Sunscreen 101: What Does SPF Mean?

sunscreen 101 difference uva uvb ray


Sunscreen 101

Everything You Need to Know About Sun, SPF and UVA/UVB


As summer winds down and the hottest month of the year hits New York, I thought I’d write a Sunscreen 101 post covering some common misconceptions about sun damage, the best sunscreen formulations and the best products out on the market.

Read on to learn about the difference between UVA and UVB rays, why I prefer physical sunscreen formulas and the right amount to apply!





What are UVA vs. UVB rays?

Sunlight is composed of two types of rays, ultraviolet A (UVA) and ultraviolet B (UVB).

Of the two, UVA is more damaging long term as it penetrates deep into the dermis of the skin. Prolonged UVA exposure causes premature skin aging, wrinkling and even suppression of the immune system. Studies have shown that UVA rays can DNA damage to skin cells over time and increase the risk of malignant melanoma.

Meanwhile, on a day to day basis most people are concerned with UVB rays, which burn the topical layer of the skin and cause immediate burns and redness. Continued topical damage to the skin from UVB rays causes skin cancer.

 Read more:  The Best Packable Sun Hats





What do SPF number mean? 

Sunscreens are classified by their sun protection factor (SPF) where each rating is a measurement of the fraction of sunburn producing UV rays that reach the skin.

For example, SPF 15 means 1/15 of the burning radiation will reach the skin if the sunscreen is applied at the even dosage of 2 milligrams per sq centimeter. Unfortunately the scale isn’t simple and intuitive:

  • SPF 15 blocks 93% of UVB rays
  • SPF 30 blocks 97% of UVB rays
  • SPF 50 blocks 98% of UVB rays
  • SPF 100 blocks 99% of UVB rays

For an easier gauge of what this means, you can multiply the SPF factor by the length of time it takes you to get burned without sunscreen. Essentially, if you would develop a burn within 10 minutes of standing in strong sunlight, wearing SPF 15 means you can avoid sunburn for 150 minutes.

The minimum recommended number is SPF 30, which blocks 97% of the sun’s rays. Keep in mind that the maximum FDA approved rating is SPF 50 so a sunscreen labeled with anything higher (70, 110, etc) is just a marketing gimmick.

The downfall of relying on SPF alone is that sunscreens with SPF measure only their ability to deflect UVB rays.





What does PA factor stand for?

I had no idea what PA stood for until I went to Japan for the first time and saw all the sunscreens labeled with this.

At first I thought it was their version of SPF but I looked closer and noticed that all their sunscreen bottles clearly stated both SPF and PA.

PA stands for the Protection Grade of UVA and explicitly measures the UVA protection a sunscreen provides. The rating system is designated by a ‘+‘ symbol and is as follows:

  • PA+ corresponds to a UVA protection factor between 2 and 4
  • PA++ to a rating between 4 and 8
  • PA+++ to a rating of more than 8


In the European Union, sunscreens are required to protect against both UVA and UVB rays. Typically they use the phrase “broad spectrum” to denote UVA protection of at least 1/3 as strong as the UVB protection.

In the US, there is absolutely no requirement (the FDA considered adding this in 2007 but it was never adopted as it was concluded it would prove “too confusing” to consumers – um, what?) so for this reason (and many others) I generally avoid American sunscreens.

In general, Asia is a great place to buy sunscreen because Asians love avoiding the sun and prize paler skintones (spot all the umbrellas on sunny days and you’ll see what I mean). Also, generally the EU has much stricter guidelines on the health and beauty industry, and products are under more stringent regulation there than in the U.S.





Are Chemical Or Physical Sunscreens Better?

There are two ways to protect against damaging sun rays.

Chemical sunscreens work by sinking into the skin pores to lie in wait for any UVA/UVB rays that penetrate the dermis. Then, using chemical actives they dissipate the harmful rays that the skin absorbs.

On the other hand, physical sunscreens sit on top of the skin and form a physical barrier to scatter and deflect sun damage. Generally, physical sunscreens are better formulated products since the chemical components are more stable.

In the US, the only chemical ingredient that has good protection against UVA rays is avobenzene, which is inherently an unstable compound and needs to be bound with other chemicals to stabilize it (typically in the US this is octocrylene).





How do I know if I’m using a Physical or a Chemical sunscreen?

By looking at the ingredients!

Physical sunscreens are formulated with zinc oxide or titanium dioxide. Chemical sunscreen ingredients usually end with -ate, -ene or -one such as ho-mosalate, octocrylene, avobenzene or oxybenzone.

