What is ozone?

Ozone (O3) is a molecule made up of three oxygen atoms. It’s a highly oxidizing gas easily recognizable by its caustic odor (reminiscent of bleach) —that’s also where ozone gets its name, from the greek word “smell”. 

It’s important to distinguish the harmful, tropospheric (ground level) ozone we are discussing, from the stratospheric ozone layer that is critical to human life on earth—protecting us from harmful UV rays.

Ozone is one of the main components of photochemical smog, the brown, noxious haze that hangs over many big cities caused by a photochemical reaction between sunlight and certain pollutants.

When and how does ozone form?

Low levels of ozone in the troposphere are naturally occurring. Its natural sources are generally anthropogenic. In fact, most of the ozone we breathe is not created directly by human activity and is mostly a result of complex interactions and reactions between other pollutants. This is why ozone is considered to be a secondary pollutant.

Ozone is created first and foremost by the reaction that occurs when nitrogen dioxide and volatile organic compounds are exposed to the sun’s UV rays.

This is why high levels of ozone are much more common during the summer and around big cities—when there is sunshine and a high concentration of precursor pollutants.

Pollution can travel incredible distances with the wind, from city to city, even country to country. As pollutants travel, they can react with sunlight to create ozone. This is why rural areas can also be affected by ozone pollution. This also explains why ozone peaks are usually observed in the afternoon and later in the day, once vehicle and other emissions have reacted with UV rays from the sun.

Ozone is found in much lower levels inside. It deteriorates rapidly after coming into contact with certain indoor surfaces, forming volatile organic compounds like formaldehyde. However, certain types of air purifiers and photocopiers can create indoor ozone. 

What are the health effects of ozone?

Ozone is an irritating gas that can cause discomfort in the eyes, nose, and throat in high enough concentrations. Ozone easily penetrates the respiratory system causing coughs, shortness of breath, and other respiratory distress. Young children, elderly, and people suffering from asthma are the most at risk of suffering these health effects when exposed to ozone.