 9.88. ×. 10
7
. ibers,. the. 8-hr. time-weighted. concentration. cannot. exceed.
1.20.×.10
5
.ibers/m
3
,.and.the.maximum.1-hr.exposure.should.not.exceed.9.64.×.10
5
.
ibers/m
3
.for.ibers.with.an.effective.diameter.less.than.5.µm.
Currently,. no. published. physiological. or. epidemiological. studies. describe. the.
effect. of. SWCNT. inhalation. in. humans.. The. available. studies. were. performed. in.
rodents..It.is.assumed.in.this.analysis.that.a.safe.level.in.rodents.equates.to.a.safe.
level.in.humans..Other.studies.with.inlammatory.ibrous.material.appear.to.indicate.
that.the.cross-species.comparisons.are.valid.[33]..However,.it.remains.an.uncertainty.
if.this.relation.will.hold.true.for.SWCNTs.
Histological. examination. of. the. lesions. associated. with. SWCNTs. in. the. deep.
lung.suggests.that.the.degree.of.granuloma.formation.and.resulting.inlammation.is.
independent.of.the.amount.of.SWCNT.within.the.granulomas.[34]..This.is.similar,.
within.limits,.to.observations.with.other.ibrous.inlammatory.agents.such.as.asbes-
tos.[33]..However,.this.qualitative.observation.has.not.been.tested.directly..It.may.be.
that.larger.SWCNT.tangles.have.a.greater.inlammatory.potential.than.smaller.ones..
If.this.is.the.case,.however,.the.differences.in.magnitude.are.of.an.order.that.they.did.
not.present.obvious.histological.differences.in.the.current.available.studies.
Further. uncertainty. exists. in. the. derivation. of. the. SWCNT. elimination. rate.
based.on.two.observations.by.Muller.et.al..[31].at.an.exposure.rate.different.from.that.
used.as.the.toxicity.threshold..Scaling.the.exposure.to.the.projected.risk.threshold.
required.an.assumption.of.irst-order.kinetics..Regression.of.the.one-dose.observa-
tions.suggests.strongly.that.the.elimination.does.follow.irst-order.kinetics..However,.
it.has.not.been.repeated.or.demonstrated.for.other.SWCNT.exposure.rates..It.is.cur-
rently.an.assumption.and.therefore.represents.an.uncertainty.in.this.derivation.
. known toxiCitY oF nanoMateRials
The.study.of.the.toxicity.of.nanomaterials.is.in.its.infancy.and.the.literature.is.grow-
ing.rapidly..It.is.useful.to.examine.the.literature.to.date.for.the.six.types.of.nanoma-
terials.that.are.the.focus.of.this.topic.to.understand.the.types.of.effects.that.might.
occur..Table.9.1.offers.a.brief.review.of.the.literature.
As.shown.in.Table.9.1,.a.number.of.studies.have.investigated.the.potential.human.
health. implications. associated. with. exposure. to. nanomaterials.. Although. many. of.
the.results.are.very.preliminary,.there.are.indications.that.the.six.major.engineered.
nanomaterials.can.elicit.an.oxidative.stress.response.in.certain.biological.test.sys-
tems.[37,.39,.54-56]..This.is.seen.as.measured.indication.of.cell.membrane.damage,.
