The remaining parts of your skin that are not protected from the sun in any other way should be protected by proper use of sunscreens (sun blocks, sun creams, sun lotions, etc.).

However, the availability and increased use of high protection factor sunscreens since about the 1970s has also been accompanied by a dramatic rise in the incidence of melanomas, the most serious form of skin cancer. There may be several reasons for this:

  • Until recently, sunscreens have been designed to protect against sunburn, and therefore primarily block only UVB, not UVA. UVA penetrates more deeply into the skin and can therefore be more damaging. It also suppresses the body’s immune system. Sunscreen users could therefore have a false sense of security, the reduced risk of sunburn causing them to stay longer in the sun. This effect could well be exacerbated by the widespread misapplication of sunscreens: people often do not apply it sufficiently thickly, nor reapply it sufficiently often.
  • Sunscreen use inhibits vitamin D production in the skin, and vitamin D is known to reduce the incidence and severity of many forms of cancer, including melanomas.
  • Some chemicals commonly used in sunscreens, once absorbed into the skin and then subjected to UV radiation are broken down to highly reactive forms that can then damage the DNA in the cells of the skin, possibly leading to cancer. This is similar to the effect of UVA on the natural chemicals in the skin, only there are now even more highly active chemicals around to do the damage.

Therefore it is best not to rely too heavily on sunscreens for protection: use sunscreens to supplement other methods such as clothing and seeking shade. Certainly do not use them to prolong sunbathing activities.


The effectiveness of a sunscreen is measured by its Sun Protection Factor (SPF) – although some would say that this should more accurately be Sunburn Protection Factor.

However, the SPF only measures UVB protection (UVB is the type that causes sunburn). In the UK UVA protection is indicated by an additional star rating – from 0 to 5 stars – which gives the ratio of UVA to UVB protection. Thus a high SPF sunscreen with few stars can actually give better UVA protection than a low SPF product with more stars. Other countries provide UVA ratings in different ways, if at all – there is no international standard.

The SPF rating gives the number of times exposure duration has to be increased for the same effect as unprotected exposure at the same sun intensity. Thus for a SPF of 15, 15 hours exposure is equivalent to 1 hour unprotected, and for SPF 30, 30 hours gives the same effect.

Applied properly, a sunscreen with an SPF of 15 should be sufficient, even in the tropics, except for those unusually sensitive to UV radiation who burn very easily. Some organisations, particularly in the USA and Australia, recommend an SPF of 30 or more.

How to Choose

When choosing a sunscreen, you therefore need to check both the SPF (15 or more) and the UVA rating. You also need to consider the degree of water resistance offered. It is generally better to choose at least a water resistant product, so that it will resist being washed off by sweating. If you intend to swim or take part in sporting activities (and therefore likely to sweat a lot), then a properly waterproof product will be better.

The method of application – cream, lotion, spray, etc. – is a matter of personal preference. You should also choose one that is comfortable on your skin, since you are likely to be wearing it for prolonged periods.

The effectiveness of sunscreen products also slowly degrade over time: check the expiry date and do not use out of date products. However, if you are using it properly, a bottle of sunscreen is unlikely to last very long.


To be effective, sunscreen should be applied evenly and liberally. Many people do not apply sunscreen sufficiently thickly, and instructions on the bottles are often inadequate.

A covering of at least 2 mg per square cm of skin is needed. This corresponds to a teaspoonful for each arm, leg, the front of the body, and the back of the body, together with half a teaspoonful for the face, neck and ears. This adds up to about 35 ml, or an eggcup or shot-glass full, for the whole body. A typical bottle size often only contains enough for about six such applications.

For maximum protection this should be applied about 20–30 minutes before going into the sun. Some also recommend re-application after about 30–40 minutes in the sun. After this, the sunscreen is gradually absorbed into the skin and loses its effectiveness. Sunscreen should therefore be reapplied about every 2 hours to maintain protection.

Sunscreen should also be reapplied after swimming, bathing, sweating heavily, or rubbing or drying off with a towel or handkerchief. Waterproof sunscreens often also need reapplication more frequently while swimming.

Make-up or insect repellents, if needed, should be applied on top of sunscreen, not underneath.

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