UV radiation
Sunlight is composed of different photons of light. UV radiation has broad spectrum, ranging from 40 to 400nm (30-3eV), which is divided into Vacuum UV (40-190nm), Far UV (190-220nm), UVC (220-290nm), UVB (290-320) and UVA (320-400nm), of which the latter two are medically important. There are two distinct subtypes of UVA radiation. Short-wave UVA (320-340nm) and long-wave UVA (340-400nm), the latter constituting most of UVA radiation. The amount of exposure to UVA usually remains constant, whereas UVB exposure occurs more in the summer.
UVC light has the highest energy and is the most damaging yet thankfully is present high in the ozone layer. UVA penetrates deep into the skin and causes wrinkling and other visible signs of ageing. This is called photo-damage. UVB has higher energy than UVA and is considered the primary source of hazardous sunburn. All types of UV light can interact with our DNA which can lead to mutations and later can possibly lead to skin cancer.
How does sunscreen stop UV then?
Sunscreen is a chemical filter than can either absorb or reflect UV photons before reaching our skin. It is important to note that many sunscreens before only used to protect against UVB because it is UVB which brings about that burning effect on our skin. Yet nowadays sunscreens protect against both UVB and UVA since it has been observed that UVA increases the risk of skin cancer. Sunscreens which protect against both UVB and UVA are referred to as broad-spectrum.
It is important to keep in mind that no sunscreen completely blocks UV rays! Yet when using sunscreen and other measures such as wearing protective clothing, we can reduce the risk of skin cancer and early skin ageing by reducing how much of that UV is actually reaching our skin.
Sunscreens work in relation to the sun protection factor, that is, SPF. SPF is a measure of the fraction of the sunburn-producing UV rays which reach the skin. The SPF of a sunscreen is tested in the laboratory on humans by exposing human skin on different spots of light which is equivalent to around 10 minutes in the sun. The different spots of light are brought about by a machine which is known as a sun stimulator. The SPF number is calculated simply by the number of seconds it takes for the exposure for example spending 10 minutes in the hot sun in August you will only get a mild erythema, that is, you turn pink yet when applying a sunscreen SPF 30 the risk of mild erythema should be 30 times as low. Therefore using this principle to get mild erythema when applying SPF 30 it would take around 300 minutes to do so. On the other hand SPF 50 will give you 500 minutes of sun exposure protection which is around 9 hours of the day. Therefore having a high SPF number will give you better and prolonged sun protection.
What sunscreen should we therefore use?
One should use sunscreens with an SPF of at least 15 yet this is not enough since it may stop some UVB rays but not UVA rays. Therefore a sunscreen should be broad-spectrum 15 or higher to protect against both UVB and UVA rays. On another note sunscreens do not tend to work perfectly on the beach when compared in a lab under strict controlled measures because sometimes we tend to not apply sunscreen as thick as we do in the lab. On average a person applies only a third of the recommended amount and hence the sunscreen does not work efficiently.
It is also important to note that tattoos do not protect against sun damage or skin cancer. It is still suspectable to get skin cancer at the tattoo area and in fact various dermatologists believe that tattoos can interfere with skin cancer diagnosis if they are at the site of cancer. Therefore, it is also important to apply sunscreen on the tattoo region.
Rebecca Caruana is a medical student