How does sunscreen work, and what does SPF, UVA and UVB mean?

It looks as though summer has finally arrived in the UK, albeit slightly delayed. That means Brits will be heading to beaches, garden parties and alfresco lunches – and they might also be more concerned about sun protection during this time of year.

Some of the biggest sunscreen brands in the UK have failed safety tests, revealing that they offer less protection than they claim to.

In an annual safety test by consumer group Which?, Asda Protect Moisturising Sun Lotion SPF30 High, Calypso Press & Protect Sun Lotion SPF30 and Bondi Sands SPF 50+ Fragrance Free Face Sunscreen Lotion all failed the watchdog’s tests to assess their SPF and UVA protection.

From SPF, to UVA and UVB, here's what you need to know about sun protection.

How does sunscreen work?

Sunscreen ingredients prevent UV rays from damaging your skin. Physical blockers contain minerals such as titanium dioxide or zinc oxide that sit on the surface of the skin and reflect UV rays away from it.

Chemical sunscreen ingredients form a thin protective film that absorbs UV radiation before it penetrates the skin. These are sometimes referred to as organic sunscreens.

What does SPF, UVA and UVB mean?

SPF, which stands for Sun Protection Factor, is a measure of how long a person can stay in the sun before its UVB rays start to burn the skin. The number represents how much UVB it allows to reach your skin.

UVB rays primarily affect the outer layer of the skin, known as the epidermis. They are responsible for sunburns and some skin cancers.

The sun also emits UVA rays, which can penetrate into the lower level of the skin, called the dermis.

A way to distinguish between UVA and UVB is to remember that A is for ageing and B is for burning.

Typically, SPF numbers refer only to UVB rays but some sunscreens can protect against UVA as well.

The amount of UV rays filtered depends on the level of SPF. SPF15 blocks 93 per cent of UVB rays, SPF30 blocks 96.7 per cent of UVB rays, and SPF50 blocks 98 per cent of UVB rays.

The number tells you how long you're protected from the sun for. For example, if you typically burn after being outside for 30 minutes and have an SPF 15 sunscreen you plan to use. You’d multiply the minutes (30) by the SPF (15).

However, no sunscreen offers 100% protection. Most sunscreens will wear off from exposure to the elements as well as sunscreen not being applied as directed (it is recommended that you reapply every two hours).