SmellslikeChlorine?
ByStephanA.Hubbs,PE
Theysay“thenoseknows,”butIsaythenosecanbeconfused.Chlorineodorsareagoodexample.
Severaldifferentchlorineodorscanarisefromvariouschlorine-based
substancesandindifferentcircumstances.Theyarenotallsimplydueto
“chlorine.”Aprimeexampleistheirritatingsmellcommonlyattributedto
chlorinearoundsomepoorlymanagedswimmingpools.Thatsmellisfrom
acoupleofchemicalcompoundsinthechloraminefamily.Some
chloraminesformwhenchlorinedisinfectantsreactchemicallywith
nitrogen-basedsubstancesfromthebodiesofswimmers,includingurine.
Thepoolsidepronouncementof“toomuchchlorineinthepool”maybe
moreaptlydescribedas“toomuchpeeinginthepool.”Ironically,the
Thenoseisanextremely
odorcouldsignalthatmorechlorineisneededinthepool.
NotOneChloramine
Chloraminesstartoutasammonia–NH3—whichlookslikeathree-legged
sensitive“detector”that
sendsinformationtothe
brainwhereitis
interpreted.
stoolwiththenitrogenatomasthe“seat”andahydrogenatomattheendofeach
“leg.”Ammoniaiscommonintheenvironment,andalthoughhousehold
ammoniahasaverysharpodor,ammoniahasnoodorattheverydilutelevels
typicallyfoundinwater.Whenchlorineisaddedtowaterinsufficientamounts,it
breaksammoniadownintonitrogen(N2)gasandhydrogen(aswaterorH2O).But
iftheamountofnitrogenincreases(frompeeinginthepool,forexample),the
balancebetweenchorineandnitrogenisdisturbedandtheammoniaisonlypartiallydeconstructed.
Whenthreechlorineatomsreplacethehydrogenatomsontheammonia
molecule,theresultingcompoundistrichloramine,apungent,irritating
compoundthatisoftenmistakenfor“toomuchchlorine”inthepool,evenat
verylowconcentrations.Trichloramineanditssisterchemicaldichloramine
(withtwochlorineatomsandonehydrogenformingthe“legs”)areresponsible
fortheodoryoumightsmellwhenyouenteranenclosedpoolareainwhich
thereispoorairflowthroughthebuilding.Besidesjustsmellingbad,thesesame
twochemicalscanalsoturnswimmers’eyesred.
Theproblemwithtrichloramineanddichloraminegoesbeyondthatirritating
smell.Whentrichloramineispresentinthepool,thelevelof“freechlorine”
availabletodisinfectthewaterandprotectswimmersfrommicrobialdiseaseisgreatlydiminished.
That’swhyit’simportanttocheckpoolwaterregularlyforthepHandthepresenceoffreechlorine1.
Poolmanagersareobligedtodojustthat,andpoolpatronscandothesamewitheasy-to-usepooltest
strips.
Trichloramineanddichloraminehaveathirdsister–monochloramine.Asthe
namesuggests,inthiscompound,onlyoneofthehydrogenatomsonthe
ammoniahasbeenreplacedwithchlorine.Thischemical,however,carriesno
odorandisaneffectivedisinfectantthathasbeenusedsuccessfullybymany
watertreatmentplantsformanydecades.Whatadifferenceaminorchangeina
chemicalformulacanmake!
Monochloramineistypicallyformedinsourcewaterbyfirstaddingchlorinetobreakdownany
ammonia(intonitrogenandwater)andthenaddingbacktraceamountsofammoniaincarefully
monitoredamountstoproduceonlymonochloramine.
BleachSmell
Unlikethechloraminefamilyofcompounds,chlorinebleachisasolutionofwaterandsodium
hypochloritewiththechemicalformulaNaOCl[bleachcontainssodium(Na),oxygen(O)andchlorine
(Cl)].Theodorofbleach(bleachthatisnotinfusedwithafragrance)isnotnearlyaspungentasthatof
di-andtrichloramines.Whenthechlorineinbleachcombineswithnitrogen-basedsubstances,
however,smellydi-andtrichloraminescanformbychemicalreaction.Thiscouldhappeninthe
swimmingpoolenvironment,forexample.
ChlorineGas
Chlorineisacommon,naturallyoccurringelement,butduetoitsreactivity,itusuallyoccursinnature
combinedwithotherelements,suchassodiumincommontablesalt,NaCl.Itisalsoproduced
industriallyaspartofthe“chlor-alkali”processinwhichbothchlorineandsodiumhydroxide(NaOH)are
generatedbyapplyingelectricitytosaltywater.Inthatprocess,chlorineisproducedasagasconsisting
oftwinatomschemicallyboundtogether,andisrepresentedbychemistsas“Cl2.”Bothchlorineand
sodiumhydroxideareusedtohelpproducehundredsofeverydayproducts,includingwatertreatment
chemicals,PVCpipes,pharmaceuticals,paper,aluminum,siliconchipsforcomputersandeventhe
titaniummetalusedinjointreplacements.
Chlorinegasisarespiratoryirritantthatthehumannosecandetectatverylowlevels(0.2-0.4partsper
millioninair;justforreference,onepartpermillionisequivalenttofourdropsofinkina55-gallon
barrelofwater).Attheselowconcentrations,chlorinegassmellsverymuchlikehouseholdbleach.
Whenlevelsrisetotherangeof1-3ppm,however,mildmucousmembraneirritationisnotedand
higherlevelexposurebecomesincreasinglydangerous.2Thatiswhythechlor-alkaliindustrytakes
1
Hypochlorousacid,orHOCl,foryouchemists!
TheChlorineInstitute,Inc.,Pamphlet63:FirstAid,MedicalManagement/SurveillanceandOccupationalHygiene
MonitoringPracticesforChlorine,Edition8,June,2011.
2
extensivemeasurestoensurethesafeproduction,handlingandtransportationofchlorinegasandeven
thelessreactivesodiumhypochloriteandcalciumhypochlorite.
NotJustOneChlorineCompound
Chlorineisareactiveelementthatisfoundinmanydifferentcompoundsexhibitingdifferentbut
sometimessimilarproperties,includingodor.Itiscreditedtohavedramaticallyimprovedpublichealth
overthepast100yearsthroughitsabilitytodisinfectwater.Ihopethisdiscussionhelpstoclarify“the
smellofchlorine.”Nowyournoseknows!
StephanA.HubbsretiredfromwatertreatmentoperationsattheLouisvilleWaterCompanyin2004.Hewas
involvedinthedevelopmentofthefirstchlorineby-productsregulationin1975-1979andremainsanactive
volunteerinthedrinkingwatercommunitytoday.
www.waterandhealth.org