Not all sunlight is created equal

When it comes to the sunlight (or, more precisely, electromagnetic radiation) that reaches Earth, the light you can see is only one part of the picture. Electromagnetic radiation is divided into different types according to its wavelength, and includes infrared, visible and ultraviolet.

Ultraviolet (UV) radiation, as you can tell from its name, lies beyond the violet end of the visible light spectrum and has shorter wavelengths than visible light—between about 100 and 400 nanometres. It’s UV radiation that we need to worry about when it comes to sun damage to our skin. UV radiation comes from sunlight directly reaching our skin (think sunbathing), but it can also reach our skin indirectly, for example when UV radiation is reflected by white concrete, dry sand or water. So that beach umbrella you’ve just purchased will only partly protect you from UV.

UV radiation can be further divided into UVA and UVB. UVB has shorter wavelengths (280–315 nanometres) and higher energy, and is the major carcinogen and tissue hazard. UVB is responsible for sunburn, and can cause skin cancer and eye damage. UVA (315–400 nanometres) has the longer wavelengths of the two and is lower in energy. It causes damage to the epidermis and throughout the dermis and is considered to cause the longer-term damage responsible for skin cancers and melanomas. UVA is the form of ultraviolet produced in most solariums, and causes wrinkles and ageing of the skin.

It used to be thought that only UVB was a problem when it came to harm caused by the sun. Today, however, we know more about the damaging effects of UVA. The most effective sunscreens filter UVA as well as UVB. If your sunscreen is labelled ‘broad spectrum’, it’s one of those that protect against UVA and UVB damage.

To get the best protection from your sunscreen, remember to apply it liberally (really go to town—don’t hold back), and frequently (at least every two hours).