I prefer physical formulations for a couple reasons.

One, they form an effective physical barrier immediately, meaning no wait time to activate so that age old rule of waiting 30 minutes before jumping into the pool doesn’t apply.

Two, they tend to be more friendly to those with sensitive skin. If you ever get burning eyes from drippy sunscreen, it’s likely the chemical ingredients are irritating you.

Third, they break down more readily in nature and are more environmentally friendly. Some chemical formulas like oxybenzone are damaging to coral reef structures when dissolved in water. In fact, Hawaii has banned oxybenzone and octinoxate as they have a negative effect on coral reef health.





What’s the best way to apply sunscreen?

The order of sunscreen application depends on whether you’re using a chemical or physical.

  • If chemical, apply first before anything else (makeup, lotion etc) because the formula works by sinking into the skin pores to be able to capture sun rays.
  • If you’re using a physical sunscreen, make sure its the last thing you apply so that it can properly form a physical outer barrier.


Suncreen SPF ratings are classified on a proscribed dosage level of about 1 ounce every 2 hours. For body sprays, this means about 30 to 90 seconds of spraying for a nice even layer.

On a side note, there are concerns about the effectiveness of sprays.. If swimming or sweating, apply more frequently as the sunscreen can get washed off.

Essentially, you should be using a massive amount of sunscreen and reapply often. The general rule of thumb is that if you’re wearing shorts and a t-shirt, you should be applying a shot glass worth of sun lotion (crazy, right?) on every inch of exposed skin.





Is sunscreen really waterproof?

Technically, no.

The FDA bans the words “waterproof” or “sweatproof” on labels. Instead, sunscreens are allowed to be labeled “water resistant” and they much specify the exact amount of time they will hold up when exposed to water.

They have 2 tests, where sunscreens should be either

  • Water resistant for 40 minutes
  • Water resistant for 80 minutes

Generally, if you’ll be swimming or getting wet, you want to re-apply after you towel off, since toweling will remove suncreen from your skin.





I hope that was helpful! I’ve found that applying sunscreen everyday has prevented new sun freckles from appearing on my cheeks – and also remembering to double cleanse to remove the sunscreen at night has kept my skin clearer.




  1. morgan
    August 13, 2016 / 10:48 am

    Really like this post, I learned so much!

  2. thea
    August 14, 2016 / 2:52 am

    I really enjoyed this post, it was so helpful! thanks for sharing

  3. Lacey
    August 18, 2016 / 2:23 pm

    Such helpful info! Thanks for putting this out there. Sun protection is SO important!

  4. anna cobbs
    August 18, 2016 / 2:24 pm

    Super informative post!! I’ve been trying to take better care of my skin so this was very helpful!!

  5. Alex
    August 18, 2016 / 2:32 pm

    Such an informative post! Thanks for sharing!!

  6. ashley
    August 18, 2016 / 2:38 pm

    Such a great and informative post! I am such an advocate for sun protection!

  7. Kate
    August 18, 2016 / 2:46 pm

    Great post, so much good info!

  8. Bailey
    August 18, 2016 / 5:10 pm

    This post is so helpful! I didn’t realize how much I truly don’t know about sunscreen, and it’s really so important!

  9. Stephanie
    August 18, 2016 / 5:52 pm

    WOW, I learned so much from this post! Definitely bookmarking it for future reference. I had no idea I didn’t know anything about sunscreen lol, just like Bailey said. Thanks for sharing this info, super helpful.

  10. Carolyna Bauer
    August 18, 2016 / 6:31 pm

    Great post on Sunscreen and it’s importance. I should put some every morning (even in winter)!

  11. Lawren Bagley
    August 18, 2016 / 9:29 pm

    Such an informative post, love it! Sunscreen is so important, & I use it daily!

  12. Greta
    August 18, 2016 / 9:48 pm

    Such a good post! I’ve been starting to take better care of my skin and wear some kind of sunscreen every day. So important!

  13. Brittany Boone
    August 19, 2016 / 12:31 am

    This is such an informative post, I always wondered on the SPF numbers

  14. August 19, 2016 / 11:55 pm

    What an informative post, and this is truly all the information ever needed when it comes to sunscreen. Such a good collaborative post!

  15. Laura
    May 26, 2019 / 12:15 pm

    Wow this was really helpful and the Japanese rating system of PA was so interesting! Now I’m all about UVA protection.

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