What is SPF? an inside look into your Minimum Erythematous Dose (MED)
SPF (sun protection factor) reflects the measure of time that you will be protected from UVB light that reaches us from the sun.
So how long you will be able to stay in the sun before you burn? Realistically you can only assess the real protection you are getting from a sun protection product when you know how long it will take you to turn red or darker. This time is called the Minimum Erythema Dose or MED.
If my MED is 10 min and my SPF is SPF 25, it will mean that when I have my sunscreen on; in the right amount; it will take me 250 minutes before I will burn (My personal MED X the SPF = Potential protection time). If I used an SPF sunscreen of 50 this will go up to 500 min. The challenge is that if I applied my sunscreen once only and spent 500 minutes in the sun, I would probably too late and burnt my sin. This is not the ultimate way to estimate your protection but only give us an idea. Hence why all product houses suggest that you re-apply your sunscreen every two to three hours when you are in direct sunlight.
The intensity of sunlight will affect the time that you can spend in the sun. The higher the intensity the less protection you will have. Cloud cover also changes this. On a cloudy day the UVB light is actually scattered so you might take longer to burn. The problem is that UVA light passes thorough clouds and may still cause harmful damages. The UVA frequency of light is the light that causes you to go brown without feeling any burn. The unfortunate truth is that UVA Light is very damaging as it is able to move deeper into the skin. This frequency is the most aging frequency.
Another factor to keep take into account when you are looking at your SPF is to consider if you are going to be in the water or sweating at all. Water will wash off sunscreens and sunblocks as they are only water-resistant, not waterproof. You will need to re-apply your sunscreen as soon as you come out of the water or if you have been sweating.
The rule of thumb regardless of MED or your daily activity is to re-apply your sunscreen every two to three hours if you are going to be in direct sun.