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People can see SPF in most specifications and the three letters is an abbreviation of sun protect factor， and the number after it is a measurment of how well your skin can be kept well covered by suncream. SPF15 means that it can blochk 93% UVB， while SPF30 deals with 97% ， as for SPF50 ， block 98% UVb.
SPF also means something else. if your skin get sunburnt in 10 minutes， the number behind SPF tells you the times that caused by the same sunburnt after you wear sunscreen. For example， if it
Takes ten minutes to get sunburnt for bare skin， then it will take 150 minutes to get sunburnt after you wear suncream with signal of SPF15. but the sunlight is always changeable， its intensity varies with time， besides， it is very difficult to wear even suncream， so it is wrong to think you can stay in the strong sunligt since you have wore SPF30 suncream. The number is only a index of how skin can be freed from sunburnt， it does not mean how long you can stay outside.
SPF also means something else. if your skin get sunburnt in 10 minutes， the number behind SPF tells you the times that caused by the same sunburnt after you wear sunscreen. For example， if it
Takes ten minutes to get sunburnt for bare skin， then it will take 150 minutes to get sunburnt after you wear suncream with signal of SPF15. but the sunlight is always changeable， its intensity varies with time， besides， it is very difficult to wear even suncream， so it is wrong to think you can stay in the strong sunligt since you have wore SPF30 suncream. The number is only a index of how skin can be freed from sunburnt， it does not mean how long you can stay outside.