Name: Oxygen

  • Symbol: O
  • Atomic Number: 8
  • Relative Atomic Mass: 16.00
  • Category: Non-metal
  • Appearance: Clear, colourless gas at room temperature
Above: Image ©

Many organisms, including humans, need oxygen to survive. But did you know that this element can also be dangerous? Ozone, which is made up of 3 oxygen atoms, protects you from skin cancer when it’s high above the Earth’s surface. But when it gets too close, it can harm your lungs!

Diatomic oxygen

Oxygen is a diatomic element. At room temperature, an oxygen atom will always be paired with another oxygen atom. So, in nature, oxygen is found as the molecule O2.

Places where oxygen is available for breathing are called aerobic environments. The oxygen found in aerobic environments is mainly produced by a process called photosynthesis. As they produce energy for growth, plants and other organisms absorb light from the Sun and carbon dioxide from the air, releasing oxygen in the process. Between 50 and 85 per cent of the oxygen in Earth’s atmosphere comes from phytoplankton living in the ocean. 

6CO2 + 6H2O → C6H12O6 + 6O2

Carbon dioxide + Water + Light → Sugar + Oxygen

Chemical equation for photosynthesis in plants (ZooFari, Wikimedia Commons)

Helpful and dangerous ozone

Did you know? Not all photosynthesis is done by plants. In fact, cyanobacteria were responsible for producing the first oxygen in Earth's atmosphere 2.4 billion years ago.Oxygen is also found in the atmosphere in the form of a compound called ozone (O3). In the stratosphere—20 to 50 kilometres above the ground—ozone protects the Earth from harmful ultraviolet radiation. In other words, ozone is the Earth’s sunscreen! When there is a decrease in stratospheric ozone and more ultraviolet radiation reaches the surface, rates of skin cancer increase.

Unfortunately, large amounts of chlorinated hydrocarbons were used in hairspray, refrigerants, and other aerosols before the 1990s. These chemicals were broken down by the Sun to produce chlorine radicals. Radicals are highly reactive atoms that have a valence electron. Chlorine radicals react with stratospheric ozone to create O2 and chlorine monoxide (ClO). This caused a significant reduction in the amount of ozone over Antarctica, often called the “hole in the ozone layer.”

Closer to the surface, instead of protecting you from sunburns, ozone poses a threat to human health. For example, it can cause chest pain, breathing problems, and bronchitis. Ground-level ozone is most often found in cities during morning and evening rush hours. It is one of the pollutants found in smog.

Did you know? Ground-level ozone is a key ingredient of smog. Every year in Canada, smog causes thousands of people to get sick or die prematurely.However, vehicle exhaust is not the only source of ground-level ozone. For example, researchers have found concentrations in rural Utah that are similar to those found in Los Angeles during morning rush hour. It turns out that these high levels of ground-level ozone were caused by off-gassing of hydrocarbons at nearby oil and gas wells. Snow cover also played a part. Specifically, sunshine reflecting off the snow caused the hydrocarbons to break down faster than normal.

Who knew an otherwise helpful molecule could do so much damage when it’s in the wrong place! You need oxygen to breathe and ozone protects you from ultraviolet radiation, but breathing ozone is bad for your lungs. So the next time you take a breath, make sure you’re breathing O2 and not O3!

Learn more!

Websites with information on sources of oxygen used for breathing:

Websites with information on the health effects of ground-level ozone:

Ground Level Ozone: Health Effects (2014)
US Environmental Protection Agency

Smog (2014)
Environment Canada

Heidi Kavanagh


I am a teacher in Labrador. I have a Bachelor of Science (Honours) in Earth Sciences with minors in Environmental Science and French and a Masters in Environmental Science. I finished off my education with a B.Ed. for students grades 7-12.

I was a very active Let's Talk Science volunteer from the Memorial University site in St. John's, NL. Additionally, I was the rural, remote and aboriginal outreach coordinator at that site and am very passionate about delivery Let's Talk Science programs across NL. 

I am also a Science Editor with CurioCity. I love everything science!


Je travaille comme enseignante au Labrador et aussi comme réviseure scientifique pour CurioCité. J'ai un baccalauréat ès sciences avec spécialisation en sciences de la Terre et mineure en sciences de l'environnement, ainsi qu'une maitrise en sciences de l'environnement et un baccalauréat en éducation (7e à 12e année).

J'étais une bénévole très active de Parlons sciences au site de sensibilisation de l'Université Memorial, à Saint-Jean de Terre-Neuve. De plus, j'étais la coordonnatrice de sensibilisation aux communautés rurales, éloignées et autochtones. Je travaillais pour assurer la livraison des programmes de Parlons sciences partout à Terre-Neuve-et-Labrador. 

J'adore des sciences!

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