New scientific data published by L'Oréal researchers show that the accumulation of low doses of UV rays day after day leads to significant biological damage to the skin when the latter is not protected.
"The abuse of UV rays is dangerous to the skin" - each season, we are reminded of this warning by manufacturers of sunscreen products and by dermatologists during prevention campaigns. These harmful effects are revealed by studies simulating the summer sun near its zenith blazing down on the Earth in a clear sky. This demonstration is admittedly effective, but it is based on a disaster scenario. It must be recognized that only part of the world population is likely to be exposed to this type of radiation. The other part-the majority-is quite rarely or even never subjected to such doses of UV rays. Based on these findings, a L'Oréal research team has evaluated the biological effects of exposure to daily UV radiation in non-extreme conditions that are closer to reality for each of us.
Simulating UV radiation in non-extreme exposure conditions
"To study the effects of repeated exposure to daily UV rays on our volunteers, we used a solar simulator duplicating the UV radiation of sunlight with a UVA/UVB ratio of 24. This ratio, characteristic of non-extreme conditions of exposure, was calculated in a diversity of locations and weather conditions (see the sidebar at the end of the article)", Sophie Seité, first author of the publication, explained.
The dose of UV rays administered during each exposure is very low, 0.5 MED (minimum erythema dose), half the dose of UV rays necessary to cause a rash. "These are realistic exposure conditions! This dose is the one we are exposed to during the day in mid-April in Paris", S. Seité noted.
The investigators evaluated the biological effects of exposure repeated up to 19 times during the four weeks of the study on a battery of molecular cell markers of the epidermis and dermis known to respond to UV-induced stress.
Erythema is a poor indicator of skin damage
Most effects identified after repeated exposure, such as reduced hydration, generation of "sunburn" cells, alteration of Langerhans cells (immune defense cells of the skin), occur without or before the appearance of a rash. Therefore, avoiding sunburn does not systematically prevent such damage.
After 19 exposures, the researchers observed significant changes in skin color and in epidermal morphology, including increased thickness; they also noted an accumulation of p53 protein ("the guardian of the genome") in the nuclei of keratinocytes and a reduced number of Langerhans cells. In the dermis, deposits of lysozyme on elastic fibers increase, whereas deposits of glycosaminoglycans taking part in hydration diminish.
Data supporting day-to-day protection from sunlight
The results of this study are in support of day-to-day protection of the skin to counter the harmful effects of low doses of UV rays due to repeated exposure. As indicated by S. Seité, "it is not appropriate to use SPF 50+ products to be protected from daily UV rays. However, coverage of the entire solar spectrum is essential and provides effective protection against UVAs, in particular." By way of a reminder, most UVA rays cross the epidermis and 20 to 30% of them reach the dermis. Although they carry less energy, they are 100 times more abundant than UVBs; they are involved in premature skin aging and in skin cancers.
Seité S, Medaisko C, Christiaens F, Bredoux C, Compan D, Zucchi H, Lombard D, Fourtanier, A; Biological effects of simulated ultraviolet daylight: a new approach to investigate daily photoprotection ; Photodermatol Photoimmunol Photomed 2006; 22: 67-77
The solar spectrum
The solar spectrum includes several wavelengths: UV rays (UV-R, 280-400 nm), visible light (380-780 nm) and infrared rays (700-1,000,000 nm). Although the share of UV rays represents less than 7% of the total solar radiation of 280 to 2,500 nm received on the surface of the Earth, UV rays have the biggest biological impact on the skin.
The spectrum of daily UV radiation
A study by Christiaens et al. has made it possible to define the spectrum of UV radiation representing non-extreme conditions of exposure to sunlight. The values were determined on different dates and in different locations with the aid of a specific calculation model. This spectrum is appropriate and was simulated for laboratory studies of the effects of sunlight on the skin, each day and in the long term. The simulator of daily UV rays was designed in as practical a manner as possible by simply adapting common sunlight simulators. Moreover, the method of implementation of this simulator has been patented.
Christiaens F, Chardon A, Fourtanier A, Frederick J; Standard Ultraviolet Daylight for non Extreme Exposure Conditions; Photochem Photobiol. 2005, 81; 874-878