It's easy to head out into the summer heat thinking you're protected because of all that sunscreen you've slathered on. You've judiciously checked the labels and decided on a sunscreen with an SPF rating of 30 or more. Slap it on and soak up the sun without fear of burning.

But Canadian dermatologists say you could be setting yourself up for trouble. There are 75,000 new cases of skin cancer in the country every year.

"We estimate that from one in five to one in seven people will develop a skin cancer during the course of their lifetime," Vancouver dermatologist Dr. Jason Rivers told CBC News. "Unfortunately, the incidence rates have more than doubled in the last 20 years for all types of skin cancer."

"Further, in North America, someone dies from a skin cancer every hour, so this is obviously a problem that we have to deal with because it is largely preventable."

In spring 2011, Environment Canada forecast that UV levels will be "a little stronger than normal" — roughly three to four per cent higher than levels before 1980, for southern Ontario.

Rivers says one of the biggest mistakes people make about sunscreen is they don't use enough of it. If you buy one tube in the spring and still have some left at the end of summer, you're not using enough, unless, of course, you've spent the whole summer indoors. Rivers says a 120-millilitre tube of sunscreen is only good for about four or five applications.

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About a shot glass full of sunscreen per application remains your best protection against the harmful rays of the sun. (Evan Vucci/AP)

You should be applying approximately a shot glass full of sunscreen about half an hour before you go outside. You should also re-apply sunscreen every few hours, especially if you're swimming, riding, running or playing other sports.

Dr. Peter Vignjevic, a dermatologist and assistant professor of dermatology at McMaster University in Hamilton, Ont., people need to apply about two to three teaspoons per arm and three to four per leg to be properly protected.

"Most people don't use it in nearly those amounts," Vignjevic said. "Those are the amounts you need to use if you actually want to get the SPF that's on the bottle."

Another problem is that many people don't purchase the proper kind of sunscreen. You should get a sunscreen that offers broad protection from both UVA and UVB rays.

There are three types of UV radiation:

  • UVA: Not as powerful as UVB, but these rays penetrate more deeply into the skin. UVA rays are responsible for contributing to photodamage and wrinkling of the skin, premature aging, and skin cancer.
  • UVB: Primarily affects the skin's outer layers and is thought to be the primary cause of sunburn, skin aging and skin cancer. UVB rays tend to be more intense during the summer months.
  • UVC: strongest, most dangerous rays. But they're normally filtered out by the ozone layer and do not reach the surface of the Earth.

Rivers says dermatologists recommend using a sunscreen at least Sun Protection Factor (SPF) of 30 or more. But that sunscreen should also offer protection against UVA rays.

SPF numbers were introduced in 1962 to measure a sunscreen's effect against UVB rays. Those numbers start at two. Currently, the maximum protection a sunscreen offers is an SPF rating of 70. In European Union nations, the maximum is 50+.

How do they calculate those SPF numbers?

A test group of 20 people is checked to see how long they take to develop sunburn without sunscreen. They're retested later with the sunscreen. The "with sunscreen" number is divided by the "without sunscreen" number to derive the Sun Protection Factor (SPF).

To figure out how long a sunscreen protects you, multiply the minutes it takes you to burn without sunscreen by the sunscreen's SPF. The result is how long that sunscreen will protect you from sunburn.

The formula is very rough and depends on a lot of factors. However, if it normally takes you 10 minutes to develop sunburn, a sunscreen with a SPF rating of 35 will keep you sunburn-free for 350 minutes — if used correctly.

A sunscreen that carries an SPF rating of 30 will protect you from 96.7 per cent of the UVB rays your body could be absorbing. Apply a sunscreen with an SPF rating of 50 and you're protected from 98 per cent of the UVB rays your body could be absorbing. Go beyond 50 and the increase in protection value is marginal.

In the United States, the Food and Drug Administration (FDA) is expected to establish new rules for sunscreen labelling that includes UVA ratings. The new labels are expected to be in place by the summer of 2012. 

Exposure to ultraviolet (UV) radiation can cause premature aging and at worst, skin cancer. Artificial sources of UV radiation, such as sun lamps and tanning booths, can also cause skin cancer.

Ultraviolet light can cause skin cancer in a variety of ways. Each time skin is exposed to the sun and becomes tanned or burned, damage is done to individual cells and to DNA.

Some cells die and some repair themselves by getting rid of the damaged DNA. Cells that cannot repair themselves eventually become defective. UV radiation affects the body's immune system by making it more difficult for the system to destroy defective cells. Defective cells that are not destroyed can slowly grow and produce a tumour.

