All you need to know: Sunscreens explained, how to best protect yourself this summer

Knowing how easily your skin burns is your best weapon in the fight to protect yourself from the sun and the dangers associated with it.

Sunburn is caused by excess exposure to ultraviolet (UV) radiation from the sun and increases the risk of skin cancer.

The simple suggestion of slip, slop, slap and wrap was thrust upon Kiwis from a young age. However, many of us continue to burn.

But it's not just a case of failing to follow the instructions. Consumer NZ recently revealed nine out of 20 sunscreens failed to meet their label claims.

The products, all sold in New Zealand, fared better than those tested in 2018, when only four of 19 lived up to expectations.

Two types of UV light are proven to contribute to the risk of skin cancer: ultraviolet A (UVA) and ultraviolet B (UVB).

Both types of UV light were proven to contribute to the risk of skin cancer. Photo / Getty Images

UVA has a longer wavelength and is associated with ageing skin, while UVB's shorter wavelength is associated with burning skin.

Sunscreens are measured by their sun protection factor (SPF), which records the sunscreen's protection level from UVB rays.

If your skin normally burns after 10 minutes in the sun, applying SPF 15 sunscreen will protect you for about 150 minutes.

SPF 30 extends that to 300 minutes and SPF 50 to 500 minutes.

The protection rating is a rough estimate which depends on skin type, the intensity of sunlight and the amount of sunscreen used.

University of Auckland school of pharmacy senior lecturer Dr Manisha Sharma said the best methods of sunburn protection could be easily followed.

"The first thing the consumer should realise is that the use of sunscreen is one of the ways to protect from the sun and is not the only way," she said.

"People can protect themselves by using protective clothing [sunglasses, wide-brimmed hats], use shades and should try to avoid the sun during peak hours [10am to 4pm]."

But people should balance protection against the sun with exposure, as it is important to keep vitamin D levels up, Sharma said.

A woman applying sunscreen to her legs. Photo / 123RF

Basking in the New Zealand sun was hazardous due to the country lying so close to the "ozone hole" which had formed over the South Pole.

The ozone layer, in theory, should act as a natural sunscreen for Earth, Sharma said, but the protection level over New Zealand was quite low.

"Therefore, more protection is necessary here from the strong sun UV rays compared to the rest of the world."

Among those to fail the Consumer NZ sunscreen test was a Cancer Society Sunscreen SPF 50+ product which was immediately withdrawn.

Following testing at two different labs, the product was found to provide a maximum SPF that was 20 units less than it claimed to be.

Banana Boat, Sunsense Ultra and Marine Blue were other big-name brands to fail the rigorous testing.

SPF testing is generally undertaken on humans and there would always be variability as people burnt at different rates, Sharma said.

Common protocol required testing on at least 10 people, a sample size unlikely to be representative of the entire population.

Sunscreen should then be applied generously around 15 to 30 minutes before exposure to the sun and then after every two hours.

Sunscreen wears off due to sweating and while swimming. Even water-resistant products can be washed-off if the person stays in the water for a long time.

Sharma said when it came to buying sunscreen products, people should look to select those with a broad-spectrum, to fight off both UVA and UVB rays.

Meanwhile, it was worth noting SPF scale was not linear but logarithmic, meaning a higher SPF did not provide greater protection. For example, SPF 60 will not provide twice as more protection then SPF 30.

"In general, SPF 15 product is recommended for daily use and SPF 30 for outdoor activity to have adequate protection," Sharma said.

"However, it is important to know your skin type and choose the product accordingly, as different skin will react differently."

How to be sunsmart this summer

Sunscreen should be applied generously around 15 to 30 minutes before exposure to the sun and then after every two hours. Photo / Sylvie Whinray

• Apply sunscreen at least 15 minutes before exposure, then every two hours after that.

• Try to use half a teaspoon of sunscreen on your face, ears and neck, and one teaspoon on each arm and leg.

• Seek shade from the sun, either under a tree or an umbrella wherever possible.

• Slap on some protective clothing and accessories such as a wide-brimmed hats, long-sleeved shirts and sunglasses.

• Avoid the sun during peak hours between 10am and 4pm. You can also check the UV index of the day online.

• Remember it's important to expose your body to the sun for the synthesis of vitamin D, so try and balance protection and exposure.

Who is at risk of getting sunburnt?

• The fairer your skin, the higher the risk of you getting sunburnt. UV radiation can damage kin of any colour.

• The closer you live to the equator is also a risk factor, with the level of UV increasing towards the tropics.

• UV radiation also increases 4 per cent for every 300m in elevation, so being at a higher altitude comes with its risk.

• Cloudy skies reduce UV radiation. However, 80 per cent of radiation can still break through light cloud cover.

What are the signs of sunburn?

• Red, warm skin that is tender to touch.

• Skin peeling around four to seven days after exposure. Signs usually start three to five hours after exposure.

• Severe cases of sunburn can cause blistering, swelling, chills and a high temperature.

How do I treat sunburn?

• Sunburn can generally be treated at home with aloe vera or other moisturisers.

• If a child or baby has sunburn, you're burnt over a large area, or feel faint, dehydrated and have severe blistering, seek medical advice.