Absorption-of-Toxicants-through-the-Skin

Absorption ofToxicants through the SkinabstractHuman skinis notvery permeableand thereforeis relativelygood barrierfor separatingorganismsfrom theirenvironment.However,some chemicalscan beabsorbed byskin insufficient quantitiestoproduce systemiceffects.To beabsorbed throughthe skin,a toxicantmust pass throughthe epidermis ortheappendages sweatand sebaceousglands andhair follicles.Chemicals that are absorbedthrough theskinhave topassthroughseveral celllayers atotal ofseven beforeentering thesmall bloodand lymphcapillariesin the dermis.The mostimportant considerationsregarding thedermal absorptionof xenobioticsrelate tothestratum corneum.The firstphase ofpercutaneous absorptionis diffusion of xenobioticsthrough therate-limiting barrier,the stratum corneum thatconsists ofabout75%to80%moderately lipophilicmaterials.It isclear thatall toxicantsmove acrossit bypassive diffusion.Moreover,human stratumcorneumdisplays significantdifferences instructure andchemistry fromone regionof thebody toanother,and thesedifferences affectthe permeabilityof theskin tochemicals thatdepends onboth thediffusivityand thethickness ofthe stratum corneum.The secondphase ofpercutaneous absorptionconsists ofdiffusionofthe toxicantsthrough thelowerlayers oftheepidermisstratum granulosum,spinosum andgerminetivum and thedermisthatcontain aporous,nonselective aqueousdiffusion medium.Then thetoxicants enterthe systemiccirculationand lymphaticcapillaries.And thisrate ofdiffusion dependson blood flow,interstitial fluidmovementand perhapsinteractions withdermal constituents.As isreported,some agentscould injurethe stratumcorneum andincrease itspermeability,whichmay enhancethe absorptionof sometoxicants.And thestratumcorneumthat ispartially hydratedismore permeablethan thatwhich isdry.Having beenemployed instudying thedermal absorptionof toxicants,considerable speciesvariationshave beenobserved incutaneous permeability.Species differencesin percutaneousabsorptionare accountfor thedifferential toxicityof insecticidesin insectsand humans.According toDugard,theappendages sweatducts,pilosebaceous ductsandthestratumcorneumthatarehighlyspecies-dependent areimportant indermal absorptionof chemicals.Besides,the cutaneousbloodflowisanother potentialrate-limiting stepin thedermal absorptionof chemicals.。