Increasing use of high sun protection factors in sunscreen products

UV filters: so that we can expose ourselves to the sunlight without getting wrinkles

Scientists have discovered that each and every episode of sunburn can contribute to the development of additional wrinkles even 15 to 20 years later. And statistics show a startling increase in the frequency of skin cancer. Whereas earlier sunscreen products generally relied on only one or two filters to provide protection against the harmful UVB component of the sun's rays, products with combinations of four or more filters and high sun protection factors are now becoming established. Efforts are also being made to reduce the amount of UVA radiation, which can cause chronic skin damage.

1. There's a lot to learn about suntanning

Of the sunlight that reaches the earth's surface, around 39 percent is composed of visible light, 56 percent of infrared radiation, and the remaining 5 percent of ultraviolet radiation. Of the latter, 98 percent is composed of the longer-wave UVA rays that are responsible for direct suntanning, while the remainder is composed of the shorter-wave UVB rays that cause indirect suntanning. The very short-wave UVC rays are absorbed in the ozone layer of the stratosphere at an altitude of 30 to 35 km, with the result that only tiny amounts reach the earth's surface in high mountain ranges. These rays tan the skin strongly, but also burn it, and are so energetic that when artificially produced they are used for the purpose of sterilisation.

UV rays can stimulate blood flow and metabolism in the skin and can produce vitamin D3, which is important for calcium balance, from the corresponding provitamin, which is present in skin fat.

The basal layer of the human epidermis contains basal cells and occasional pigment-forming cells, or melanocytes. These contain melanosomes, which respond to UV radiation by producing increased amounts of melanin. This pigment is transported to the cornified cells of the superficial layer of the epidermis, where it imparts a brown colour to the skin.

Triggering of increased cell proliferation leads to gradual thickening of the horny layer of the epidermis. The "Lichtschwiele" formed in this way blocks some of the UVB rays and thus provides the skin with some degree of protection against sunburn.

Excessive exposure of the skin to UVB rays nevertheless causes acute damage in the form of sunburn (erythema). Chronic UVB-induced damage leads to degeneration of the connective tissue of the dermis (corium) and in turn to premature skin aging. Far more serious, however, is the development of skin cancer.

The UVA component of sunlight initially causes spontaneous pigmentation. With more prolonged exposure, this leads to a suntan that persists for some time. In a solarium these processes are accelerated by application of large amounts of energy.

UVA rays potentiate the biological action of UVB rays. In addition, these longer-wave rays penetrate into the deeper layers of the skin and can also cause damage there.

2. The UVA component is less variable

The intensity and composition of the UV rays that reach the human body are influenced by a number of factors. These include the length of their path through the earth's atmosphere (as determined by season, time of day, and geographical latitude), height above sea level, and air purity.

The content of UVB rays in sunshine is greatest at latitudes of up to 30 degrees north and south of the equator. At an altitude of 1000 meters the content of UVB rays is 15 percent higher, whereas the content of UVA rays is almost the same as at sea level. At latitudes of greater than 55 degrees (cf. Switzerland: 47°), very little UVB radiation reaches the earth's surface during the winter months.

As the amount of UVB radiation reaching the earth's surface is directly dependent on the angle of incidence of the sunlight, the risk of sunburn is greatest in the middle of the day. Sunbathing should therefore be avoided during this period. The amount of UVA radiation reaching the earth is less influenced by the position of the sun, hence UVA radiation is present throughout the year. Even an 83 percent cloud cover only halves the intensity of UVA radiation.

Window glass and some types of plastic that block UVB rays do not interfere with the passage of UVA rays.

3. The skin also needs protection from UVA rays

Eighty percent of sunscreen products are supplied in the form of emulsions. Other forms include aqueous, alcoholic, or oily solutions, aerosol foaming creams, and lip salves. The sun protection agents that they contain are substances that absorb, and in some cases also scatter and reflect, UV rays.

The European positive list currently includes about twenty UV filters for use in sunscreen products, though in practice only about half of these can be used. Most commonly used are substances that absorb UV rays with wavelengths in the region of 300 nanometers, as these burn the skin very easily, while allowing suntanning by permitting the passage of a proportion of UVA rays.

In chemical terms, the most commonly used organic UVB filters are cinnamic acid esters, methylbenzylidene camphor, and a water-soluble benzimidazole. Salicylic acid esters, which are also used but whose filter action is less pronounced, provide good solvent properties for the sparingly soluble filter components.

Other components include micropigments such as titanium dioxide and zinc oxide. These contribute to light protection by absorbing, scattering, and reflecting UV rays. Zinc oxide influences UVA rays more than does titanium dioxide, which acts mostly in the UVB range. Microfine oxides such as these can be used only in limited amounts, as otherwise they make the skin appear white. An optimal protective effect can be achieved only if the particles are extremely finely ground and can be distributed evenly over the skin.

4. Sun protection factors: a numbers game?

The sun protection factor (SPF) of a sunscreen product indicates how long UVB radiation takes to cause a certain degree of erythema (reddening) on skin to which that product has been applied as compared with the time it takes to cause the same degree of erythema on unprotected skin. It is calculated as the ratio of the UV dose required to cause sunburn on sunscreen-protected skin to the UV dose that causes sunburn on unprotected skin.

Similar standards for the determination of SPF have now been established in Australia, the USA, and Europe. An important factor in this determination is correct simulation of the spectrum of solar radiation in the open air.

The current trend in sunscreen products is to higher and higher SPFs. Most products have SPF ratings of 15 to 25, though in the USA, for example, products with SPF ratings of over 50 are also available. Australia recently set an upper limit of SPF 30, and in fact, values above 40 bring scarcely any additional benefit, because even with an SPF of 33 about 97 percent of the rays that cause erythema are filtered out.

Although the UVA component of solar radiation predominates, it makes itself less apparent as sunburn than does the UVB component. Attempts have therefore been made in recent years to develop suitable methods for measuring the extent to which a product affords protection against UVA radiation. Nevertheless, it is likely to be some time yet before a universally valid and standardised system of classification becomes available.

As each filter has specific absorption characteristics and causes greatest absorption within a given range of wavelengths, sun protection agents can differ greatly in terms of their chemical structure. When these filter molecules absorb UV radiation energy, the absorbed UV rays raise electrons to higher energy states. The electrons thus raised to more peripheral orbitals return to their original energy state not directly, but via a number of intermediate steps, releasing the energy difference mostly in the form of longer-wavelength rays and light or heat.

5. Strict safety regulations for the positive list

Before a new filter can be marketed, it must be shown to be harmless to health, effective, and safe to use. The legal regulations that apply to UV filters are considerably stricter than those that apply to many other cosmetic substances. The cost of the skin and allergy tests prescribed for the development of a new substance can thus easily exceed two million Swiss francs.

PARSOL® 1789, a Roche product, is the only UVA filter currently marketed throughout the world. It is a component of virtually every sunscreen product marketed in Europe and Australia. In April of this year it was approved for use in the USA and therefore seems likely to be used increasingly there too.

The solar spectrum