What SPF Do I Actually Need?

SPF is one of the more misunderstood numbers in everyday health, partly because it looks like a simple scale and is not, and partly because the way it is tested bears little resemblance to how anyone actually uses sunscreen.

What SPF measures

Sun Protection Factor describes protection against UVB, the wavelength primarily responsible for sunburn.

The number indicates how much longer it takes skin to redden compared with no protection, under controlled conditions. Skin that would burn in 10 minutes unprotected would, in theory, take 300 minutes under SPF 30.

That is a laboratory figure and it is not a promise about an afternoon on a beach.

The percentages

SPF	UVB filtered
15	about 93 percent
30	about 97 percent
50	about 98 percent
100	about 99 percent

The scale flattens quickly. Going from 15 to 30 removes over half the remaining exposure. Going from 30 to 50 removes about a third of what is left, and from 50 to 100 rather less again.

This is why UK guidance settles on at least SPF 30 rather than pushing for the highest available. It is also why very high numbers are of limited extra value and carry a risk of false confidence.

UVA, the half the number ignores

SPF describes UVB only. UVA penetrates more deeply, contributes heavily to skin ageing, and also plays a part in skin cancer risk.

In the UK and Europe, UVA protection is shown separately, either as a star rating from zero to five or as the letters UVA inside a circle, which indicates the product meets a minimum ratio of UVA to UVB protection.

Look for both. A high SPF with poor UVA protection leaves a significant gap, and because UVA does not cause immediate reddening, there is no feedback to tell you.

The quantity problem

This is the single largest issue and it is not about the product at all.

SPF is tested at an application of 2 milligrams per square centimetre of skin. Studies of how people actually apply sunscreen consistently find between a quarter and half that amount.

Protection does not decline proportionally; it declines faster. Applying half the tested amount of an SPF 30 gives considerably less than SPF 15 in practice.

For a full adult body, the tested quantity works out at roughly 6 to 8 teaspoons. A practical way to hit it is a teaspoon each for the face and neck, each arm, each leg, the front of the torso and the back.

Most people do not use anywhere near that, which means the effective protection from a bottle of SPF 50 applied thinly may be lower than SPF 30 applied properly.

Reapplication

Sunscreen degrades in sunlight, rubs off on clothing and towels, and washes off in water or with sweat.

Reapply at least every two hours, and immediately after swimming or towelling, even with products labelled water resistant. Water resistance is tested for a defined immersion time and does not mean permanent.

One application in the morning does not last the day, whatever the SPF.

The bits people miss

Ears, the back of the neck, the tops of the feet, the hairline and any parting, the backs of the hands, and the lips, which need a balm with SPF since ordinary sunscreen is unpleasant there.

Around the eyes is also commonly under-applied, and it is a common site for skin cancers.

Beyond sunscreen

Sunscreen is the third line of defence, not the first.

Shade, particularly between 11am and 3pm when UV is strongest.

Clothing, which provides consistent protection that does not wear off. A closely woven long-sleeved top outperforms any sunscreen because it does not depend on application technique.

A wide-brimmed hat for the face, ears and neck.

Sunglasses meeting a UV protection standard, since eyes are also damaged by ultraviolet.

Sunscreen covers what these cannot, rather than replacing them. Guidance is explicit that sunscreen should not be used as a way to extend time in strong sun.

Cloud, water and altitude

Up to 80 percent of ultraviolet radiation passes through light cloud, which is why overcast days produce burns from people who felt no heat and saw no glare.

Water, sand and snow all reflect UV upward, increasing exposure to areas that would otherwise be shaded, including under the chin and nose. Altitude increases intensity by roughly 10 percent per thousand metres.

The vitamin D question

Sunscreen does reduce vitamin D synthesis, which has prompted reasonable concern.

The practical resolution is that short periods of unprotected exposure in summer, well short of burning, are enough for most people, and that a daily supplement from October to March is advised in the UK regardless. Deliberately burning to make vitamin D trades a small nutritional gain for a measurable increase in cancer risk.

Expiry and storage

Sunscreen degrades. Most carry a shelf life of around three years unopened, and a period-after-opening symbol showing how long they remain effective once used.

Heat accelerates the breakdown, which is unfortunate given where sunscreen is usually kept. A bottle left in a hot car or on a beach all summer loses effectiveness faster than the date suggests.

The practical test is that a bottle you are using properly should not last multiple summers. Applying the recommended quantity, a household gets through a standard bottle in a few outings. A bottle that has survived three years is evidence of under-application rather than good value.