How to create any sun protection factor (SPF)

Executive Summary Video

Sun Protection Factor or SPF tells you what fraction of sunlight reaches your skin through the cream/lotion.

Sunlight is made up of many colours as well as invisible rays such as ultraviolet (UV) and infrared rays. Amongst the UV rays, a specific frequency range is called UV-B rays. They are instrumental in skinburn due to their skin penetration ability. In this article, when I refer to sunlight, I would be talking about UV-B rays.

SPF is calculated based on the sunlight blocked when 2 mg of cream is applied to 1 cm² of skin. SPF 20 means 1/20^th of the sunlight reaches the skin and the rest is filtered out when such a coat is applied. In other words, 5% of the sunlight reaches the skin and 95% is absorbed by the sunscreen.

If you apply 4 mg instead of 2 mg, the coat will be twice as thick. And it will block far, far more sunlight.

By understanding simple science, you can literally create any SPF level provided you can practically manage a proper coating of sunscreen or lotion onto the skin.

My Views

1. Let us understand basic physics. The amount of sunlight absorbed at any point in a coat of lotion is the function of sunlight itself. The more the sunlight, the higher is the absorption. A physicist would know that such light rays will decay in an exponential manner as they penetrate the sunscreen layer. That is, if the rays reduce to 1/3^th within a certain thickness, they will decay to 1/9^th in double the thickness. Using this property, you can theoretically get any SPF you want from any cream SPF.
   By applying a coat twice as thick—4 mg per square cm—of an SPF 20 cream will allow 5% of 5% sunlight to pass through.
   5% x 5% = 0.25%
   That would be equivalent to SPF 400. So by simply applying a thicker or thinner coat, you can create any SPF, if you can take care of the practicality of controlling such thickness.
2. One reason to choose a high SPF is because most people apply less than half as thick a coat as is recommended. So if they use an SPF 25, they end up getting SPF 5 level of protection (square root of 25 = 5).
3. The SPF number mentioned on a bottle is roughly equivalent of using 6 teaspoons or 2 tablespoons of sunscreen for the whole body of an average adult. It is also equivalent of using half a teaspoon each on arms, plus another half a teaspoon on face and neck together.
4. SPF decisions should also be made based on the skin type. White-skinned people get skinburns much faster with the same amount of sunlight and so they need a higher SPF.
5. SPF decisions also depend on the UV Index, which indicates how strong the outdoor ultraviolet radiation is at that time.
6. Ultraviolet rays are invisible to the eyes. So we cannot tell if they are falling on our skins. Even if one is sitting in a shade, one receives ultraviolet rays reflected from various surfaces including the ground and the water.
7. Ultraviolet rays penetrate water up to at least 1 meter of water. So sunscreens are important even if one is swimming. In such cases, water-insoluble sunscreens are needed.
8. Sunscreens lose their efficacy after about two to three hours and need to be reapplied.
9. In people with darker skin, UV-A rays are more important than UV-B. UV-A rays cause premature photoageing of the skin, while UV-B rays cause skinburns.
   In white-skinned people, UV-B rays will cause the skin to burn so quickly that they don’t have to worry about UV-A rays related damage. However, in dark-skinned people, UV-B rays cause very little damage because the layer of melanocytes (pigment making cells in the skin) absorbs a lot of them. So they can sit in the sun for long durations comfortably without realising that UV-A rays may be causing permanent damage to their skins.

For More Reading

• Wikipedia: Sunscreen
• US FDA: Sunscreen: How to Help Protect Your Skin from the Sun
• MedicalNewsToday: Which sunscreen should I use?
• US Environmental Protection Agency: UV Index
• Mayo Clinic: Best sunscreen: Understand sunscreen options
• On this website: How to Prevent Wrinkle Formation

First published on: 16th December 2021
Image credit: Nikolaj Erema on Pexels