A rating of 15 means 93 per cent of UV rays are blocked

Everyone knows that an SPF rating of 60 provides double the protection of SPF 30 or does it? Studies over the years have shown that sunscreen with an SPF, or sun protection factor, of 30 blocks about 97 per cent of ultraviolet rays.

A rating of 15 means 93 per cent of UV rays are blocked, and anything higher than 30 remains in the 97 or 98 per cent range.

In 1999 the US Food and Drug Administration recommended that sunscreens with an SPF higher than 30 be labelled 30+, mostly to prevent people from developing a false sense of security that might lead them to spend more time in the sun. What many people do not realise is that the amount of sunscreen applied plays an enormous role.

A study in The British Journal of Dermatology this year found that applying less than two ounces over the entire body at one time can leave people with an SPF rating far lower than what is on the bottle.

Some studies have shown that people typically apply just 10 percent of the amount recommended. It is also important to look for sunscreens that protect against UVA and UVB radiation. SPF ratings apply only to UVB rays, and some sunscreens provide no protection at all against UVA rays, which penetrate the skin more deeply.

For broad protection, look for sunscreens with avobenzone, zinc oxide or titanium dioxide, all of which block UVA.