Nanogirl Michelle Dickinson: Can you trust your sunscreen?

COMMENT: With summer finally warming up and the school holidays about to start, it's a good time to remember to apply and reapply sunscreen.

Usually, people pick their sunscreen by the SPF number on the bottle, with those seeking higher protection looking for sunscreens with a higher SPF number. This week, however, Consumer NZ testing found that a sunscreen labelled as SFP30 was only measured as SPF6 in its lab studies, and six out of the 10 sunscreens that it tested failed to meet their respective SPF label claim.

So how is the SPF number determined in our sunscreens? Can we trust them, and does New Zealand need to do more to protect its people?

SPF on sunscreen stands for sun protection factor and is a measure of protection against the ultraviolet B (UVB) rays from the sun. These shorter wavelengths can cause sunburn, which may lead to skin cancer. The SPF number doesn't represent the strength of a sunscreen, but instead represents the amount of time a sunscreen can protect for.

Ideally, a sunscreen labelled as SPF 15 would protect you so that you could stay in the sun 15 times longer than you could without any sunscreen on before burning.

The Consumer NZ testing this year and in previous years showed that many of the sunscreens available in our stores do not provide the sun protection factor they claim.

Because New Zealand classes sunscreens as a cosmetic, the standard used to rate sunscreens (AS/NZS 2604:2012) is voluntary and manufacturers are not required to meet it to sell their products here. Compare that with the US where they are classed as an over-the-counter medical product.

To determine the SPF number of a sunscreen, a testing laboratory will mark out squares of the same size across the back of a paid volunteer. Some of the squares will be left as bare skin whereas others will have a layer of sunscreen applied to them. After 15 minutes to allow the sunscreen to dry and be absorbed, the volunteers are them placed under a UV lamp and the squares exposed to light for pre-determined amounts of time.

Twenty-four hours later the skin is analysed for any redness to determine the minimum amount of exposure needed to cause sunburn on the volunteer with and without the sunscreen covering.

Then it can calculate the SPF number, for example, if an SPF factor 15 sunscreen is being tested then the sunscreen will pass if the patch of sunscreen-covered skin can last 15 times longer than the unprotected skin before turning red.

If you normally burn after 10 minutes of sun exposure then using SPF factor 15 should let you be in the sun for 150 minutes without burning. The challenge is that typically only 10 to 20 paid volunteers are tested for an SPF number calculation - a very small sample size unlikely to be representative of the population.

Another challenge is that sunscreen manufacturers aren't required to carry out regular testing between different batches of their sunscreen. This means that simple things like changing the supplier of its ingredients could create variations in the effectiveness of the product, which aren't noted in the new batches.

Most of us probably don't know the date when we open our sunscreen bottles, but sunscreen – like many products – deteriorates over time and may not be as effective after being opened for a while.

In a country with one of the highest rates of skin cancer and melanoma in the world, maybe its time to tighten up our SPF testing rules and take our sunscreens more seriously.

In the meantime, hopefully, better consumer understanding of the limitations of the system will allow sun-smart choices.