Ozone can be both helpful and harmful, depending on where it is found in the atmosphere. 

What Is Ozone?

Ozone molecules are made of oxygen atoms, but they are different than the oxygen molecules in the air we breathe.

The oxygen molecules in our atmosphere are each made of two oxygen atoms. Scientists use the symbol O2 to indicate this oxygen compound. O2 makes up approximately 21% of the atmosphere we breathe. A single atom of oxygen is unstable - it wants to combine with something else. That is why oxygen is almost always found in pairs as O2.

Ozone is made of three oxygen atoms and indicated with the symbol O3. It is not stable and is prone to losing an oxygen atom, becoming an oxygen molecule (O2).

Ozone was discovered in 1839 by professor Christian Friedrich Schoenbein at the University of Basel, Switzerland. The word ozone comes from the Greek word ozein, meaning "to smell." Ozone has a pungent odor.

Ozone in the stratophere's ozone layer blocks harmful ultraviolet radiation from the Sun.

Where Is Ozone Found in the Atmosphere?

Ozone is both good news and bad news! Ozone in the stratosphere protects us from the Sun's harmful ultraviolet rays. However, ozone in the troposphere, closer to Earth’s surface, is a pollutant and hazardous to our health.

In the stratosphere, the concentration of ozone about 25 km (15 miles) above the Earth's surface is usually less than ten parts per million (ppm). This means that for every million molecules of air, only ten molecules are ozone. Even though ozone exists in minute quantities, it is vital for life on Earth. The ozone layer filters out ultraviolet radiation from the Sun, while allowing other wavelengths of light to travel through the atmosphere.

Ozone the troposphere occurs naturally in very small amounts - about 10 to 30 parts per billion (ppb). This means that for every billion molecules of air, only 10 to 30 molecules are natural ozone. However, most of the ozone in the troposphere is from pollution. Complex chemical reactions between sunlight and pollutants emitted by motor vehicles and industry create ozone in the troposphere.

The haze from pollution in Los Angeles looks orange at sunset.

Tropospheric ozone and other air pollutants are common in urban areas in late afternoon.
Credit: UCAR

According to the U.S. Environmental Protection Agency, ozone levels over 70 ppb pose a high danger to human health, although no amount of ozone is safe to breathe. Breathing ozone into your lungs causes damage to the lung tissue and narrows the passages that let air in and out of the lungs. Plants are also damaged by ozone in the troposphere and items such as rubber and nylon break down in the presence of ozone. In the troposphere, ozone also acts as a potent greenhouse gas.

 - Learn more about ozone chemistry in our atmospheric chemistry section. 

 - Learn more about ozone in the troposphere

 - Reports about our Boulder ozone garden are in the UCAR SciEd blog.

 - Teaching about ozone pollution? Check out the ozone education resources in the Air Quality Teaching Box.