How to Understand SPF Numbers

SPF as most people are aware stands for ‘sun protection factor’. This is the number then that appears on the bottles of sun block and sun tan lotion. The higher the number of course, the more protection the sun block provides against UV rays and the safer you should be from getting sun burn, but it can also be more complex than this.


UV rays, are the rays from the sun that cause ageing and wrinkling as well as causing sun burn. There are two types of UV rays however, these being UVA and UVB. It is UVA that causes premature ageing and wrinkling and wrinkles, while it is UVB that can cause sun burn. UV rays can also cause skin cancer as they cause cell damage and can eventually result in mutations in skin cells. When these skin cells reproduce these damages are then copied identically and will spread around the body as cancer.

Types of SPF

SPF can work in two ways – chemical or physical. Sun tan lotions or sun-screens as they are known use this method and will still allow for some tanning. These chemical sun blocks provide a thin layer of chemicals designed to filter out the absorption of UV rays but do not completely block them. Meanwhile ‘physical’ sun blocks block the sun completely. These are the products that are known as ‘sun blocks’ and these contain substances such as titanium dioxide and zinc oxide. While sun block provides the greater protection and ensures you do not get sun burn, it can be bad for oily skin as it is a lot thicker, and also prevents tanning to a greater degree. At the same time it is important to bear in mind that the sun’s rays are also very good for the skin – encouraging the production of vitamin D for example which has many positive effects on the skin and on the body.

Understanding SPF Numbers

When you choose your sun block or sun screen it will contain an SPF number which will range from 2 to 60, which 60 being the ‘strongest’ and offering the greatest protection. These numbers however are proportionate, so that SPF 4 is not twice as effective as SPF 2. For instance, while SPF 2 will absorb 50% of the UV rays, and SPF 15 absorbs 94% and an SPF 30 absorbs 97%.

Calculating SPF

In order to give yourself the correct amount of protection from the sun’s rays you need to use the correct SPF. This should generally be based on the amount of time that is going to be spent in the sun, so that SPF 30 should be used in any situation where you intend to spend more than 20 minutes outside. Bear in mind that even on cloudy days, 80% of the sun’s rays pass through the clouds, so you should still use some sun screen or sun block to protect your skin. When you are going out for the day for regular usage then, an SPF 30 will generally be adequate, however if you are going out sunbathing or to the beach where you will be spending lots of time directly in the sun, then you might as well go for the safest option and go for an SPF 60.

While many foundations and moisturisers contain sun screen, these often are only around SPF 15 and not enough to protect your skin for extended amounts of time. However it does mean that you can use a lower SPF sun tan if you are using it in conjunction with other cosmetics. Another thing to take into consideration is whether you will be getting wet. Here you need to look out for ‘water proof’ and ‘water resistant’ sun tan lotions, and if you intend to go swimming etc, then you should re-apply the protection. Anything marked as ‘water resistant’ will last for 40 minutes underwater, while those that are ‘waterproof’ will last 80 minutes. As a rule then it is a good idea to opt for waterproof sun tan lotion and to re-apply every hour if you are getting wet.

Despite popular belief, skin type has no bearing on the type of sun screen or sun block you need. It is true that some skin types will be less prone to burning than others, however the type of UV rays do not differ and as such you are still doing damage to your skin unless you use adequate protection.

Comments 2
  1. The article refers to "sun tan" lotions a few times, when really the lotions people need to use are "sun screens". Potayto – potahto, perhaps, but since this is a "health guidance" site, I think (totally my opinion) that it would be better to go with "sun screen" lotion.

    Otherwise, I appreciate the information. It seems clear and easy to understand. Thanks!

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