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SPF explained | How to make sense of SPF

There are several misconceptions about sunscreens.



Photo: Wix

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First things first: What does SPF stand for?




How do sunscreens work?

There are several misconceptions about sunscreens. The most prevalent one I encounter as a Dermatologist, however, is how they work.

Chemical sunscreens work by absorbing light. Once the sunscreen maxes out its ability to absorb light, the rest flows over to your skin. Imagine this as a cup on your skin. The cup fills with ultraviolet light (UV). Once the cup fills, the rest of the UV overflows onto your skin.

Have you ever said, "I literally wore tons of sunscreen and reapplied and still got sunburned!" The answer may be that the ability of your chemical sunscreen to work was maxed out while in the sun. There is no indicator we have to say when this occurs. This is actually why we tend to recommend reapplying sunscreen every 2 hours, every if you are not swimming or sweating. The hope is to replenish the ability of your sunscreen to work at its maximal effectiveness. The challenge your sunscreen will face however is when you are in regions with intense UV exposure as we approach closer to the equator for example.

This is the distinction with physical sunblocks such as zinc or titanium. Physical sunblocks block UV by shielding the skin directly. They do not absorb UV, they block it physically.

What do the SPF ratings mean?

SPF ratings are so poorly understood by consumers. SPF is a value assigned to the sunscreen product and regulated by the FDA to inform the consumer of the sunscreen's ability to prevent sunburn when it is used. Sunburns are triggered by UVB light. The SPF is only telling us the amount of UVB blocked by the product.

One of the most common misconceptions I hear from my patients is that the SPF ratings tell us something about the time of sun exposure that could lead to a sunburn. This is not accurate. SPF is a rating focused on the amount of UVB exposure but not how much time you can spend in the sun. The amount of UVB exposure varies based on the time of day you are outdoors making it impossible for the SPF to provide a value that you could apply to the amount of time you can spend in the sun based on the time of day.

SPF ratings tell us the percentage of UVB prevented from reaching our skin.


%UVB Blocked











Examining the chart above can help us understand why SPF is so confusing. So many of my patients eager to protect their skin from skin cancer will tell me they only wear SPFs above 50. The reality is that it is difficult to say how much more protection they are getting from their product. The higher SPFs tend to cost more so the concern is that well-intentioned consumers are spending more but not getting enough added protection to justify the cost.

How reliable are the SPF ratings on sunscreens?

SPF values are regulated by the FDA and as such should be considered reliable. This means that whether you have a sunscreen lotion, cream, or spray, the SPF value listed should be accurate based on the product's ability to block UVB.

The important note to make here is that the SPF value listed is based on optimal use. One study found that although we should be applying 2mg/cm of sunscreen, most apply only 20 to 50 % of this amount of sunscreen in real-world circumstances. This means that the actual effectiveness of the sunscreen applied could likely be 20 to 50% lower than the SPF value listed. There are a few ways to address this discrepancy. The best way to approach this is to learn to apply sunscreen generously and reapply it routinely to maintain its effectiveness. Although it is true that purchasing higher SPF values may adjust for this difference, it may make more sense to apply the products you already have appropriately.

Also, remember that sunscreen can only work on areas it is applied to. I routinely find sunburn patterns reflecting the missed areas of application.


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