Sunscreen not the only answer

But Rivers cautions that the evidence for sunscreen preventing skin cancer is somewhat weak.

"This is why we don't underscore it as a first-line protection against the sun," he said. "What we do know is that there is some laboratory evidence and also some epidemiological evidence that sunscreens will help to reduce the risk of squamous cell carcinoma."

Squamous cell carcinoma is the second most common form of skin cancer. It's caused by chronic overexposure to the sun and causes small tumours on the face, neck, bald scalp, hands, shoulders, arms and back. But it is highly treatable when identified early.

How much radiation is my body absorbing even with sunscreen on?

SPF protection can be confusing. A sunscreen with an SPF rating of 2 protects your body from about half the UVB radiation that hits it. But double the SPF number and you're not doubling the protection.

An SPF 4 sunscreen will protect you from 70 per cent of the UVB radiation directed at your body. By moving up to an SPF 30 sunscreen, you're blocking 96.7 per cent of the UVB rays your body could be absorbing. But go much above 30, and the increase in protection is marginal.

An SPF 50 sunscreen protects you from 98 per cent of the UVB radiation you could be facing.

Basal cell carcinoma is the most common form of skin cancer in Canada. It is also the least dangerous but requires treatment as without it, the cancer will continue to grow. The growing tumour will destroy surrounding skin tissue and eventually cause disfigurement.

Basal cell carcinoma is caused by exposure to UV radiation. You're at greater risk if you are fair-skinned with blond or red hair and you usually burn when you're out in the sun.

Malignant melanoma is a less common but more dangerous type of skin cancer. If it's found early, the cure rate is about 90 per cent. But if it's left to grow too long, it will eventually reach the blood stream or the lymphatic system and invade other organs, which can lead to death. 

Melanoma can begin as a small brown or black freckle-like spot on your skin's outer surface. A change in the shape or colour of an existing mole or spot on your skin could also signal the development of melanoma. Rivers says there's no evidence yet that sunscreen will protect you from developing malignant melanoma.

Confusion over vitamin D

Because of fear of cancer, you want to protect yourself from the sun's damaging rays, but you've also heard that exposure to the sun is the best way to get your daily dose of vitamin D. The Canadian Dermatology Association says most Canadians get enough vitamin D in the spring, summer and fall from their exposure to the sun in their normal daily routines.

The Canadian Cancer Society says it's not prepared to change its recommendations on exposure to the sun, as "skin cancer is the most frequently diagnosed cancer in Canada." The agency recommends vitamin D supplements if you're not getting enough of the sunshine vitamin.

"Sunscreen shouldn't be used as a way of prolonging your time outdoors so you can get that great golden tan," says Rivers. "A golden tan means that you've been roasted."

Here are some tips on protecting your skin and your health.

  • Use a broad-based sunscreen with UVA and UVB protection with a minimum Sun Protection Factor (SPF) 30 (look for the CDA logo — Canadian Dermatological Association).
  • Re-apply sunscreen periodically throughout the day, and frequently if you're exercising in the heat.
  • Reduce your exposure to the sun, particularly between 11 a.m. and 4 p.m., when the sun's rays are at their peak.
  • Wear clothing that protects as much skin as possible.
  • Wear a wide-brimmed hat and wraparound sunglasses with UVA and UVB protection.
  • Look for shaded areas during outdoor activities.
  • If you are taking antibiotics or acne medication, be especially cautious because they can increase your risk of sunburn.
  • If you use self-tanning creams, remember that you must still apply sunscreen. Your skin may be darker, but that doesn't mean you're protected.
  • Check your daily local forecast for "UV ratings" and cover up accordingly.

How to read the UV rating

0 to 2 (low): Minimal protection for normal activity. Most people can stay in the sun for up to 1 hour during peak hours without burning.

3 to 5 (moderate): Cover up. Wear a hat, sunglasses and sunscreen if you're outside for more than 30 minutes.

6 to 7 (high): Protection is required. Reduce your exposure to the sun between 11 a.m. and 4 p.m. Light-skinned people can burn in less than 15 minutes.

8 to 10 (very high): Take full precautions and avoid the sun between 11 a.m. and 4 p.m. Fair-skinned people can burn in less than 10 minutes. Wear clothing with tightly-woven fabric as UV rays can pass through the spaces of loosely knit clothes.

11+ (extreme): Take full precautions and avoid the sun between 11 a.m. and 4 p.m. Fair-skinned people can burn in less than 5 minutes. A UV Rating this high is extremely rare in Canada - but fairly common in tropical areas.