Environmental Engineering Reference
In-Depth Information
the. results. are. characteristically. subtle,. requiring. innovative. and/or. very. sensitive.
testing.methodologies.
The.natural.physiological.variability.within.a.population.means.that.individuals.
may.react.differently.upon.exposure..The.reasons.given.for.this.variability.often.are.
physiological,. such. as. internal. genetic. differences,. or. environmental.. The. gender.
of. an. animal,. the. species,. or. its. age. can. make. a. very. signiicant. difference. in. the.
response. following. exposure. to. a. chemical. or. nanomaterial.. Younger. animals. are.
generally.more.susceptible.to.toxicants.than.older.animals,.partly.due.to.the.fact.that.
they.weigh.less.and.therefore,.pound.for.pound,.will.get.a.larger.dose.than.would.an.
adult.animal..Similarly,.there.are.some.strains.of.mice.that.are.very.resistant.to.the.
toxic.effects.of.heavy.metals,.whereas.other.strains.are.overly.sensitive..The.results.
of.these.variations.in.sensitivity.can.be.observed.in.the.classic.dose/response.curve,.
which.is.typically.an.S-shaped.function..Plotted.on.a.graph,.with.the.dose.on.the.x-
axis.and.the.percent.of.organisms.affected.on.the.y-axis,.the.cause.of.the.inlections.
in.the.S-shaped.curve.are.due.to.the.presence.of.sensitive.individuals.in.the.low.dose.
ranges.and.tolerant.individuals.in.the.high.dose.ranges.
8.1.4 e
xpOsure
A.complete.exposure.pathway.must.exist.for.an.animal.to.be.affected.by.a.chemical.
or.nanomaterial..This.means.that.a.mechanism.must.exist.to.transfer.the.compound.
or. nanomaterial. in. question. from. the.
source
. in. air,. water,. soil,. or. sediment. to. the.
receptor
.organism.in.question..
Without exposure, there can be no risk
..Therefore,.
and.this.is.a.critical.factor.as.nanotechnology.evolves,.as.long.as.nanomaterials.are.
properly.handled.and/or.contained,.risk.and/or.hazard(s).will.be.negligible.
Scientists.use.the.term.“fate.and.transport”.to.refer.to.processes.that.affect.a.sub-
stance.as.it.travels.from.the.source.to.a.potential.receptor..As.described.in.Chapter.6,.
various.processes.can.change.the.nature.and.concentration.of.a.nanomaterial,.which,.
in.turn,.can.change.its.potential.to.induce.toxicity.
Partitioning.from.one.phase.of.media.to.another.is.an.extremely.important.phe-
nomenon.that.can.affect.the.properties.(and.often.the.quantities).of.a.nanomaterial.
within. an. environmental. medium.. Partitioning. typically. is. expressed. in. terms. of.
a. ratio. or. partition. coeficient. (e.g.,. water-to-sediment,. soil-to-water,. water-to-air,.
water-to-biota,.etc.)..For.example,.a.bioconcentration.factor.(BCF).is.the.ratio.of.the.
concentration.of.a.substance.in.ish.tissue.to.the.concentration.in.a.waterbody.
Weathering,.which.includes.the.variety.of.chemical.reactions.and.physical.atten-
uation.processes.that.occur.after.a.chemical.is.released.into.the.environment,.will.
generally.decrease.exposure,.bioavailability,.and/or.toxicity..The.exceptions.to.this.
are.compounds.or.materials.that.resist.degradation,.such.as.mercurials.or.arsenicals,.
some.types.of.commercial.pesticides,.polychlorinated.dioxins.and.furans,.and.poly-
chlorinated.biphenyls,.to.name.just.a.few.examples.
Another. important. underlying. principle. in. ecological. toxicology. is. the. differ-
ence. between. exposure. and. dose.. An.
exposure
. is. the. sum. total. of. a. compound. or.
nanomaterial.that.reaches.an.ecological.receptor,.but.the.
dose
.is.a.smaller.percentage.
of.the.total.material.that.actually.enters.the.body..Bioaccessibility.and.bioavailability.
