That bottle of SPF30 sunscreen sitting next to you on your beach towel may be less help than you think. Experts say New Zealanders put too much faith in their sunblock - and are likely to be doing themselves damage because of it.
Dermatologist Paul Salmon, of the Skin Centre in Tauranga, says people believe if they cover themselves in sunscreen, it is a licence to spend as much time in the sun as they want.
"People think: 'Great, I can wear my broad-spectrum sunscreen, put on my bikini and go to the beach all day and not get burned'. But sunscreen should really be used for the areas that can't be covered with clothing. Covering up is far better protection."
A Cancer Society spokeswoman agrees, saying: "It's not a big shield you put on and go outside."
Dr Salmon says it's New Zealand culture to wear a T-shirt and shorts and lie in the sun.
He says although you may get away with that in the Northern Hemisphere, "it's really bad in New Zealand". And it is especially a problem for children.
"They get a lot of sun exposure ... we're not particularly good at covering them up."
Concerns about the chemicals in sunscreen have prompted a move towards organic and homemade varieties, but neither expert says there is evidence of any toxicity or that chemicals in sunscreen penetrate skin. The NZ Dermatological Society says chemical-absorbing sunscreens often contain a combination of ingredients to provide coverage against UVB and UVA radiation. Some are combined with physical blockers, such as titanium dioxide and zinc oxide. Some organic formulations may degrade when exposed to sunlight and therefore not perform as well as expected, says the society.
Dr Salmon says there is no real cause for concern about the chemical makeup of sunscreens. "The damage done by not using sunscreen is far greater."
He says there is some talk about whether zinc nanoparticles could penetrate the skin, but that does not seem to be the case. Home-made sunscreen is not a good idea, he says. "It's quite likely that some of the other stuff in it contains chemicals, anyway."
He and the Cancer Society spokeswoman say it is important to determine the efficacy of any product. People who are worried could use zinc creams.
But sun lovers should be looking for more than just a high SPF (sun protection factor) rating, Dr Salmon says.
Sunscreens are tested at a thickness of 2mg per square centimetre and calculated on how much radiation it takes to cause a barely detectable sunburn, with and without sunscreen.
If it takes 10 minutes to burn without a sunscreen and 100 minutes to burn with a sunscreen, then that sunscreen gets a rating of SPF10.
It is a logarithmic scale so SPF30 blocks only 5 per cent more than SPF15, and SPF60 another 2 per cent.
The New Zealand Dermatological Society says the difference between an SPF15 and an SPF30 sunscreen may not be noticeable in actual use because the effectiveness of a sunscreen has more to do with how much of it and how often it is applied, and whether the wearer is being exposed to water or is sweating.
"Most people apply their sunscreen at about one third the thickness used for testing; they fail to apply it to all exposed areas of skin; and they forget to reapply it every couple of hours. Therefore, the actual protection may be less than the tests indicate." Dr Salmon says people should always wear broad-spectrum sunscreens, which block UVA and UVB rays, and he warns people to keep out of the sun between 10am and 5pm.