UV radiation
Ultra violet, commonly known as UV, is a type of electromagnetic radiation. Take a look at the electromagnetic spectrum:
Figure 1. Figure of the electromagnetic spectrum[1]
The shorter the wavelength, the higher the frequency and thus the higher the energy of an electromagnetic wave. As can be seen from the spectrum, the UV radiation lies between the visible spectrum and the X-rays. Hence, UV radiation cannot be detected by the naked eyes.
Solar UV radiation can be absorbed by the ozone layer; those which are not absorbed will then reach the earth’s surface.
Types of UV rays and their effects
There are three types of UV rays, namely UVA, UVB and UVC. UVC has the shortest wavelength out of the three, and has the most damaging effect on humans, followed by UVB and finally UVA.
UVA, however, is the reason for tanning of skin, being penetrable into the deeper layers of skin. In addition, about 95% of the UV radiation which reaches earth surface consists of UVA. UVA can not only cause ageing and wrinkling of skin, but can also lead to skin cancers, a finding which had been supported by recent studies.
UVB, on the otherhand, is only penetrable to the superficial skin layers. Similar to UVA, UVB can cause ageing and wrinkling of skin, and can significantly result in skin cancers development. However, most of the UVB rays are absorbed by the ozone layer and thus only a small amount will reach the earth’s surface.
UVC, having the shortest wavelength, will be the most damaging to skin. However, all of UVC rays are absorbed by the ozone layer and thus will not reach the earth’s surface.[2]
UVB will have a more damaging effect on the skin as compared to UVA as it has higher energy. UVB can also cause damage to DNA, leading to cell mutation which can later lead to skin cancers.[2]
Effects of UVB on our skin[3]
When DNA on our skin cells directly absorb UVB photons, the thymine base pairs next to each other in our DNA will bond together to form pyrimidine dimers, which will cause a disruption in the DNA strand, which reproductive enzymes cannot copy.
Due to the excellent photochemical properties of DNA, it will only be damaged by a tiny fraction of the absorbed photons (0.1%). DNA transforms more than 99.9% of the photons into harmless heat via a photochemical process called internal conversion, which is highly efficient. However, in the nucleus of the cell it is presumably much slower for the recovery of the damage done on the DNA.
How does our skin protect itself from UV?
The pigment responsible for absorbing UV rays is melanin, which is produced by melanocytes in our skin. When exposed to sunlight, melanocytes will increase its production of melanin to absorb the UV rays to prevent them from being absorbed by the cells.[4]
Figure 2. Chemical structure of melanin[5]
How do sunscreens protect our skin from UV?
Sunscreens contain inorganic and organic substances which can either absorb or reflect harmful UV rays from reaching the skin. Some inorganic chemicals including minerals such as titanium dioxide acts as a physical sunblock. They reflect UV rays, similar to how white paint reflects light. Organic chemicals in sunscreens such as oxybenzone absorb UV radiation. As the bonds in oxybenzone adsorb UV radiation, it gets broken and releases the UV radiation absorbed in the form of heat.[6]
Figure 3. Chemical structure of oxybenzone[7]
Sunscreens with SPF (Sun Protection Factor) 15 or higher protects your skin by prolonging the time taken for the effect of UVB rays to have an effect on your body. If it takes 20 minutes for unprotected skin to start turning red, using a sunscreen with SPF 15 increases the time taken to about 5 hours.
References:
1. Leader Tech, Inc. (2017). The Basics of the Electromagnetic Spectrum. Retrieved from: https://leadertechinc.com/blog/basics-electromagnetic-spectrum/
2. World Health Organization. (2017). UV Radiation. Retrieved from: http://www.who.int/uv/faq/whatisuv/en/index2.html
3. Kohler Research Group. (2001). Ultrafast Photodynamics of Nucleic Acids. http://cbc-wb01x.chemistry.ohio-state.edu/~kohler/dna.html
4. Marmur, E. Q What happens to skin when it is exposed to ultraviolet light? Retrieved from: https://www.sharecare.com/health/healthy-skin/what-happens-skin-ultraviolet-light
5. Glentham Life Sciences. (2017). GK4761 Melanin. Retrieved from: http://www.chemspider.com/Chemical-Structure.4884931.html
6. How Does Sunscreen Work? (2010). http://www.livescience.com/32666-how-does-sunscreen-work.html
7. Structure of Oxybenzone. https://upload.wikimedia.org/wikipedia/commons/thumb/9/92/Oxybenzone.svg/1200px-Oxybenzone.svg.png