No one thinks of applying sunscreens before going out when it is raining or overcast. But do you know that your skin gets damaged by sunrays even during that period? Worse, you may spend more time outdoors with the skin unprotected, causing additional harm. Here is the interesting science behind that.
Sunscreen protects your skin from the sun’s ultraviolet (UV) rays. So if there is no harsh sunlight, you would logically think that sunscreen won’t be needed.
But ultraviolet rays are invisible. How do you know that they are not falling on your skin when there is a cloud cover? Your reasoning may be that the brighter the visible light, the stronger would be the invisible UV rays. Good guess but wrong science!
Skin Layers
Your skin has three main layers:
- The top layer—the epidermis—protects your skin from outside chemicals, pathogens, and UV rays. It has cells that produce pigments that give colour to the skin.
- The middle layer—the dermis—gives firmness and suppleness to your skin.
- The bottom layer—the hypodermis—prevents your skin from rubbing against muscles underneath it.
The top two layers are affected by excess sunlight.
Sunlight and Ultraviolet Rays
Sunlight is composed of 50% visible light, 40% infrared light, and 10% ultraviolet light. The latter two are invisible to your eyes.
What if something filters most visible rays out of the sunlight, and allows the rest unhindered? What if that something is your cloud cover? Let’s explore a bit more.
In sunlight, the UV rays are the most energetic ones and cause maximum skin damage. They are divided into three parts in increasing order of frequencies: UV-A, UV-B, and UV-C. Each one causes a different type of skin injury.
UV-A Rays and Skin Damage
Of the three types, UV-A rays penetrate the deepest, all the way to the skin’s middle layer, which is made of collagen and elastin proteins giving the skin firmness and elasticity, respectively.
The energy of UV-A rays breaks these protein strands. With repeated and long exposures, the skin loses its strength, becoming saggy, wrinkled, thick, and leathery. It may also develop freckles and brown spots. This is called photoageing or premature skin ageing due to sunlight.
This skin damage is cumulative—if you expose your skin to UV-A rays for four hours today, and another three hours two days later, the total skin damage will be equal to seven hours of continuous exposure to UV-A rays.
UV-A rays are not absorbed by the atmosphere or the cloud cover and they fall almost unhindered on our skin. Their intensity does not change during the day or throughout the year. They even penetrate windows as glass cannot absorb them.
If you are out in the rain during the daytime, the UV-A rays still fall on your skin with full intensity through thick clouds—the premature ageing of your skin continues unabated.
The darker the skin, the lesser the UV-A damage because the skin pigment melanin absorbs some UV rays. White skin allows 55% of UV-A rays to pass through to the dermis, while black skin allows only 17.5%. In other words, average black skin will need thrice as long an exposure as average white skin for the same amount of photoageing.
UV-A rays increase the risk of deadly skin cancers called melanomas. While they do not cause skin cancer directly by breaking the cell DNA, they build oxidative stress in the skin which causes these cancers.
Incidentally, tanning beds mainly use UV-A rays and are not safe for long-term use.
UV-B Rays and Skin Damage
UV-B rays are of higher energy than UV-A ones and dump their power into the top skin layer. Such a concentrated burst of power burns the skin, accompanied by pain, swelling, redness, and blisters.
UV-B damage is not cumulative. If you sit in the sun for fifteen minutes today, and again for fifteen minutes three days later, you will not get a skin burn equivalent to thirty minutes of UV-B exposure.
The atmosphere absorbs most UV-B rays coming from the sun. As a result, the UV rays in the sunlight reaching you have only five per cent as UV-B radiation and the rest are UV-A rays. That proportion drops further as the sun gets closer to the horizon.
UV-B rays are the strongest from late morning to mid-afternoon when the sun is directly overhead.
The window glass absorbs UV-B rays and so if you sit behind such a window, you will not face any UV-B ray damage.
The darker the skin, the lesser the UV-B damage. White skin allows 24% of UV-B rays to pass through, while black skin allows only 7.4%. In other words, average black skin will need thrice as much exposure as average white skin to get the same amount of sunburn.
UV-B rays increase the risk of some skin cancers—basal and squamous cell carcinomas, which are less dangerous than melanomas caused by UV-A rays. But they induce this damage directly by breaking the cell DNAs, as their wavelengths are similar to DNA strand lengths.
Read on this Website: Do You Need Higher SPF Sunscreens?
UV-C Rays and Skin Damage
UV-C rays are the most dangerous rays.
However, they are completely blocked by the ozone layer in the earth’s atmosphere and don’t reach us. You need to worry about their presence in the sunlight only if you are an astronaut in space!
There are certain lamps and lasers that emit UV-C rays. One example is the mercury vapour lamps used for lighting the stadiums. They can cause severe skin burns if you happen to be close to them—a rare possibility.
Here is how to remember the dangers of UV rays for the skin:
- UV-A: A for Ageing;
- UV-B: B for Burning.
Sunscreens For Cloudy or Rainy Days
As discussed, UV-B rays and sunburn damage goes down significantly when the conditions are overcast. On the other hand, UV-A rays and skin ageing continues unrestrained during heavy rains, too. To worsen matters, people may spend time outdoors without applying sunscreen when the day is cloudy or rainy.
In summary, you need sunscreen to protect against UV-A rays, rain or shine. But it will need to be different from the one that protects against sunburn or UV-B rays.
Since using multiple sunscreens would be cumbersome, it is better to have one with UV-A and UV-B protection together.
UV-A Rating
Sunscreens were originally developed for preventing sunburns in white-skinned people. So they were rated for their ability to protect against UV-B rays. That number is called the Sun Protection Factor (SPF). The higher the SPF, the better the UV-B absorption by the sunscreen.
However, SPF ratings do not indicate a sunscreen’s UV-A protection ability; a separate UV-A rating is needed for that.
There are different ways to judge UV-A protection, the main one being how long it takes for the skin to tan with sunscreen applied compared to without one. If it takes ten times longer exposure, the sunscreen is said to have a UV-A Protection Factor (UVA-PF) of ten.
Unfortunately, different countries have distinct UV-A ratings. I will cover that in a separate article. Look for one with the best UV-A rating in your country, and use it every day.
Summary
- When it is cloudy or rainy, you still need to apply sunscreen; but it should be capable of protecting against UV-A rays.
- Modern sunscreens offer protection against both UV-A and UV-B rays.
- Different countries have their own way of rating UV-A protection.
- Look for a sunscreen with a high UV-A rating in your country.
To Read More
- Cancer Research, UK: How does the sun and UV cause cancer?
- Twindly: Understanding the label: UVA protection on sunscreen
- Skin Cancer Foundation: UV Radiation & Your Skin
- World Health Organisation (WHO): Radiation: Ultraviolet (UV) radiation
- NHS, UK: Sunscreen and sun safety
- Cleveland Clinic: Ultraviolet Radiation and Skin Cancer
- Read on this Website: How To Create Any Sun Protection Factor (SPF)?
First Published on: 26th June 2023
Image Credit: gpointstudio on Freepik