The Science of Sunscreen

The Science of Sunscreen

How could something that feels so good be so wrong? As a dermatologist, I see the heartbreak of sun damage every day in my office. I see it in the form of skin cancer which can be disfiguring and even life-threatening. And, I see it in the form of premature aging, with skin wrinkling, increased broken blood vessels, and blotchy discoloration.

Many of my patients in their 50’s tell me they wish they knew in their 20’s what they know now, which brings me to my best advice ever: “nothing looks more beautiful in your 50’s than sun protection in your 20’s.”

However, many of my younger patients tell me that they worry about the safety of sunscreen, for themselves and for the environment, while many many patients come in with dark tans but swear up and down that they wore sunscreen, they insist “my skin just tans easily”. On further questioning, we often learn that they only applied the sunscreen once, first thing in the morning, and they didn’t think they needed it at all if it wasn’t sunny out. If you’re that person, this blog post is especially for you.

Fortunately, sunscreen has become an indispensable part of our lives, allowing us to safely spend more time outdoors while protecting our skin from the harmful effects of the sun's ultraviolet (UV). But sunscreen is only one part of being sun-safe, as I’ll explain.

The history of sunscreens is a fascinating one, with significant advancements in both safety and efficacy over the years. The concept of sun protection dates back centuries, however, the modern sunscreen industry only began to take shape in the early 20th century when tanned skin began to be considered as a status symbol of the wealthy with leisure time, but with excessive sunbathing came painful sunburns and so the sunscreen industry was born, to help people spend more time in the sun without burning. Skin aging and skin cancer were not yet clearly connected to excessive sun exposure. That didn’t start to happen until the 1960’s!


Safety of Sunscreens:

Sunscreen safety has come under attack in recent years. Concerns have mostly revolved around certain chemical (absorbed) sunscreen ingredients, such as oxybenzone and octinoxate, which some say are endocrine disrupters, which is based on un-repoduced animal data. while others worry they may have adverse effects on coral reefs and marine ecosystems. However, it is important to note that the FDA has approved their use in sunscreens since the 1960’s and had to go through rigorous testing to get there. Mineral-based sunscreens, containing titanium dioxide and zinc oxide, in 1999. Sunscreen safety is so difficult to get through the FDA that we have not had any new sunscreen filters approved in the US for over 20 years!


Difference Between Sunscreen Approvals in the US and EU:

UV filters are the active ingredients in sunscreens that provide protection against UV radiation. In the US, the FDA regulates sunscreens as over-the-counter drugs, while in the EU, sunscreens are considered cosmetic products which is why there are many more sunscreen filters available in the EU than there are here in the US. Going through the FDA-approval process is costly and can take years, but the process is built around proving safety, so, hopefully, it builds confidence around the products we have available here.


Understanding Sun Protection Factor (SPF) and UV Filters:

Sun Protection Factor (SPF) measurements were developed in the 1970’s as a way to standardize the level of protection a sunscreen offers against UVB rays, which are the ones that are primarily responsible for sunburns, and the UVA star rating system was created in 1980, but has fallen out of favor. We now just recommend looking for the words "broad spectrum" on the label to show that the product has both UVA and UVB protection. 

What we commonly call chemical sunscreens contain organic UV filters which are absorbed into the skin to help protect against the damage from UV rays, while physical sunscreens use mineral UV filters that sit on the surface of the skin and mainly reflect UV rays. Each type has its advantages and disadvantages, but there are many options available, and there is a right sunscreen for everyone, you just have to figure out which one is right for you. 


The higher the SPF number, the longer it takes to burn, in general:

For example:

  • SPF 15 blocks about 93% of UVB rays
  • SPF 30 blocks about 97%
  • SPF 50 blocks about 98%

Put differently, by going from SPF 30 to SPF 50, you get 1.5 times the UV being blocked by the filter, so there is a significant difference between SPF 30 and 50. 


Types of Sunscreens:

There are two types of sunscreens: 

  1. a) Physical Blockers (inorganic): These sunscreens contain active ingredients such as zinc oxide or titanium dioxide. They sit on the surface of the skin and work mostly by reflecting UV rays from the skin's surface.
  2. b) Absorbed (also called chemical or organic) Blockers: These sunscreens contain ingredients that absorb UV rays, such as aminobenzoic acid, avobenzone, octisalate, and octocrylene. They sink into the skin more which makes them more cosmetically elegant, but some worry about their safety, as mentioned above.


Recommended Sunscreen Application:

SPF levels are determined under strict, controlled conditions to allow for proper standardization; however, no one uses the amounts used in testing in the real world. This means you’re not really getting the SPF number that is listed on the label- think of those leggings that get stretched as we put them on and become see-through if over-stretched. It’s the same with your sunscreen, if you don’t use enough, you won’t get proper coverage and damaging UV rays will get through. Also, if you happen to be one of the rare people who apply more than the recommended amount of sunscreen thinking that it will increase your UV protection, there is no need to do that; you will just be getting the same SPF value of your sunscreen, over-applying will not increase the protection beyond that. Also, I’d love to meet you. You would be the one in a million who does that!

In general you should use: 

Ideally you want a sunscreen that is: 

  • SPF of 30 or higher
  • Water resistant (up to 40 minutes) or very water resistant (up to 80 minutes)
  • Provides broad-spectrum coverage (UVA and UVB coverage) 
  • Reapply every 2-3 hours, more often if you’re swimming or sweating it off


How much should you apply?

First thing to know is that all skin tones and types can burn in the sun, so everyone needs proper sun protection. Here’s an easy guide for how much to apply to different parts of the body:

  • Face: Apply around a quarter of a teaspoon or two finger lengths of sunscreen to cover the face. 
  • Arms: Use about two finger lengths of sunscreen for each arm.
  • Torso and Back: Apply about four finger lengths of sunscreen each to cover the front and back torso.
  • Legs: Use about four finger lengths of sunscreen for each leg.
  • Full Body: As a rough estimate, a shot glass or two tablespoons of sunscreen should be sufficient to cover the entire body.


Sunscreen in moisturizer or makeup is fine for the winter but in the summer months when you’re spending more time outdoors it’s better to use a proper sunscreen under your makeup and consider a powder sunscreen for your reapplication throughout the day, as needed.

Also, sunscreen is only one component of being sun smart. This includes:

  • Avoiding midday sun when possible
  • Seeking shade
  • Wearing sunglasses
  • Wearing sun protective clothing
  • Avoid tanning beds


One important aspect to keep in mind is that you need to fully remove any remaining sunscreen at the end of the day. I love a salicylic acid cleanser to help clean deep into your pores and also gentle exfoliating pads to ensure complete removal of product left on the skin after washing.


Bottom Line:

Sunscreen plays a crucial role in safeguarding our skin from the harmful effects of UV radiation. Proper sun protection is a vital component in protecting your skin from the ravages of skin cancer and premature aging. There is no one perfect sunscreen for everyone, but there are many excellent options for every skin tone and skin type, so it is important to look for one that is right for you.

Understanding the history, recommended application for different body parts, types of sunscreens, safety considerations, and the difference between sunscreen approvals in the US and EU can help us make informed decisions about sun protection. Remember to choose a sunscreen with an appropriate SPF, apply it generously, and reapply regularly to ensure effective sun protection and maintain healthy skin.

Remember, sunscreen application, regardless of the SPF rating, should be accompanied by other sun-safe practices such as seeking shade, wearing protective clothing, and avoiding sun exposure during peak hours for optimal protection.

Sunscreen is an important part of being sun smart, but it’s also become confusing because there are so many mixed messages about safety, environment issues and more.


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