Pragmatic Approach for Health Risk Assessment of Bauxite Tailing Waste Materials - Environmental Contamination: Health Risks and Ecological Restoration - page 107

as.shown.by.anecdotal.evidence.gathered.from.the.local.residents.who.were.
subjected.to.high-dust.activities.
PM 2.5 . concentrations. were. not. measured. directly. in. our. study.. However,.
the.particle.size.distribution.analysis.showed.that.less.than.10%.of.the.bulk.
tailings. were. PM 2.5 ,. and. of. the. PM 10 . fraction,. there. was. about. half. of. the.
ine.dust.that.was.PM 2.5 ..We.extrapolated.that.the.average.PM 2.5 .was.half.of.
48.5.μg/m 3 . to. yield. a. value. of. 24.25.μg/m 3 .. This. value. is. within. the. NEPM.
PM 2.5 .standard.of.25.μg/m 3 .and,.hence,.poses.no.risk.
It. is. generally. considered. that. respirable. particles. have. an. aerodynamic.
diameter.of.<3-4.μm,.whereas.most.particles.larger.than.5.μm.may.be.depos-
ited. in. the. tracheobronchial. airways. and,. thus,. may. not.reach. the. alveolar.
(IARC. 1997).. The. clearance. rates. for. respirable. particles. deposited. in. the.
respiratory. bronchioles. and. proximal. alveoli. (deep. lung). are. slower. and.
more.likely.to.injure.the.lung..The.clearance.kinetics.of.quartz.particles.in.
humans.remains.inconclusive.to.date.(IARC.1997).
Exposure. to. excessive. concentrations. of. respirable. ambient. air. particles.
can. adversely. affect. human. health.. The. speciic. particle. size. fractions. of.
the.chemical.or.biological.components.are.the.most.dominant.sources.that.
might.be.responsible.for.adverse.health.effects.and.are.not.fully.studied.for.
their.potential.to.cause.health.hazards..For.bauxite.tailings.tested.thus.far,.
it.appears.that.the.metal.and.metalloid.levels.present.limited.health.risk.as.
discussed. earlier.. We. then. turned. our. focus. on. air. particulates,. with. spe-
cial.reference.to.crystalline.silica.(e.g.,.quartz).component.present.in.bauxite.
waste.materials.
There.are.many.studies.both.epidemiologically.and.in.animal.experimen-
tations.linking.silica.(Quartz:.CAS.No..14808-60-7).exposure.to.cancer.and.
noncancer.health.effects.(WHO.2000)..Silicosis,.lung.cancer,.and.pulmonary.
tuberculosis. are. associated. with. occupational. exposure. to. quartz. dust. at.
relatively.large.doses.over.a.long.time..However,.to.date,.there.is.no.known.
adverse. health. effect. associated. with. nonoccupational. exposure. to. quartz.
dust.(WHO.2000)..Some.of.the.inherent.problems.with.many.epidemiologi-
cal.studies.include.lack.of.accurate.exposure.data.and.confounding.factors,.
which. may. make. it. dificult. to. interpret. accurately. the. observations. made.
during.such.studies..Quartz.was.found.to.induce.pulmonary.tumors.in.rats.
but.not.in.hamsters.or.in.mice..This.demonstrates.species.differences.in.their.
response.to.exposure.to.chemicals.
The.risk.estimates.for.silicosis.prevalence.for.a.working.lifetime.of.exposure.
to.respirable.quartz.dust.concentration.of.about.50.or.100.μg/m 3 .in.the.occu-
pational.environment.vary.widely.(i.e.,.2%-90%)..Using.methods.described.
by. USEPA. (1996),. the. BMD. (bench. mark. dose). analysis. predicted. that. the.
silicosis. risk. for. a. continuous. 70. year. lifetime. exposure. to. 8.μg/m 3 . is. less.
than.3%.of.healthy.individuals.not.compromised.by.other.conditions.such.
as.respiratory.diseases.and.pollution.in.ambient.environment..The.highest.
average.quartz.content.in.bauxite.tailings.was.found.to.be.about.6%.of.the.set.
value.of.48.5.μg/m 3 ,.leading.to.a.calculated.concentration.of.less.than.3.μg/m 3 .