What is the main job of sunscreen and how does it work?
Sunscreens are skin care products with active ingredients intended to absorb or reflect UV. The active ingredients available are generally broken down into two categories: physical sunscreens and chemical sunscreens.
Products containing physical sunscreen ingredients work by blocking UV from interacting with the skin. They reflect UV away from the skin. Examples of physical sunscreens include products that contain zinc or titanium. Often times these can be referred to as sunblocks because they are physically blocking UV from interacting with the skin.
Products with chemical sunscreen ingredients contain compounds that absorb UV to prevent it from interacting with the skin. These can work well but may not always be able to absorb all of the UV that your skin is exposed to allowing some UV rays to come through. There are a number of chemical sunscreen ingredients with the most common being oxybenzone, avobenzone, octisalate, octocrylene, homosalate and octinoxate.
UV from the sun (and other sources) is broken down into UVA and UVB rays. UVA penetrates deeper in the skin and causing premature aging of the skin (fine lines, freckles, discoloration, thinning of the skin, sagging, etc). Think UVA Ages the skin. UVB rays are primarily responsible for causing sunburns and reddening the skin because these rays hit the skin in more superficial layers. Think UVB Burns the skin.
Physical sunscreens tend to block both types of UV as they are broad spectrum while chemical sunscreens can vary.
The main use of a sunscreen is to prevent your skin from experiencing damage from harmful UVA and UVB rays. It’s important to note that the use of a sunscreen is not just to prevent a sunburn. This is an important point to emphasize as many people who do not burn easily in the sun are under the false impression that they do not need to use sunscreens. I often hear- ‘I don’t really get sunburns so I don’t have to use them.’ This is a problem because research and practice has shown that those with a tendency to tan easier than burn still get skin cancer but it is often diagnosed at a later stage as many are simply thinking they are not at risk for it.
If your sunscreen is doing its job then you will prevent premature aging of the skin, sunburns and skin cancer.
How do UVA rays cause aging?
UVA rays penetrate deeper into the skin into the dermis where collagen and elastic fibers are and can damage the DNA of of our cells. It has also been shown to damage the elastic fibers directly in our skin. The elasticity of our skin is the property that allows it to ‘snap’ back into place after making facial expressions like smiling or frowning or simply after pinching the skin. Think of the thin skin under our eyes- if you pinch it then release and the skin has a little lag time before it settles back into place then you have a loss of elasticity. You can blame UVA exposure for this.
How does skin cancer occur?
When it comes to skin cancer it’s important to remember that there are multiple types of skin cancer based on which cells in the skin is affected.
Most superficial skin cells are keratinocytes arranges in five layers. If the top layer of keratinocytes is affected then squamous cell carcinoma develops. This can be caused by both UVB and UVA, however these cells tend to be more susceptible to UVB induced damage to their DNA. If the basal or bottom layer of keratinocytes in the epidermis is affected then basal cell carcinoma develops. There are of course other factors, including genetic, that can increase our susceptibility to skin cancer but UV still plays a key role.
The pigmented cells in the skin, melanocytes, produce melanin to contribute to our skin color and tanning of our skin. Although this melanin can in part serve as a shield to protect the DNA of our cells, the melanocytes themselves are susceptible to UV damage.
There are other cells in our skin- touch receptors called Merkel cells that can cause in merkel cell carcinoma, immune cells called lymphocytes that can cause skin forms of lymphoma, oil producing cells called sebaceous glands that can lead to sebaceous carcinoma, etc. Of these Merkel cell carcinoma, a rare and aggressive form of skin cancer is thought to be strongly linked to excessive UV exposure.
Can you get UV damage even on a cloudy day or through windows?
Yes and yes.
It is very common for even the most diligent of sunscreen wearers to mistake cloudy days for days they do not need to wear sunscreen. Once you have had a sunburn on a cloudy day you learn pretty quickly that the clouds do not block all of the UV light. Clouds only appear to block about 20% of UV rays. The other 80% are still coming through. There is false sense of security from the shade provided by these clouds as they are blocking some visible light to darken the skies and some infrared rays to cool the air, however the residual UV coming through can still burn or damage your skin.
Windows are another source of misunderstanding when it comes to UV exposure. If a window specifies it has a UV filter for both UVB and UVA then you are likely ok. The filters can be clear and do not necessarily requiring tinting. It’s also true that just because a window is tinted does not mean it’s specifically blocking UV rays. There is thought to be a left sidedness to some skin cancers because of the car windows in some cases. Front windshields tend to block both UVB and UVA but the side windows may not. Remember this when you are an airplane looking out the window- with this altitude you are at much higher risk for UVA exposure. Also think about this when you buy sunglasses- make sure they specify broad spectrum coverage to avoid the damaging effects of UV to the eyes.
How is UV damage cumulative?
UV damage has been shown to be cumulative, meaning every exposure adds up to more and more damage. Once the DNA of our cells in damaged then our cells get to work with DNA repair activities. Although we have some remarkable methods of resilience, if the DNA repair mechanisms are constantly being put to work then at some point they will fall behind or fail. The best way to visualize this is to think of a town after a storm comes through that causes some damage. If there are no other storms that come through for a long time then the town will have the time and resources to repair the damage and move on. However, if the town is subjected to one storm after the next, before they are finished repairing from the last, at some point the time and resources to fully recover will be lost. This is exactly what is happening to the DNA of our cells with chronic cumulative and repetitive UV exposure- every time your DNA starts to repair itself, another exposure triggers further damage making it harder and harder to heal.
Beyond aging/skin cancer prevention, what are some other skin
benefits of wearing daily sunscreen?
Aside from protecting our skin from aging and skin cancer, the mineral sunblocks such as zinc or titanium also serve an added role of protecting the skin from irritation and breakdown.
When it comes to choosing the right sunscreen, I tend to recommend sticking with a product that is broad spectrum, easy to use and affordable. The bottom line is just that we want you to use it and take away any barriers to protecting your skin. As long as you have broad spectrum coverage, blocking both UVA and UVB rays then the brand and price point do not matter. Oftentimes the cheaper the better because you will be more likely to use it and use it consistently. It’s ok to choose a version you like- creams, lotions, sprays, sticks, etc
What tips would you give someone on choosing a sunscreen and reapplying during the day?