Skin aging is determined not only by genetic aspects but mainly by environmental factors and the lifestyle of each individual. Solar UV-radiation, nicotine and alcohol consumption can cause free radical formation in human skin. In small concentrations free radicals are important for signaling processes, but if their value is exceeding the critical radical concentration, these highly reactive molecules are able to destroy cells or cell compartments. The human organism has developed a protection system against the destructive action of free radicals in form of the antioxidative system. Antioxidants can neutralize free radicals before they can damage the tissue or tissue components.
At the Center of Experimental and Applied Cutaneous Physiology spectroscopic methods were developed to determine the concentration of antioxidants in human skin noninvasively. Based on these measurements the action of free radicals could be estimated by the degradation of the antioxidants. Most antioxidants are not produced in the human organism. They have to be ingested by a healthy diet rich in fruits and vegetables.
The antioxidative potential of the human organism is a fingerprint of the volunteers as determined by their nutritional and stress behaviour. Stress factors like sleepiness, illness, alcohol consumption or psychological stress is reducing the concentration of antioxidants in the skin. This was demonstrated by comparing differences in the nutritional and stress behaviour of German and Korean citizens. It is well known from the literature that UV irradiation generates free radicals which are reducing the antioxidants in the skin. The same effect could be detected for infrared light, the energy of which is insufficient to produce free radicals by itself. It was found that the infrared radiation stimulates mitochondria which produce free radicals. These results were confirmed by optical spectroscopic methods and also by paramagnetic resonance spectroscopy. Based on these findings it could be demonstrated that 50% of the free radicals produced by sun radiation in human skin are formed by visible and infrared light. As people applying sunscreens with high protection on the beach are protected against sunburn, these people stay much longer in the sun than if they remained unprotected. In this case the free radicals produced in the unprotected visible and infrared spectral range of the sun radiation form amounts much higher than the critical radical concentration responsible for skin damage. Protection mechanisms of sunscreens in the visible and infrared spectral range are discussed in the presentation