Environmental Engineering Reference
In-Depth Information
In.risk.assessment,.dermal.penetration.follows.the.concentration.gradient..How-
ever,.the.penetration.of.the.stratum.corneum.is.extremely.rate.limiting..As.a.result,.
an. attenuating. gradient. forms. across. this. layer.. Studies. with. polysaccharide. mic-
roparticles. demonstrated. this. gradient. with. almost. no. subdermal. penetration. [16]..
The.gradient.is.dificult.to.model.based.on.the.multifactorial.nature.of.the.diffusion.
dynamics..Furthermore,.particulate.matter.that.does.reach.the.epidermal.and.dermal.
layers.is.subject.to.phagocytosis.by.Langerhans.cells.and.other.macrophages,.which.
results. in. transport. to. the. lymphatic. system. rather. than. the. central. compartment..
While.limiting.systemic.exposure,.lymphatic.transport.may.result.in.inlammation.
and.hypersensitization.reactions.not.immediately.associated.with.the.point.of.con-
tact.with.the.causative.nanomaterial.[17].
. exPosuRe and eFFeCts thRough inhalation
Generally,.most.of.the.work.regarding.exposure.to.nanomaterials.derives.from.con-
cerns. related. to. the. inhalation. of. ultraine. particles. found. in. certain. occupational.
settings,.as.well.as.ultraine.aerosols.resulting.from.combustion..Scientists.have.spe-
ciically.linked.serious.chronic.diseases.to.the.inhalation.of.ultraine.particles..These.
diseases. include. Clara. cell. carcinomas. (polycyclic. aromatic. hydrocarbons),. meso-
thelioma.(asbestos),.and.berylliosis.(beryllium)..General.syndromes.associated.with.
exposures.to.aerosols.include.black.lung.(coal),.emphysema.(combustion.products),.
and.metal.fume.fever.(zinc,.tin,.and.other.transition.metals).
Relatively.stable.aerosols.consist.of.a.suspension.of.nonvolatile.particles.ranging.
from.10.nm.to.25.micrometers.(µm)..Typically,.aerosol.particles.less.than.500.nm.
deposit.with.a.pattern.more.like.that.of.a.gas.than.a.particulate.suspension..Hence,.
diffusion.governs.deposition.and.can.be.expected.to.occur.throughout.the.respira-
tory.tract,.including.the.alveoli..Deposition.depends.on.the.adherence.and.residence.
time.of.the.nanoparticles..Particles.between.500.nm.and.25.µm.demonstrate.a.slow.
depositional.pattern.where.the.majority.may.be.deposited.in.the.upper.airway,.but.
some. penetrate. to. the. deep. lung.. Particles. larger. than. 25. µm. tend. to. be. deposited.
through.gravitational.deposition.and.will.settle.in.the.nasopharyngeal.region.where.
the.low.velocity.is.reduced.[18].
9.4.1 m
eChanisms
FOr
a
dsOrptiOn
and
r
emOval
The.lux.rate.(
J
).from.the.inhaled.atmosphere.to.the.respiratory.epithelium.can.be.pre-
dicted.through.a.modiication.of.Fick's.law.of.diffusion,.which.is.expressed.as.follows:
D
dc
dx
J
= −
.
.
(9.8)
where.
dc
.is.the.concentration.gradient,.
dx
.is.the.distance.across.the.concentration.
gradient,.and.
D
.is.the.diffusion.coeficient..In.the.case.of.inhalation,.the.separation.
distance. is. a. function. of. the. size. and. shape. of. the. air. space.. Because. an. inhaled.
nanomaterial. is. distributed. within. the. three-dimensional. air. space,. concentration.
requires. integration. over. the. lateral. and. longitudinal. directions. based. on. the. con-
centration.gradient.relative.to.a.given.location.along.the.airway..This.usually.can.be.
