Sunscreen, our white, sticky guardian against those harmful UV rays, has oozed its way into Australian life, to the point where it would be impossible to imagine summer without it. We all know the importance of slip, slop, slapping, but very few of us really understand what we’re slopping on, and how it works in slapping skin cancer right in the epidermis.

Given Monday’s report in The Age that the Cancer Prevention Centre of the Cancer Council of Victoria is pushing to endorse SPF50+ sunscreens into Australian markets, Crikey thinks it’s time to ask the dumb questions, and disprove a few myths along the way. Professor Dallas English, chair of Epidemiology at the University of Melbourne, is here to help.

How does sunscreen work?

The purpose of sunscreen is to protect skin from UVB rays, which is essentially solar radiation that has the potential to alter DNA in skin cells, leading to melanomas. However, not all sun protection creams are the same. Sunscreen is designed to absorb UVB rays before they reach the skin, whereas sunblock (like zinc cream) does the opposite: it literally acts as a barrier, reflecting the rays away. Nowadays, some sunscreens actually contain elements of sunblock, offering both an absorbent and a reflective shield against UVB rays.

What does the SPF rating of sunscreen actually mean?

It is commonly believed that the SPF factor refers to the amount of time you can spend in the sun without getting burnt, relative to your skin’s natural burning time. For example, if you put SPF 30+ on, you can stay in the sun 30 times longer than you would otherwise. This is not true! SPF is a rating of what percentage of UVB rays the sunscreen protects you from. For example:

  • SPF 15+ allows one in every 15 rays in, therefore offering 93% protection.
  • SPF 30+ allows one in every 30 rays in, therefore offering 97% protection.
  • SPF 50+ allows one in every 50 rays in, therefore offering 98% protection.

So, realistically, what advantage does factor 50 have over factor 30? As Professor English puts it, “the technical term for that is that there’s bugger all difference”.

Is there a risk that people will feel “invincible” in factor 50?

The short answer is that, yes, there is a risk that SPF 50+ sunscreen will lead people into a false sense of security. According to Dr English, “there is some evidence that people who wear high SPF sunscreen spend more time outdoors than people who wear sunscreen with lower SPF”.

So, knowing that factor 50 could in fact contribute to an increase in sun-related medical problems, why is there a push to have it introduced into the market? Professor English has a theory that this is a marketing ploy to encourage people to spend more on “stronger” sunscreen. “Manufacturers always like to do that,” he said. “They’re the ones pushing it.”

So what is the best way to keep protected from skin cancer?

Regardless of the SPF rating of the sunscreen, the unanimous advice is that it should be re-applied at least once every two hours. Of course, if the skin has been exposed to water, or heavy amounts of sweat, re-application should be more frequent.

Professor English, however, has an even simpler philosophy — “sunscreen is a last resort”. So, if you really want to avoid skin cancer, stay inside when the sun is at its strongest, wear clothing that gives good coverage, and stay in shade wherever possible. It’s not hard.