SPF – Why is it Important? How is it Calculated?

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If you have been using sunscreen to secure your skin against the harmful sunrays, you might be familiar with the term ‘SPF’. In case you are wondering what this term means and how SPF is calculated, we have explained these aspects in this article.

The product label of a sunscreen should mention the SPF. This is a number that indicates the level of protection that the sunscreen would provide you from UVB rays. The label should also mention whether the product has passed the broad-spectrum test to protect you against UVA radiation of the sun.


What is SPF?
SPF stands for sun protection factor, a parameter that demonstrates the level of protection that the sunscreen would provide you against UV rays. As you know, exposure to these harmful radiations might cause your skin to sag, degrade the skin quality, or even lead to skin cancer. The number mentioned on the label expresses whether the protection is greater or lesser.

Dermatologists recommend using sunscreen with the SPF level at least 15. To give you an idea, here’s how you can determine the level of protection you can get from your sunscreen.

  • SPF under 15: Low protection against UV rays
  • SPF between 15 and 29: Medium protection against UV rays
  • SPF between 30 and 49: High protection against UV rays
  • SPF over 50: Very high protection against UV rays

How is SPF calculated?
Now that you know SPF states the amount of protection it would provide to your skin, let’s find out how it is calculated. SPF is based on the time that the UV rays take to reach the skin through the sunscreen and affects it, compared to the time needed to damage the skin in the absence of sunscreen.

So, you need to divide the dose of sun radiation required to redden the skin with the amount required to redden it when you don’t use sunscreen.

SPF= dose of radiation to cause sunburn while using sunscreen/ dose of radiation to cause sunburn without using sunscreen

This calculation is made by applying 2 mg of the cosmetic product on skin surface for each square centimeter. In case the harmful radiation from the sun takes 15 times longer duration to harm the skin compared to the case without sunscreen, the SPF is said to be 15.

Theoretically, when you don’t use sunscreen, the unprotected skin starts becoming red after 10 minutes. If you are using a sunscreen with SPF 30, it would take 300 minutes or 5 hours of sun exposure to cause this damage.

Which SPF value should you choose?
The higher the SPF value, the greater protection you get. This, however, doesn’t imply that using a sunscreen with the highest SPF value would completely protect your skin. Some factors that might influence the level of protection include:

  • Time of the day
  • Weather conditions
  • Type of the skin
  • Amount of sunscreen used
  • Other individual factors

While an SPF 15 sunscreen can block 93% of the harmful UV rays, a sunscreen with SPF 50 can block 98%. Therefore, it would be logical to use a sunscreen with higher SPF value if you experience prolonged exposure to the sun.