What is SPF in sunscreen creams? I go to 2500m-4000m altitude. What should I use? And I've heard that SPF more than 55 is not for men. Does more SPF mean more sun protection?
As it is stated in this Wikipedia article, the sun protection factor (SPF) roughly describes how the time that your skin is able to protect itself from sunburn is elongated. To take the Wikipedia example:
if a person develops a sunburn in 10 minutes when not wearing a sunblock, the same person will prevent sunburn for 150 minutes if he/she wears a sunblock with a SPF of 15
So, yes, more SPF means more protection. How much protection you need depends on how intense the sun is where you are, how long you are about to stay in the sun, and how sensitive your skin is.
In general one can say, that if you are somewhere with highly reflective surfaces (snow, water...) you need more protection than somewhere in a light forest. Also the higher you are, the more SPF you should take, as in higher altitudes less of the UV radiation is absorbed by the atmosphere. Therefore one cannot give a definite recommendation on what SPF you should use.
Also if your skin requires an SPF of 50 or higher, you will need it – male or not male – and in high altitudes on a glacier you might need it.
If you are totally unsure about what is suitable, I would recommend seeing a dermatologist to find out which skin type you are and what protection will be needed for it.
I really like the existing answer, but I feel like it is a bit misleading in regards to what SPF really means. Reading that quote makes it seem like SPF is a linear relationship between SPF and the time before you will get burned. That isn't strictly true. A more extended quote from Wikipedia is (emphasis mine):
The sun protection factor (SPF rating, introduced in 1974) is a measure of the fraction of sunburn-producing UV rays that reach the skin. For example, "SPF 15" means that 1/15th of the burning radiation will reach the skin, assuming sunscreen is applied evenly at a thick dosage of 2 milligrams per square centimeter (mg/cm2). A user can determine the effectiveness of a sunscreen "by multiplying the SPF factor by the length of time it takes for him or her to suffer a burn without sunscreen."a Thus, if a person develops a sunburn in 10 minutes when not wearing a sunscreen, the same person in the same intensity of sunlight will avoid sunburn for 150 minutes if wearing a sunscreen with an SPF of 15.a It is important to note that sunscreens with higher SPF do not last or remain effective on the skin any longer than lower SPF and must be continually reapplied as directed, usually every two hours.b
So as you can see the SPF 15 gives 150 minutes of protection is more of a math exercise to see equivalent exposure levels. It isn't extendable to say that applying SPF 50 sunscreen one time will give me 500 minutes of protection since sunscreen needs to be periodically reapplied.
The other bit that I wanted to add was why people say that after SPF 50 the benefits are negligible. This is because you get less benefit as the SPF goes up. Using the 1/SPF rule from above you see:
- SPF 15 blocks 93.33% of harmful UV rays
- SPF 30 blocks 97.67% of harmful UV rays
- SPF 50 blocks 98.00% of harmful UV rays
- SPF 75 blocks 98.67% of harmful UV rays
- SPF 100 blocks 99.00% of harmful UV rays
- SPF 200 blocks 99.50% of harmful UV rays
As you can see you get big gains from using some sunscreen, and then as the numbers get higher you get less bang for the buck (practically speaking). To back this up consider this recommendation from the American Academy of Dermatology (Emphasis mine):
Dermatologists recommend using a sunscreen with an SPF of at least 30, which blocks 97 percent of the sun’s rays. Higher-number SPFs block slightly more of the sun’s rays, but no sunscreen can block 100 percent of the sun’s rays. Currently, there is not any scientific evidence that indicates using a sunscreen with an SPF higher than 50 can protect you better than a sunscreen with an SPF of 50.
It is also important to remember that high-number SPFs last the same amount of time as low-number SPFs. A high-number SPF does not allow you to spend additional time outdoors without reapplication. All sunscreens should be applied approximately every two hours or according to time on the label, even on cloudy days, and after swimming or sweating.
Instead of boosting your SPF beyond 50 you should consider your skin's sensitivity to the ingredients, and the required reapplication interval.
As other answers have already explained what SPF is, I will focus on an aspect of high SPF sunscreens that has not been discussed here yet.
There are two ways sunscreen can work. The first is by passively blocking UV light (e.g. titanium dioxide and zinc oxide based sunscreens. The second type relies on various organic compounds to actively react with UV rays and absorb them. Very high SPF sunscreens (more than 50 SPF or so) are of the absorbing type.
There are a few downsides to actively absorbing sunscreens. First, some of the organic chemicals used may have health risks and have even been banned in some countries. A more practical downside is that to achieve a very high SPF, one must use a wider array and higher concentrations of UV absorbing organic chemicals. A side effect is that high SPF suncreens are more unpleasant if you happen to get them in your eyes (which is easy to do). I have experimented with very high SPF sunscreens to see if there was any benefit at higher altitudes and getting very high SPF sunscreen in your eyes is quite unpleasant. By contrast, my eyes do not get irritated when I use 50 SPF titanium and zinc oxide based sunscreens.
Although it won't be an issue for most, due to the longer list of UV absorbing chemicals in higher concentrations, very high SPF sunscreens are also more likely to irritate the skin and some users have reported adverse reactions that they did not experience with lower SPF sunscreens.
The bottom line is that there are disadvantages to very high SPF sunscreens and there is no real benefit. As mentioned in other answers, SPF 50 already gives you 98% protection. The highest available SPF I have seen is 110 which provides about 99.1%. This simply is not enough to make a significant difference.