Sun Protection Factor

The sun protection factor (SPF) indicates by how much the sunscreen extends the time until erythema, i.e. redness of the skin, occurs under exposure to sunlight, or in other words, how long the body’s auto-protection time is prolonged. This time naturally depends on the pigmentation type of an individual’s skin, the geographical latitude, and the time of day. People with reddish-blond hair and freckles, i.e. photobiological skin type I, often have an auto-protection time of only 5 to 10 minutes in summer. So a sunscreen with an SPF of 30 for UVB radiation can protect the skin for about 3 hours. However, people should never stay out for this maximum period and expose themselves to the sun until a sunburn reaction occurs as already well before this, i.e. after about half the time, chronic light damage and skin ageing start to occur. Renewed application of a sunscreen with the same SPF does not extend the time of protection. Nevertheless, it is still advisable to keep reapplying the sunscreen because the substances contained in it are degraded by exposure to sunlight. Additionally, the concentration of the sunscreen on the skin is reduced by sweating and swimming and by being rubbed off by clothing, bathing towels and sand.

A good idea of the protection and filter effect of the individual sun protection factors is given by the following table:

Lichtschutzfaktor UVB-Absorbtion

SPF = sun protection factor / UVB absorption (%)

Choosing the right sunscreen
Important factors in choosing the right sun screen are your individual pigmentation type and the intensity of the sun’s radiation you will be exposed to. The radiation intensity depends on the geographical latitude, the time of day and the time of year, the weather, and also the amount of reflected radiation, e.g. through sand, water, snow, etc.

The auto-protection time of the skin varies strongly according to geographical latitude. In the Mediterranean region it falls by about 30 %, and in tropical countries by as much as 50 %. Altitude must also be taken into account as UVB intensity in particular increases with altitude. As a rule of thumb: UVB radiation increases by 15 % for each 1000 metres of altitude. It should also be remembered that a good half of all UV radiation occurs between 11 a.m. and 2.00 p.m. During this time, it is best to avoid the sun altogether and sit in the shade, or even better, indoors.

All these factors therefore have to be taken into consideration when choosing the right sunscreen. For bathing holidays, sport, or in hot, humid countries, water-resistant sunscreens must be used. The ones with the best water-resistance are liposome preparations and hydrodispersion gels, lipogels, oils and water-in-oil emulsions that contain silicon oils.

For winter sports, sunscreens with a very high SPF should be used as in winter the skin is not accustomed to sunlight. Moreover, the sun’s intensity increases significantly at higher altitudes and is also amplified by being reflected off the snow. For protection against the cold, water-in-oil emulsions, pastes or lipogels should be preferred. These provide protection against freezing. Hydrogels and oil-in-water emulsions are not suitable as they increase the risk of freezing because of their high water content.

Protection against the sun is especially important for children and adolescents. Recent studies have shown a close correlation between the number of sunburns people suffered in their childhood and youth and the likelihood of developing skin cancer later on in life. In the first year of their lives, babies should not be exposed to sunlight at all but should always be protected by clothing and a sun canopy. Later, in addition to clothing, if possible with textiles that do not allow UV radiation to pass through, they should also be protected with sunscreens with a high SPF (30 to 40). Sunscreens with reflecting substances (pigments such as titanium oxide, zinc oxide, iron oxide) should be preferred as these are not absorbed by the skin. Light-absorbing (chemical) sunscreens are absorbed only to a very small extent by adult skin, but children’s skin is more pervious. There are at present no indications that the absorption of chemical sunscreens by the lower layers of the skin has any harmful effects. Nevertheless, physical (pigment) sunscreens should be preferred for children.

When applying sunscreen, the lips should not be forgotten as they cannot protect themselves by producing their own pigment. Lipsalves with a sufficiently high light protection factor (SPF 50) should be used.