夏天到了，各类防晒产品都争相登场。据说有一款SPF70的防晒霜在国外面世了，那是不是说我们涂了这个防晒霜以后就可以尽情地在太阳底下活动了呢？这个SPF到底代表什么呢？真的是SPF值越高，防晒时间就越久吗？
Thissummer,sunscreenswithSPF70hitthemarket.Thefair-skinnedoftheworldmightbetemptedtotakethisasanopeninvitationtoslathersomeon,jumpontoaloungechairandbakefor12hourswithoutgettingburned.Thehigherthenumber,thebetter,right?Beforeyouthrowcautiontothewind,though,youshouldknowafewthingsaboutSPF.Whatisit,exactly?Whatdothenumbersmean,andhowhighcantheygo?Weusesunscreentoblockultravioletlightfromdamagingtheskin.TherearetwocategoriesofUVlight--UVAandUVB--thatweconsiderintermsofsunscreen.UVBcausessunburn,andUVAhasmorelong-termdamagingeffectsontheskin,likeprematureaging.SPF,orsunprotectionfactor,numberswereintroducedin1962tomeasureasunscreen'seffectagainstUVBrays.
Todetermineasunscreen'sSPF,testersroundup20sun-sensitivepeopleandmeasuretheamountofUVraysittakesthemtoburnwithoutsunscreen.Thentheyredothetestwithsunscreen.The"withsunscreen"numberisdividedbythe"withoutsunscreen"number,andtheresultisroundeddowntothenearestfive.ThisistheSPF.
SPFnumbersstartat2andhavejustrecentlyreached70.TofigureouthowlongyoucanstayinthesunwithagivenSPF,usethisequation:
MinutestoburnwithoutsunscreenxSPFnumber=maximumsunexposuretime
Forexample,ifyouburnafter10minutesofsunexposure,anSPFof15willallowyoutobeinthesunforupto150minuteswithoutburning.Butbeforeyougrabyourcalculatorandheadforthebeach,youshouldknowthatthisequationisnotalwaysaccurate.Peopleusuallyusefarlesssunscreenthantheamountusedintesting.Intherealworld,theaveragesunworshipperuseshalftheamountofsunscreenusedinthelaboratory,whichcouldresultinasunburninhalfthetime.