Trusty sunscreen does hard yards against rays

Melanoma is the most serious kind of skin cancer and New Zealand has the highest incidence rate in the world. With over 90 per cent of all skin cancer cases attributed to excess sun exposure and personally witnessing several sunburned people last weekend, I thought it might be timely for a scientific reminder of the importance of slip, slop and slapping that sunscreen on.

Sunlight contains high energy ultraviolet UVB and UVA photons, which are shorter in wavelength and higher in energy than visible light, meaning that we can't see them with our eyes.

With wavelengths of 280-315 nanometres, UVB rays are the shorter of the two and only affect the surface layers of our skin. Low-level exposure of UVB rays cause your skin to respond by producing more melanin, the pigment which gives your skin a darker appearance when you have a suntan. High levels of exposure to UVB rays trigger an inflammatory response where your blood vessels dilate, making your skin turn red while your immune system releases a flood of cytokine proteins causing you to feel pain in the burnt area.

UVA rays are longer at 315-400 nanometres and penetrate much deeper into our skin down to the connective tissue, causing wrinkles and premature skin ageing. Without sunscreen on, the energy from the sun's radiation goes into the fat and proteins of our skin, generating free radicals which can damage and mutate the DNA in our cells. Because the turnover of skin cells is relatively fast, there's a high probability that these genetic mutations carry through to the next generation of cells resulting in a cumulative collection of mutations with time. Combined with cellular damage, these two factors are thought to initiate skin cancer as well as cause wrinkles and age spots.

Sunscreen consists of a mix of chemicals which both block and absorb the sun's rays. The inorganic chemicals include minerals such as zinc oxide or titanium dioxide which act as a physical sunblock and reflect or scatter the light away from the skin. These minerals are white in colour and used to be large enough that you could see them as a white hue sitting on the skin. Over the past decade, the mineral particle size has been decreased enough that they are too small for us to see, which is why most sunscreens are now almost invisible on the skin.

Sunscreens also contain organic chemicals made from carbon, such as oxybenzone, which absorb the UV radiation through their chemical bonds. These organic molecules release the absorbed energy as heat and act as a protective surface layer.

No sunscreen can block 100 per cent of the sun's rays, so the SPF (sun-protection factor) refers to how long it will take for a person's skin to turn red from UVB exposure. Sunscreen with an SPF of 15 will prevent your skin from getting red for approximately 15 times longer than usual, so if you start to burn in 10 minutes without sunscreen, sunscreen with SPF15 will prevent burning for about 150 minutes. The SPF refers only to how well the sunscreen protects against UVB radiation. Previously, UVA was considered relatively harmless in comparison to UVB, but recent research shows it also causes cell damage so sunscreens labelled with broad-spectrum protection are designed to shield against both.

As UV rays can pass through clouds, you can burn even if the sun isn't shining, so a daily routine of applying sunscreen every two hours in addition to your summer icecream will hopefully help us to reduce the incidence of skin cancer and prevent those painful summer burns.

• Dr Michelle Dickinson, also known as Nanogirl, is an Auckland University nanotechnologist who is passionate about getting Kiwis hooked on science