Get Sun Smart—The Truth About SPF Ratings and Your Summer In The Sun

Dr. Lindsey Mcilvena, MD

In her day job, treating patients living with chronic disease, nutrition and lifestyle are not after-thoughts, they’re central to helping her patients get well. She’s also our go-to expert on plant-based diets.

How much do you really know when it comes to staying safe in the sun? The finer details of sun safety can be a little confusing, but we’re here to help. Read this short article and you’ll have a basic understanding of the science of sun exposure and the truth about the SPF rating system for sunscreens.

UV radiation

This term gets thrown around a lot. Within the sun’s visible, warm light there are invisible ultraviolet (UV) rays. When they come into contact with the skin they can damage the inner workings of our skin cells.

There are three main types of UV rays:

UV-A rays damage the DNA in skin cells, causing aging of the skin (wrinkles!) and increasing the risk of skin cancer.

UV-B rays are higher energy and also cause skin cancer and burns.

UV-C rays are even more energetic but are blocked by the earth’s ozone layer and never reach us down here at ground level.

Because of their particular wavelength, UV-A rays are able to penetrate through car and home windows, while UV-B rays bounce off. Many people think they are safe from the sun while they’re in the car but that’s only the case with UV-B rays.

The intensity of UV rays is highest between 10 am and 4 pm, during the summer, and in areas that lie closer to the equator. UV rays can penetrate through clouds, and in some cases, their intensity is increased on a cloudy day. UV rays can bounce off sidewalks, water, and snow, and reflect back up to your skin and cause just as much damage.

SPF Ratings and UV Rays

First and foremost, you’ve got to understand that SPF ratings only refer to the sunscreen’s ability to block UVB, not UVA. A sunscreen labeled “broad spectrum” means that it blocks UVA as well, but there is no rating system for the ability to block UVA.

Also, SPF ratings are a little misleading. An SPF of 15 blocks 94% of the UVB, an SPF of 30 blocks 97% of UVB, and SFP 45 blocks 98%, but there’s a catch. When the testing is done in the lab, the sunscreen is put on at a thickness of 2mg per square centimeter of skin. They basically cake it on at a thickness no one would ever actually use at the pool, beach or on a hike!

In reality, most people usually put on about a third as much. So really, if you’re putting on SPF 45 at a normal thickness (actually rubbed in), you’re really only getting a level of about SPF 15—blocking 94% of the rays. By that logic, SPF 15 is really only like an SPF of 5, blocking only a little over half the sun’s harmful UVB rays. So look for products that are at least SPF 30. SPF 50 is great too, but as you can see from above, once you get to 30 and higher, not much more UVB is blocked. There’s not that much of a difference between blocking 98% and 98.5% of UVB.

Check our other article for specific SuperFoodsRx sunscreen recommendations. We’ll also address some questions about sun risk factors.


Schaumburg, July 2014, American Academy of Dermatology Statement on Sun Exposure.

American Cancer Society, April 2017, What is Ultraviolet Radiation.

Dutera, et al, July 2014, Determination of SPF of Sunscreens by Ultraviolet Spectrophotometry, Journal of Pharmaceutical Sciences.

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