Our Essential Sunscreen (Part II) - The Science and Scaling of things
- cosmeticelement
- Jun 3, 2022
- 4 min read
Apart from the joy and warmth, sunshine also contains powerful ultra violet rays that literally cooks our skin.
UV rays are a spectrum of light invisible to our naked eye. Some animals and insects have the ability to see them. It is broken down to UVA,UVB and UVC according to their wavelengths measured in nanometers(nm).
UVC rays are scattered by our atmosphere, mainly by the ozone layer and rarely hits our skin. UVB is stopped by our skin and causes burns and skin cancer while UVA is powerful enough to penetrate deep into our skin and causes premature aging. Both causes skin to darken, but with different effects. Tanning from UVA rays are more immediate and fades typically within days while UVB tans takes up to 72 hrs to develop but can persist over months. Our bodies are more capable of reversing DNA damages done by UVA than UVB.
[A graph of UV ray wavelengths in nanometers]
Sunscreens work by either absorbing or reflecting UV rays.
Chemicals with light absorbing qualities are referred to as chromophores. While there are chemicals with the ability to absorb the full spectrum of UVA and UVB rays, most only absorb a fraction. A typical chemical sunscreen formula have to contain more than one kind of chromophores to be able to absorb all UV rays.Take Homosalate for example, the chemical absorbs UV rays with the wavelength between 295-315 nm only. This does not cover all UVB rays and none of the UVA rays, other chemicals have to be added to cover the unabsorbed UV rays.The picture below illustrates some common types of chemical absorbers used in the market and the spectrum of UV rays they can absorb.
There are decidedly less ingredients used in commercial sunscreen as UV reflectors. Ones currently being used are zinc oxide and titanium dioxide. In theory, how they work is that the ingredients sits on the surface of your skin like an amour and scatters UV rays, preventing them from touching your skin. This is a bit of a misconception. It is true that the ingredients do reflect UV rays, but the grade of the mineral compounds used in commercial sunscreen caused them to behave more like UV absorbers.
[Advanced Knowledge! The compounds absorbs UV rays through a process called band-gap absorption. For zinc oxide, the absorbed UV ray is transformed into infra-red rays and released as harmless heat. For titanium dioxide, the process is not as smooth due to its photocatalytic effect. The mineral compound is typically covered in aluminum oxide to prevent formation of free radicals.]
What is SPF and PA+ ?
In 1962, Greiter introduced the SPF( sun protection factor) to measure UVB. The SPF number is not an absolute indicator but a faction. It indicates the faction of UVB rays that will be absorbed by your skin. If you apply SPF 30 sunscreen, only 1/30 of UVB ray will reach your skin. By simple calculation, that means 97% of UVB rays are blocked. SPF 50 provides 98% of protection,SPF 100 provides 99% while SPF 130 provides 99.99%. The recommended SPF for city use is 25-30. All sunscreen have to be reapplied every 2 hrs for maximum effectiveness. The higher the SPF number does not indicate longer durability. The fabric sunblock scale UPF uses the same scaling as SPF.
For UVA, the PA (protection grade of UVA) scale was introduced by the Japanese cosmetic industries based on the persistent pigment darkening (PPD) method. PPD, like SPF, is not an absolute measurement. It measures the times of UVA absorption cut downs. A 2 PPD means you absorb 2 times less UVA than normal. There are 4 levels in the PA scale:
The American cosmetic market do not use the PA scale. They indicate the presence of UVA protection in the products by using the word broad spectrum on the packaging of the sunscreen. No specification is given on the effectiveness and durability of the UVA protection in the products.
Why is it so important to put on sunscreen?
It's because the UV rays in the sun causes damage to our DNA. UVB breaks down the collagen in your skin and causes wrinkles. While our body is designed to repair the damage by the sun, not all can be fixed. Cases of skin cancer went up hundred folds since the sun bathing and tanning salons visits started. Research shows the risk of developing skin cancer increases 75% if a person uses tanning beds before the age of 30!!
It is important that we protect ourselves against the sun.
It is recommended to stay out of direct sunlight from 10am to 4pm. Babies under the age of 6 months should be kept out of the sun since their skin is not yet mature. Always wear sunscreen when staying for long periods in the sun. And the sunscreen application formula?
1 teaspoon for the head area
1 teaspoon per arm
2 teaspoons per leg
2 teaspoons for the whole torso.
Apply sunscreen at least 20 minutes before any sun exposure to ensure better absorption.
Remember to re-apply every 2 hours.
Related Topic in CER:
Source and Further Information:
Bernd Herzog, Photoprotection of Human skin
Dupont, Gomez and Bilodeau, Beyond UV radiation: A skin under challenge. http://www.idcdermo.com/files/documents/8b/50/beyond-uv-radiation-a-skin-under-challenge.pdf
Photochemistry Volumn 40 published by The Royal Society of Chemistry 2012
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