Sunscreen: what you need to know

By Dr. Cheryl Rosen

Summer has finally arrived, so it’s definitely time for hats, sunglasses and sunscreen. Have you ever stood in the aisle at the drugstore facing row upon row of sunscreen, not knowing how to choose one from the next? Let’s see if we can make your next purchase a little simpler.

What does SPF really mean?

All sunscreens are labelled with a sun protection factor (SPF) number. This relates to the amount of time it takes for your skin to burn without any protection and how long it would take if you used the appropriate amount of sunscreen. An SPF is the ratio between the amount of UV that will cause sunburn in sunscreen-protected skin compared to unprotected skin.

Remember that sunscreens should not be used to extend the amount of time you spend in the sun. They should be used with other forms of sun protection, such as shade, hats and clothing, to protect you as much as possible. 

Do I need an SPF 30, 60 or 100?

It is logical to think an SPF of 30 is twice as good as an SPF of 15, and so on, but that is not how it works. To give you an idea of what the numbers mean, an SPF 15 sunscreen blocks 94 per cent of UVB rays, SPF 30 blocks 97 per cent, SPF 50 blocks 98 per cent and SPF 100 blocks 99 per cent. No sunscreen offers 100 per cent protection from UV rays.

Choose a broad spectrum sunscreen (one that protects against both UVB and UVA) with a minimum SPF of 30. When shopping, look for bottles with the Canadian Dermatology Association Sun Protection Program logo.

How much sunscreen do I need to apply?

A full shot glass (or two ounces) of sunscreen should do the trick. Try to remember to apply your sunscreen 20 to 30 minutes before heading outside. Be sure to reapply regularly to avoid sunburn, especially after heavy sweating, swimming or towelling.

Don’t forget about your lips; apply an SPF 30 lip balm too.

Does a sunscreen still work after the expiry date?

No. Sunscreens contain chemicals that eventually break down and the water in the product evaporates, compromising the effectiveness of the product. Keeping sunscreen in hot temperatures – in the glove compartment of a car or in a beach bag in the sun – can accelerate its deterioration.

Are there any sunscreens for sensitive skin?

If you have sensitive skin, try a small amount of the product on your arm and check for any reaction up to 48 hours later. People allergic or intolerant to certain compounds in sunscreens should look for products containing titanium dioxide and zinc oxide that reflect and absorb the sun’s rays and are much less likely to cause a reaction. 

Making proper sun safety part of your everyday routine will not only save you from painful sunburns, it will, most importantly, reduce your risk of developing skin cancer.  

Dr. Cheryl Rosen is the national co-chair of the Canadian Dermatology Association’s Sun Awareness Program, head of the division of dermatology at Toronto Western Hospital, as well as a professor in the department of medicine at the University of Toronto.


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