Environmental Engineering Reference
In-Depth Information
A
=
CS vdt
Y
Y
Hence
:
.
.
(9.12)
Y
dn
dt
2
3
kT
r
∫
dn
dy
=
⋅
⋅
−
CS vdt
C
Y
0
η
p
y
=
0
where.
CS
Y
.is.the.cross-sectional.area.of.the.airway.at.point.
y
,.and.
dy
.is.the.inini-
tesimal.of.the.change.in.position.within.the.airway..Note.that.the.area.is.expressed.
as.a.cross-section.rather.than.as.a.function.of.radius..This.is.because.the.presence.
of. processes. (i.e.,. projecting. portions. of. bone. or. tissue),. particularly. in. the. naso-
pharyngeal.region,.can.greatly.increase.the.potential.surface.area.of.exposure.per.
unit. time.. However,. in. the. pulmonary. region. of. the. lungs,. where. the. airways. are.
relatively.smooth,.the.exposed.area.per.unit.time.can.be.expressed.in.terms.of.π
r
2
a
dy
..
Figure.9.3. shows. examples. of. projected. deposition. rates. based. on. mass. and. iber.
numbers.for.the.bronchioles..As.shown,.the.deposition.rate.increases.with.concentra-
tion.and.decreases.with.particle.size.(diameter).
Direct. solution. of. this. relation. is. dificult. because. of. the. heterogeneity. of. the.
mammalian.airway..Predictions.of.absorption.rates.usually.involve.the.construction.
of.a.three-dimensional.passage.model.that.segments.differential.regions.of.the.air-
way.based.on.similar.diffusion.properties..These.models.generally.indicate.that.the.
number.of.particles.deposited.is.inversely.proportional.to.the.size.of.the.particle.[19]..
Therefore,.the.smaller.the.particle,.the.larger.the.amount.absorbed.as.the.result.of.
higher.rates.of.diffusion..Although.counter-intuitive,.the.relation.also.suggests.that.
the.faster.the.air.velocity,.the.higher.the.rate.of.absorption..But.note.that.this.results.
from.the.increase.in.surface.area.exposure.per.unit.time,.which.decreases.the.lon-
gitudinal.gradient,.thereby.allowing.higher.concentrations.in.deeper.regions.of.the.
airway.
Upon.adsorption.to.the.lining.of.the.airways,.particulate.matter.is.suspended.in.a.
complex.mixture.called.the.tracheobronchial.mucus..Produced.by.both.submucosal.
and.epithelial.secretory.cells,.the.mucus.comprises.a.mixture.of.glycoproteins.and.
electrolytes.within.an.aqueous.matrix..The.viscosity.of.the.mucus.varies.throughout.
the.respiratory.system,.thereby.altering.the.diffusivity.of.nanoparticles..The.mucous.
layer.in.humans.continues.from.the.larynx.to.the.end.of.the.irst-generation.bronchi-
oles..Within.the.alveoli,.Type.II.cells.also.produce.a.proteinaceous.secretion.similar.
to.mucus,.but.usually.of.a.lower.viscosity.and.higher.water.content.
The.pulmonary.mucosa.is.part.of.a.clearance.system.referred.to.as.the.respira-
tory.conveyer..This.system.of.ciliated.cells,.which.lines.the.bronchioles.and.trachea,.
traps.inhaled.particulates.in.mucus.and.sweeps.the.laden.mucosal.material.up.and.
out.of.the.respiratory.tract..Rates.of.movement.vary.from.about.0.6.mm/min.in.the.
bronchioles.to.about.10.mm/min.in.the.trachea.region.[20]..The.respiratory.conveyor.
deposits. most. of. the. material. in. the. esophagus,. which. may. represent. a. signiicant.
exposure.route.for.the.ingestion.of.nanomaterials.
Materials. with. a. suficient. concentration. gradient. to. reach. the. alveoli. are. not.
directly. subject. to. the. mucosal. conveyer. because. there. are. no. cilia. in. the. alveoli..
