Sun tan lotion looks to longer wavelengths

17 Jun 2002

Manufacturers of sun tan lotions are starting to look beyond the ultraviolet into the visible range for potential health risks from the sun.

"The definition of ultraviolet light having a wavelength below 400 nanometres is just a nominal cut-off and there is no reason why 401 nanometres should be any safer than 399 nanometres," said Mike Brown of Boots Contract Manufacturing in the UK. "We might start to look at visible and infrared radiation and whether these have any effect on the skin." Current sunscreens protect against light between 295 and 400 nanometres as very little of shorter wavelengths reaches the earth.

There is no strong evidence yet that visible light represents a problem but, so far, successive developments in sun creams have highlighted dangers associated with longer wavelengths of ultraviolet light.

The risks of sunburn and skin cancer from UVB radiation (280 to 320 nanometres) have been known for many years and sun care products aim to protect against these. More recently the long term skin ageing effects caused by the more deeply penetrating UVA radiation (320 to 400 nanometres) have been identified and products now advertise 'anti-ageing' properties in addition to the traditional UVB protection. Brown says that new challenges arise as more is understood about the interaction between ultraviolet and DNA. Some evidence suggests that ultraviolet has an adverse effect on the human immune system.

Sun filters are incorporated into other products such as sunglasses and uPVC windows. Sun protective clothing is made from textiles impregnated with sun filters and clear plastic bottles incorporate filters to avoid the contents fading. Apart from sun tan lotions, sun filters are included in other cosmetic products, such as make-up, moisturisers and shampoos. The materials in cosmetics are restricted by safety regulations.

Traditionally the filters in sun tan lotions are organic compounds that react with photons of ultraviolet light, exciting an electron. The electron loses the extra energy, in steps, as low-energy infrared radiation which corresponds to small amounts of heat. A disadvantage of these compounds is that the excited electron makes them more reactive so they can form other chemicals that cause allergies.

Several years ago Boots began to study inorganic compounds such as titanium dioxide. This compound reflects visible light in white paint and Boots discovered that by making the crystal size much smaller, the compound reflects ultraviolet. Inorganic compounds tend to spread less easily than organics and leave whitish marks on the skin so most sun tan lotion manufacturers now use a combination of both types of compounds.

Brown says that protection from visible radiation is likely to be achieved using physical sunscreens that reflect, rather than absorb, as this technology is already known. However, visible absorbers would give the skin a black or brown appearance and so could be tuned to produce an instant tan effect. "This is still somewhat science fiction, but who knows what the future might deliver?"

SH