Q. I am confused about the SPF and star labelling on sunscreens, also what to apply when. Can you clarify please? A. SPF stands for sun protection factor and relates to how long a sunscreen will protect you from ultra violet (UV) B rays (not UVA). The SPF is calculated by taking the time you would burn without using a sunscreen and multiplying it by the SPF. So if you would burn after 15 minutes in the sun, an SPF30 would protect you against UVB for about seven and a half hours, assuming you use it correctly and do not get wet.

The star rating (five is the top) applies to UVA protection. The mantra is UVA ages and UVB burns. UVA rays reach deeper into the dermis (under the top layer or epidermis), damaging collagen and elastin fibres and also damaging DNA, which may lead to potentially fatal melanoma. UVB is the primary culprit in sunburn and non-melanoma skin cancer, stopping at the epidermis.

Dermatologists recommend choosing a product with an SPF of 30-50 and four to five stars to give you good UVA/UVB protection. If you have touchy skin, choose a physical barrier (zinc oxide or titanium dioxide), rather than a chemical barrier. These used to often leave a ghostly white veil but better technology means you can buy sheer products now. (Though they may still ‘show’ on darker skins.)

However, using sunscreen correctly is a big assumption as almost no one applies the amount used in laboratory tests to determine the SPF. According to consultant dermatologist Dr. Nick Lowe (drnicklowe.com), if you are on a sunny holiday you need to apply eight teaspoons (a 10p-sized blob) for face and body, every two hours. One teaspoon each for your face, neck (front and back) and backs of hands; two teaspoons for torso, front and back; the same for legs and one teaspoon for arms. Start 30 minutes before you go out in the sun, then apply another layer just before leaving and don't rub it in too vigorously.

If you are only outside for short periods in Britain, an SPF25 is a good starting place, according to Dr. Stefanie William of European Dermatology London (eudelo.com).

If you’re looking for a tinted moisturiser with a high SPF, we suggest Epionce Daily Shield Lotion Tinted SPF50, £45, which has a physical barrier, is water resistant (to 80 minutes), and contains anti-inflammatory antioxidants, in the form of apple extract, argan oil and rice bran. Epionce founder Dr Carl Thornfeldt emphasises that the sun is not the only skin pollutant, traffic fumes are villains for skin. His mantra is to protect the skin barrier (stratum corneum) and Epionce formulations, which have been extensively tested and proved effective, fortify this essential defence system.

If you wear foundation, one innovation this summer is Garnier Ambre Solaire Sensitive Advanced Protecting and Hydrating Face Mist SPF50, which can be applied over make-up to top up your protection.

Other favourite brands include Green People, particularly for their sensitive range (greenpeople.co.uk) and Ultrasun (ultrasun.com), which includes a tan-activating, water-resistant SPF30 for face.

However, every expert emphasises that you cannot simply rely on sunscreen. Wear a big-brimmed, closely woven hat, Jackie O sunglasses (eyes need protection just as much as skin, for adults and children), cover up with a t-shirt or caftan, and keep out of the sun’s rays from 11am to 3pm when they are at their height.

Finally, never use a sunbed except under medical supervision.