by Colin Lynch

Ozone, or trioxygen, is a very important molecule comprised of three oxygen atoms.  This differs from the “molecular oxygen" that we normally breathe which only has two oxygen atoms. Read on to learn more about how ozone can both protect us by absorbing UV rays in the upper atmosphere as well as hurt us if it is formed by pollution near the Earth's surface.

The dangers of UV radiation

The sun emits a broad range of wavelengths of electromagnetic radiation. (1) Though it's maximum emission falls in the visible light region, a large amount of ultraviolet (UV) radiation is emitted and reaches the Earth. There are many different categories or UV radiation, differentiated based of the wavelength of the rays. The longest wavelength rays, 400 – 320 nm, are classified as UVA. Shorter UVB rays have a wavelength of 320 - 290 nm, and UVC rays have an even shorter wavelength range of 290 - 240 nm. Since Planck's equation states that the energy of a wave is inversely proportional to its wavelength, rays with shorter waves contain more energy and are therefore more damaging.

UV radiation directly causes damage to the nitrogenous base pairs in DNA, most commonly producing cyclobutene pyrimidine dimers (CPDs) via a common type of chemical reaction called a "photocycloaddition." (3) If enough DNA in the skin cell becomes damaged, the cell will die which causes skin irritation and redness in a phenomenon commonly known as a sunburn. This DNA damage is also known to cause mutations in crucial genes for cell growth regulation such as the tumor suppressor protein p53, oncogene ras, and hedgehog pathway regulatory gene patched. Mutations in these genes are common hallmarks for cancer. (4)

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Ozone absorption of UV rays

Ozone helps protect our skin by absorbing the most dangerous type of UV radiation: UVC. Most of this ozone in the stratosphere, a layer in our atmosphere so high (roughly 10 – 50 km above the Earth's surface that most of the weather we experience occurs below it.5 Ozone is formed in the stratosphere via a process known as the Chapman cycle. First, very high energy UV rays (with a wavelength shorter than 240 nm) are absorbed by molecular oxygen, causing the molecule to split into two individual oxygen atoms. From there, an individual oxygen atom can react with molecular oxygen to form ozone. Once ozone is formed, there are two methods in which ozone can be converted back to molecular oxygen: in one way, UVC rays (shorter than 300 nm) can catalyze the decomposition of ozone into molecular oxygen and a singular oxygen atom. This first reaction is the basis for ozone's absorption of harmful UV rays. Secondarily, ozone can react with a singular oxygen atom to form two O2 molecules.

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Certain pollutants, known as chlorofluorocarbons (CFCs), are known to cause damage to our ozone layer. CFCs were commonly found in refrigerants due to their ability to get very cold without freezing. However, CFCs can also create radicals which help catalyze the second method for ozone decomposition, where ozone combines with a singular oxygen atom to form molecular oxygen. This in turn disrupts the delicate equilibrium of ozone in the stratosphere and hinders ozone's capability to absorb UVC rays. Widespread use of CFCs in the late 20th century caused a hole in the ozone layer over Antarctica, which will lead to UVC rays reaching the surface, where they can cause serious harm.? Thankfully, a multi-nation agreement to restrict the use of CFCs in commercial products, known as the Montreal Protocol, was enacted in 1989 and has led to repair of the ozone hole.

Ozone is great at absorbing dangerous UVC rays, but it is important to remember that ozone does not absorb all UV rays! Some UVB rays and all UVA rays still pass through the ozone layer. While these rays are lower energy than UVC, they still cause damage. In particular, UVB rays are a chief cause of sunburns.2 Remember to always wear sunscreen and limit sun exposure!

Ozone on the surface

While ozone does tremendous good in the stratosphere, it is a highly toxic molecule and can cause damage if its formed near the Earth's surface. Ozone is an efficient oxidizing agent and can cause oxidative damage to the mucous membranes that line the respiratory track, such as inside the nose or mouth. It has long been known that certain pollutants, such as hydrocarbons and nitrogen oxides, can catalyze the formation of ozone in a phenomenon known as smog. Ozone, along with the many other toxic pollutants found in smog, create a big concern for those who live in large cities full of pollutants. In order to cut down on the amount of ozone formed at the surface, it's important to limit your carbon footprint and limit the use of cars which can contribute to smog-forming pollutants.

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1. "The Sun & its Energy" via Carnegie Mellon University: http://environ.andrew.cmu.edu/m3/s2/02sun.shtml
2. "UVA & UVB" via Skin Cancer Foundation: https://www.skincancer.org/prevention/uva-and-uvb
3. D. L. Mitchell, The relative cytotoxicity of (6-4) photoproducts and cyclobutane dimers in
mammalian cells., Photochem Photobiol. 1998, 48(1), 51-57.
4. M. Ichihashi, M. Ueda, A. Budiyanto, T. Bito, M. Oka, M. Fukunaga, K. Tsuru, T. Horikawa, UV
induced skin damage, Toxicology 2003, 189, 21-39.
5. "The Stratosphere - overview" via University Corporation for Atmospheric Research: 
6. P. J. Crutzen, The influence of nitrogen oxides on the atmospheric ozone content, Quart. J. R. 
Met. Soc. 1970, 96, 320-325.
7. "Antarctic Ozone Hole: 1979 to 2008" via NASA
8. H. E. Stokinger, Ozone toxicology, Archives of Environmental Health: An International Journal. 
1965, 10(5), 719-731.
9. "Ground-level Ozone (Smog) Information" via EPA: 
https://www3.epa.gov/region 1/airquality/index.html

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Colin Lynch is currently a student in the Department of Earth and Planetary Sciences at Northwestern University