SPF: Does it really work?

SPF is included on the label of every sunscreen and the higher the number indicates more effective protection from the sun. But what is SPF and what does it protect our skin from?

The sun emits two types of ultraviolet rays: UVA and UVB rays and they both affect skin differently.

UVA rays are longer and are absorbed deeper into the skin than UVB rays and can damage collagen fibers, whereas UVB rays are shorter and tend to affect the upper layers of the skin and are absorbed by skin cells which directly damages skin cell DNA. This damage to skin cell DNA causes melanin production, leading to tanning, sunburn and increases risk of skin cancer. 

Sunscreen is used to protect skin from these potential damage risks and the amount of protection provided by a product is determined by its SPF.

SPF stands for Sun Protection Factor which tests how much the product protects you from the sun’s UVB light, known to cause skin reddening or an ‘erythemal response’ which may later lead to sunburn. 

SPF is tested by measuring the ability of sunscreen to protect your skin from UVB rays by measuring the minimal erythemal dose (MED) values of the time it takes for skin to redden. The MED values for protected skin whilst using sunscreen and unprotected skin are compared to give a ratio, resulting in the SPF of a product. 

[SPF = MED of protected skin / MED of unprotected skin]

It is important to note that no sunscreen is 100% effective and products with an SPF of 15 protect against 93% of UVB rays, SPF 30 protects against 97% and SPF 50 protects against 98% of UVB rays.

A higher SPF also only means a higher amount of protection against UVB rays but…

What about UVA rays?

The counterpart of UVB light is UVA light. UVA rays are longer in wavelength and therefore are absorbed deeper into the dermis layer of skin and can damage collagen fibers, leading to wrinkle formation. 

Testing for UVA protection is much simpler and isn’t carried out on human volunteers due to lack of immediate visible effects of UVA light on humans. It is instead tested using a spectrophotometer which measures light to see the amount of UVA radiation absorbed by a sunscreen sample placed on a petri dish, of which results must fall into a certain bracket to be suitable for public use.

It is important to understand that ‘broad spectrum’ sunscreens are the ones which have been tested and shown protection against UVA and UVB rays. Mineral sunscreen ingredients like zinc oxide and titanium dioxide are known to help protect against UVA rays.

Why is it important?

We know that sunscreen is important for protecting skin from burning and reducing the risk of developing skin cancer but understanding that no sunscreen is 100% effective and the way that you apply sunscreen can also change its effectiveness. As always, using a parasol, hat and covering up is always a great addition to using adequate amounts of sunscreen with a high SPF. 

We also know that sunscreen is important for protecting skin from burning and reducing the risk of developing skin cancer but understanding that no sunscreen is 100% effective and the way that you apply sunscreen can also change its effectiveness.

When applying sunscreen to the face, the recommended amount is at least ‘two fingers’ in length of sunscreen, which may seem like a lot but that is necessary for getting the right amount of protection. As for the rest of the body, applying sunscreen generously is always best. As always, using a parasol, hat and covering up is always a great addition to using adequate amounts of sunscreen with a high SPF.