Sunscreens protect your skin from harsh sunlight by absorbing some of its rays. The more the absorption, the better the safety.
Their shielding ability is measured with a number called Sun Protection Factor (SPF). Sunscreens with higher SPF numbers offer better skin safeguarding.
Cosmetic companies pander to this ‘more is better’ mindset and promote more expensive, higher-SPF sunscreens, which may not be needed for darker skin types. Read on this website: Do You Need Higher SPF Sunscreens?
In this article, I will give you a simple trick to create a higher SPF effect using a low-SPF sunscreen: Just apply a thicker coat.
Here is the science behind it, if you wish to know why and by how much. And if you don’t relish technical calculations, skip to the summary at the end.
Sunscreens and SPF
SPF indicates what fraction of sunlight passes through the sunscreen layer. For example, an SPF20 sunscreen coating permits one-twentieth or five per cent of the sunlight to get through. How thick should that layer be?
SPF is calculated after two milligrams of sunscreen are evenly spread on a square centimetre of skin. This is equal to a twenty-micron thick layer—typical of what we apply—for most sunscreens. Of course, this is incorrect science because the sunscreen effect is based on the layer’s thickness and not its weight. But, let’s leave that part from the discussion.
Sunscreens and Physics of Sunlight Absorption
As sunlight passes through the sunscreen layer, its intensity keeps reducing. A physicist would know that this reduction is exponential in nature. That means, if a certain layer thickness reduces the light intensity to half, doubling the thickness will reduce it to one-fourth.
Let us take an SPF10 sunscreen. Applying a twenty-micron thick layer will give a ten times reduction in sunlight.
What happens if you double the thickness to forty microns? After the first twenty microns, the sunlight will drop from hundred per cent to ten per cent and over the next twenty microns, it will drop from ten per cent to one per cent. This is effectively an SPF100 sunscreen.
Where have you seen such a relationship: 10 (SPF) corresponds to 1 (thickness) and 100 to 2? Yes, logarithms!
log 10 = 1 and log 100 = 2.
So if you take the logarithm of the SPF you want, you will get how thick the new coat of SPF10 sunscreen will need to be.
Example: To create SPF20 Result from an SPF10 Sunscreen
log 20 ≈ 1.3
So if you apply 1.3 times—or 26-micron—thick layer of an SPF10 sunscreen, you will get the SPF20 effect.
Similarly, a 48 per cent thicker—or 30 microns thick—layer of SPF10 will give you SPF30 results.
Madhur’s SPF Conversion Rule 😉
To convert an SPF-M sunscreen to an SPF-N sunscreen, you simply apply a (log N / log M) times thicker coat than the standard twenty-micron layer of SPF-M sunscreen.
Example: To create SPF50 Effect from an SPF15 sunscreen
log 50 / log 15 ≈ 1.44
So instead of a twenty-micron layer, apply 1.44 times thicker—29-micron-thick—layer of SPF15 to get SPF50. Why pay exotic amounts of money for the same, if you can bear a slightly thicker coat?
- If you apply a standard 20-micron thick layer of SPF10 sunscreen, it will reduce sunlight by 10 times.
- If you apply a 30% thicker, or a 26-micron thick, layer of SPF10 sunscreen, it will act as SPF20.
- Similarly, a 48% thicker or a 30-micron thick, layer of SPF10 sunscreen, will effectively act as SPF30.
- And to convert SPF-M sunscreen to SPF-N sunscreen, apply a 20 micron x (log N / log M) thick layer of SPF-M sunscreen on your skin.
- Don’t overdo this idea! A thirty per cent thicker layer or doubling of SPF is sensible; beyond that, very thick sunscreen layers are uncomfortable, impractical, and hence avoidable. That should be the last resort idea when you are in a high-radiation situation such as a beach outing but have only a low SPF sunscreen at hand.
Things To Keep In Mind
- Apply a proper amount of sunscreen, as given later. Most people apply a thinner layer, which means they are not getting full protection.
- Use six teaspoons or two tablespoons (30 mL) of sunscreen for the whole body of an average adult. Out of that, half a teaspoon goes on each arm and another half a teaspoon on the face and neck together. These will correspond to twenty-micron layers.
- SPF decisions should be made based on the skin type. White-skinned people get skin burns much faster with the same amount of sunlight and so they need a higher SPF.
- SPF decisions also depend on the UV Index, which indicates how strong the outdoor ultraviolet radiation is at that time.
- Ultraviolet rays are invisible to the eyes. So we cannot tell if they are falling on our skins.
- Even if one is sitting in the shade, one receives ultraviolet rays reflected from various surfaces including the ground and the water. The intensity is lesser than direct sunlight but it is harsh enough that you should apply sunscreen.
- Ultraviolet rays penetrate at least one metre of water. So sunscreens are important even if you are swimming. In such cases, water-insoluble sunscreens are needed.
- When in doubt, err on the side of caution and apply sunscreen.
- Sunscreens lose their efficacy after about two to three hours and need to be reapplied.
- In people with brown or dark skin, a different type of ultraviolet rays is more important than that which causes skin burns. So SPF-based sunscreens are less relevant for them. But more about that in a different article.
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: Dimitris Vetsikas from Pixabay
Last Updated on: 4th June 2023