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What does SPF in Sunscreen really mean?

There is no doubt about the damaging effects of excessive sunlight; prolonged sun exposure leads to skin cancer. Time to stock up on a sunscreen as we get into those sunny days of summer!

With all the options out there, choosing the right product can be difficult. Stores are packed with many options, branded with anything from “Ultra-Sport” to “Hydrating”, but each and every label will report a sun protection factor, or “SPF” value.

In theory, that wearing a sunscreen with a SPF value of 30 allows you to stay in the sun for 30 times longer than if you skipped the sun protection. In practice, it is a bit more complicated.

When the sun beams on those hot summer days, your skin is being hit by both visible light and ultraviolet (UV) light, with the latter being responsible for creating biological damage. To keep things simple, we need a sunscreen that provides a barrier against both UVA and UVB rays.

Schematic representation of the electromagnetic spectrum of light, demonstrating the ultraviolet radiation (UVR) frequencies and their effect on human skin

Schematic representation of the electromagnetic spectrum of light, demonstrating the ultraviolet radiation (UVR) frequencies and their effect on human skin, taken from the Canadian Medical Association Journal.

A broad-spectrum sunscreen is one that protects against both UVA and UVB rays by using a combination of chemical and physical barriers against UV light.

SPF, however, is a unit measuring the protective ability of your sunscreen against UVB rays alone. It is determined in the laboratory by measuring minimal erythemal dose (MED) values, which is the amount of time it takes for skin to redden. Scientists will apply 2mg/cm2 of sunscreen to a patch of human skin (usually the lower back, since it has less exposure to sun normally) and compare it to skin with no sunscreen. When comparing the MED of protected skin to unprotected skin, we get a ratio, which is our SPF value.

SPF = MED of protected skin / MED of unprotected skin

So, the SPF value is a good representation of the solar energy protection that the sunscreen provides, especially if you are comparing two different bottles. An SPF value of 15 protects against 93% of UVB rays, SPF 30 protects against 97%, and SPF 50 protects against 98%. It is important to note that an SPF of 30 lets through 3% and an SPF of 50 lets in 2%. That is the same ratio as SPF 50 divided by SPF 30. Thus the added protection from an SPF 50 sunscreen is about a 67% improvement.

But remember, these values are determined in a laboratory where conditions are near ideal. In reality, there are several other factors at play: skin colour and texture, clothing protection, time of day, geographic location, and application method. The biggest problem is that sunscreens are often misused — user application is almost always insufficient, meaning that the protection provided is less than what you think. It’s possible that the application of sunscreens with higher SPF values will give users a false sense of security. Overestimating your protection might lead to infrequent application, which ends up being less protective. On the other hand, there is research that suggests that higher SPF sunscreens being more protective in real-life conditions. Accordingly, experts are urging for a minimum SPF of 50 for the best protection. However, SPF 30 is still the threshold recommended amount by Health Canada. But remember, your application method is just as — if not more — important than the level of SPF you choose, as long as it’s over 30.

As a rule of thumb, you should be using a teaspoon for your head and neck, front, back, each arm, and each leg, for a total of a shot glass worth of sunscreen for the whole body. But best practice would be to follow the instructions on the label.

Visual aid to guide the correct application of sunscreen

Visual aid to guide the correct application of sunscreen taken from the Canadian Medical Association Journal, based on information from the Canadian Cancer Society and the American Academy of Dermatology.

To simplify your next sunscreen haul, make sure you are getting a broad-spectrum sunscreen with a minimum SPF of 30. And don’t forget that once you’ve picked the right sunscreen, you need to apply it properly in order to get the protection you’re promised.

For more about sunscreen and sun safety:


Cat Wang recently graduated from McGill University with a Bachelor of Science (BSc) degree in the anatomy and cell biology program and is currently deciding on a Master’s program.

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