Almost half of the sunscreens tested are failing their most-basic requirement - protecting us from the sun as much as they say they do.

The results, which also slam some makers of "natural" sunscreens, now removed from shelves, come from a Consumer NZ survey, leading to calls for mandatory standards for sunscreens in New Zealand.

In summer, it's important to slap on sunscreen to reduce UV radiation damaging the skin. The sun exposes people to 2 types of UV rays, both of which can cause skin cancer:

• UVA penetrates deeply into the skin and can cause wrinkles and age spots.
• UVB causes skin reddening and is the main cause of sunburn.

The consumer watchdog tested 17 sunscreens and found that only nine met the promises made on the label about sun protection factors (SPF). A further six of the 17 failed the same test, though these companies were able to provide laboratory tests showing they did.

The standard requires SPF to be assessed using a test panel of 10 volunteers. Testing on humans determines a sunscreen's ability to provide adequate protection and stay on the skin without breaking down. To assess UVA protection the lab used a spectrophotometer to measure UVA radiation passing through a thin film of sunscreen on a plate.

The standard is mandatory in Australia, but voluntary in New Zealand where sunscreens are classified as cosmetics. Products that meet other international standards, such as those in the US or EU, are also allowed to be sold, as well as sunscreens that don't meet any standard.

The survey results were sent to companies and led to two producers of "natural" sunscreens pulling their products from shelves.

But what do the SPF numbers mean?
An SPF 15 sunscreen that's properly applied is meant to give you 15 times the protection you'd get with unprotected skin.
So if you were outside in the sort of sun that burns unprotected skin in 10 minutes, then SPF 15 would give you 150 minutes of protection. For SPF30 sunscreen that time would extend to 300 minutes and for SPF50 it would be 500 minutes.

That's the theory. These times vary from person to person because of skin type, activities(such as heavy exercise or swimming) and how well the sunscreen is applied. No matter how high the SPF, any sunscreen should be reapplied regularly - every 2 hours that you're in the sun. No sunscreen blocks 100% of UV rays: SPF15 blocks 93% of UVB, SPF30 blocks 97% and SPF 50 blocks 98%.

More: https://www.consumer.org.nz/articles/sunscreens