Published: 7 April, 1999

Sunblock: an invisible interceptor to UV light

How does sun block work?

The radiation that comes from the sun, known as the solar spectrum, contains the visible sunlight that we see as well as invisible components of "light." The different parts of the solar spectrum have different wavelengths. Some of the solar spectrum is blocked by the earth's atmosphere, including the ozone layer that people are concerned about losing. The part of the solar spectrum that reaches us on earth contains the invisible part, called infrared, that has wavelengths longer than those of visible light. While we don't see the infrared with our eyes, we can feel its warming effects on our skin.

Ultraviolet or UV radiation, another invisible part of the solar spectrum, is responsible for sunburn and other damaging effects to people's skin. UV light causes chemical reactions in living cells to a greater degree than the visible and infrared light. In the skin, these chemical reactions can lead to the increased production of pigments that darken the skin of light skinned people. The familiar and painful sunburn is also a result of UV stimulated chemical reactions. The development of skin cancers and cataracts on the eye are more damaging results of extended UV exposure to the surfaces of our bodies.

The dense fabric of hats and clothing reflects or absorbs much of the visible and UV light and blocks it from reaching the skin. Of course, on hot summer days when the solar spectrum is most intense, people may not be motivated to cover themselves with clothing, so other approaches to blocking the UV part of the solar spectrum have been developed.

Sun blocking skin lotions intercept the UV light before it gets to surface of the skin. The traditional white creams containing zinc oxide particles, that we mostly see on the noses of lifeguards, block not only the UV but also reflect the visible light, which is why they appear white. (I suppose that many people didn't like covering their skin with white goo.) There was probably motivation to find lotions that would block the sun's burning rays without being so obtrusive.

The chemical compound para-amino benzoic acid (PABA) was one of the first active ingredients used to block the UV light responsible for the reddening burn from the sun, while being mostly transparent to the visible light. PABA like the familiar window glass, is transparent to visible light but blocks the part of the UV solar spectrum responsible for sunburning.

Lotions containing PABA form nearly invisible layers on the skin. While sun lotions contain many ingredients to give them a desired feel, smell or water resistance the sun burn protection comes from the ingredients that block the UV light. Because some people developed allergies to the commonly used PABA newer "PABA free" formulas contain other chemical compounds that block the burning rays.

Recently scientists have realized that things are more complicated and compounds that block only the shorter wavelength part of the UV light, called UVB, can prevent burning. The compounds that block UVB may not, however, block the longer wavelength but still damaging UV light, called UVA. The UVA light penetrates the skin and causes damage including aging of skin.

Some modern sun screens contain tiny micro-particles of titanium oxide that block the UV light like the old zinc oxide creams, but the modern formulations are in a more transparent form. The best sun screens should have active ingredients that stop both the UVA and UVB parts of the solar spectrum.