As summer unofficially kicks off this Victoria Day weekend, it may be time to start re-thinking your sunscreen: according to a new report, many of them aren't as effective as you might think.

Out of 34 suncreen products tested by Consumer Reports (most of which were American brands), nearly a third fell short of the claims on their labels. Eleven of the products delivered anywhere from 16 to 70 per cent less SPF protection than promised.

"It's quite surprising, given that sunscreens are highly regulated in both Canada and U.S.," Dr. Sonya Cook, a dermatologist and lecturer at the University of Toronto, told Metro Morning guest host David Common.

The study tested for protection against both UVB radiation, which causes sunburns and contributes to skin cancer, and UVA radiation, which causes the skin to tan and leads to wrinkling and aging.

It found that "natural" or mineral sunscreens, which contain only titanium dioxide or zinc oxide as active ingredients, were less effective than chemical sunscreens, which use chemical ingredients such as avobenzone. Of the five natural sunscreens tested, none made the list of 15 recommended products.

Sunscreen tips

Whatever sunscreen you decide to use, Cook recommends checking the expiry date on the label before lathering it on—and if you can't find the expiry date, then toss it, she says.

"I think it's a good idea every year, do a little Spring cleaning of your sunscreens, and toss out the ones from last year and start fresh," she suggests.

Cook also recommends looking for products that provide broad-spectrum protection against both UVA and UVB rays and which have an SPF of least 30. She adds that creams and lotions tend to provide better coverage than sprays or liquids.

She also says that sunscreen should be your "last line of defence" against the sun's harmful rays, and advises wearing sunglasses, hats and sun-protective clothing. You should also avoid being in sun during its peak hours of 11 a.m. to 4 p.m.—but if that's not possible, then Cook advises applying lots of sunscreen on exposed areas.