Environmental Engineering Reference
In-Depth Information
... 
exposure and dose
Individual. organisms. can. be. exposed. to. nanomaterials. in. their. environment. via.
ingestion,. dermal. contact,. or. inhalation.. Each. of. these. routes. of. exposure. is. dis-
cussed.below,.as.are.the.natural.defenses.that.organisms.can.employ.to.reduce.the.
effective.dose.
Oral.exposure.of.terrestrial.organisms.is.anticipated.to.be.low..One.reason.for.
this.is.the.known.selectivity.of.the.intestinal.villi.and,.if.absorbed,.the.effectiveness.
of.the.hepato-biliary.system.in.eliminating.particulate.foreign.matter.from.the.body..
Other.reasons.pertain.speciically.to.terrestrial.organisms..Their.exposures.to.nano-
materials.in.soils.are.expected.to.be.low.because,.unless.waste.disposal.practices.are.
egregious.or.soils.are.very.close.to.a.source,.nanomaterials.would.become.sorbed.
to.micropores.in.the.soil.matrix.and.thereby.rendered.unavailable.to.the.organism..
Alternatively,.they.might.be.diluted.by.the.soil.matrix.if.water.solubility.were.higher.
and. the. nanoparticles. were. to. percolate. down. through. the. various. soil. horizons..
With.the.exception.of.invertebrates.such.as.earthworms.that.consume.soil.to.extract.
nutrients,. most. soil-dwelling. animals. (e.g.,. shrews,. mice,. voles,. gophers,. etc.). do.
not,.inadvertently,.consume.much.soil.(typically.less.than.1.or.2%.of.the.diet;.see.
U.S.. EPA's Wildlife Exposure Factors Handbook .[13])..Further,.with.a.few.excep-
tions.such.as.metal.oxides,.most.of.the.nanomaterials.being.produced.are.dificult.to.
get.into.suspension.and.will.therefore.form.agglomerates.or.precipitates,.which.are.
anticipated.to.become.part.of.the.soil.matrix.and.therefore.unavailable.for.biological.
uptake.into.an.organism.if.the.soil.were.inadvertently.consumed.
The.least.probable.exposure.pathway.will.most.likely.be.dermal,.for.several.rea-
sons.. First,. with. the. exception. of. certain. invertebrates,. such. as. earthworms,. many.
aquatic.organisms.and.the.vast.majority.of.terrestrial.organisms.have.a.line.of.defense.
above.and.beyond.the.dermis/epidermis.layer..Fish.scales.overlap.and,.because.they.
overlap.in.the.same.direction.as.the.general.motion.or.movement.(forward).of.the.ish,.
the.probability.of.dissolved.nanomaterials.being.absorbed.across.the.integument.of.the.
animal.is.anticipated.to.be.relatively.low..Different.terrestrial.organisms.have.different.
lines.of.defense..Mammals.have.thick.coats.of.fur..Birds.have.layer.upon.layer.of.down.
and.feathers.that,.microscopically,.form.unique.interlocking.networks.that.would.act.
as. an. effective. external. barrier.. Reptiles. have. thick,. horny. overlapping. scales.. Most.
insects. (the. vast. majority. of. which. are. beetles). have. a. sclerotized. dermal. layer. that.
strongly.resists.both.physical.and.chemical.attack..Because.of.their.extremely.small.
size,.one.might.anticipate.nanomaterials.passing.through.this.irst.line.of.defense..In.
short,.nature.has.equipped.most.ecological.receptors.with.layer.upon.layer.of.fur,.feath-
ers,.scales,.and/or.sclerotized.exteriors.with.coatings.such.as.oils,.fats,.and.waxes.that.
will.act.as.innate.dust.collectors..The.effectiveness.of.such.dust.collectors.depends.in.
part.on.a.physical.phenomenon.that.affects.the.behavior.of.nanoparticles..Nanomateri-
als.are.subject.to.the.random.movement.of.adjacent.molecules,.a.phenomenon.called.
Brownian.motion,.which.will.increase.the.probability.that.it.will.encounter,.and.collide.
with,.a.iltering.mechanism..This.process.is.called.diffusional.capture.[14].and.appears.
to.be.effective.for.traditional.particles.less.than.0.3.micrometers.(µm).in.size.
With.the.exception.of.aquatic.or.semi-aquatic.organisms.that.may.have.a.semi-
permeable.dermis,.such.as.amphibians,.the.respiratory.system.is.expected.to.be.the.
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