What is ozone?

Ozone (O3) is a colorless to bluish gas formed by solar ultraviolet radiation (sunlight) and oxygen molecules. The ozone molecule is made up of three oxygen atoms.¹

What is the difference between ground-level ozone and the ozone layer?

Ground-level ozone, also known as tropospheric ozone, is the main ingredient in smog.^2,3

Stratospheric ozone, sometimes called the ozone layer, occurs naturally in the upper atmosphere. Stratospheric ozone forms a protective layer over the Earth and absorbs ultraviolet rays from the sun.

90 percent of all the ozone on Earth is found in the stratosphere.

Because it is a potentially toxic air pollutant, many governments regulate ozone in an effort to reduce levels of ozone at the ground-level.

Where does ozone come from?

Ozone is a secondary pollutant. While other pollutants are emitted directly into the air by various sources, ozone is created by sunlight acting on nitrogen oxides and volatile organic compounds (VOCs) in the atmosphere.

When these pollutants react with sunlight, they create ground-level ozone.
Sources of nitrogen oxides include:⁴

• motor vehicles
• power plants
• combustion sources
• lightning strikes
• volcanoes
• oceans
• biological decay

Sources of VOCs include:⁵

• motor vehicles
• wood burning
• industrial emissions
• chemical plants
• refineries
• cleaners and disinfectants
• paints, paint strippers, varnishes, and finishes
• tobacco smoke
• forest fires

How does geography impact ozone and smog?

Ozone pollution is very common in heavily-populated basin cities surrounded by mountains, such as Los Angeles, California and Mexico City, Mexico. Smog becomes trapped in urban valleys and isn’t easily dispersed by wind.⁶

Cities like Stockton, Fresno, Bakersfield, and Visalia in California’s San Joaquin Valley experience high levels of ozone through hot dry summers. The heavily agricultural area produces nitrogen oxides emissions from the soil that can lead to smog.⁷

Heat and weak winds encourage the creation of smog, as pollutants like nitrogen oxides and VOCs react with sunlight to form ground-level ozone.

Analysis of a statistical model in 2016 published in Geophysical Research Letters found that increasing summer heat waves will likely lead to more days of unhealthy ozone levels by 2050.⁸ The model, which projected ozone increases in the United States, indicated that more days of unhealthy ozone were likely to occur in the Southwest and Northeast.

What are ozone’s commercial uses?

Ozone has several industrial uses. These include:^9,10

• purifying air and drinking water
• industrial waste treatment
• disinfecting agent
• oil use
• bleaching and waxes
• creation of other chemicals
• food hygiene
• increasing fruit and vegetable shelf-life

Industrial workers in paper and pulp mills, wastewater treatment plants, fisheries, and in outdoor areas with high levels of ozone can face health risks from ozone exposure.

How does ozone affect your health?

Ground-level ozone is a respiratory irritant and can be harmful to your health.¹¹ Older adults, children, infants, people with asthma or lung diseases, and people who exercise outdoors may be more vulnerable to the effects of ozone exposure.

Ozone can cause several short-term symptoms, including:
• shortness of breath
• pain during deep breaths
• lung and throat irritation
• wheezing and coughing

More serious ozone exposure effects can include: ¹²

• increased asthma attacks
• reduced lung function
• continued damage to lungs after symptoms have subsided
• causing chronic obstructive pulmonary disease

People with bronchitis, asthma, or emphysema may experience worse symptoms.

A 2020 study of ambient ozone pollution published in Environment International found that short-term exposure to ozone resulted in increased hospital admissions for pneumonia.¹³ The study indicated that the connection between pneumonia and ozone was particularly strong in the elderly and among smokers.

A 2008 study published in Environmental Health Perspectives determined that young children in New York State who were frequently exposed to high ozone levels were at greater risk of being admitted to the hospital for asthma than children with less exposure.¹⁴

A second study published in Environmental Health Perspectives in 2008 determined that ozone in Southern California also contributed to an increase in asthma hospitalizations among children.¹⁵

What are the environmental impacts of ozone?

Ozone can have a profound impact on sensitive vegetation. These impacts include: ¹⁶

• interference in plants’ ability to produce and store food
• physical damage
• slowed growth
• reduced habitat quality
• water and nutrient cycle changes
• increased susceptibility of sensitive plant species to disease, damage from insects, the effects of other pollutants, and harm from severe weather

A number of tree species are sensitive to ozone and may experience injury or stunted growth, including:^17,18

• black cherry
• quaking aspen
• ponderosa, loblolly, and white pine
• red alder
• yellow and tulip poplar

Ozone is difficult to manage due to its pervasiveness, the complexity of ozone chemistry, and the challenges involved in determining how much ozone contributes to pollution.¹⁹

Cities around the globe are challenged by air pollution despite government intervention in reducing pollutants. Poor air quality harms humanity and creates a terrible strain on economies. Visit our Cost of Air Pollution counter and find out how clean air can support a better quality of life.