Use of Ozone Against Odors

Ozone is most effective against Volatile Organic Compounds (VOCs) that contain unsaturated carbon-carbon bonds, such as alkenes. It is generally not effective against saturated hydrocarbons like alkanes and can even react with certain VOCs to produce harmful byproducts like formaldehyde. 

Ozone is a strong oxidant that can break down VOCs with certain chemical structures. Research has shown its effectiveness on:

• Alkenes and unsaturated VOCs: This category includes compounds with double or triple carbon bonds, which are easily broken by ozone. Examples include:

  • Limonene: Found in air fresheners.

  • Terpenes: Present in some cleaning agents and essential oils.

  • Cooking oils: The unsaturated VOCs emitted during cooking.

  • Acrolein: An irritant found in secondhand tobacco smoke.

• Aldehydes and ketones: While ozone can react with these oxygenated VOCs, the reaction can sometimes increase their concentration, as they can also be created as byproducts. 

Ozone is not a suitable solution for many common VOCs, and its use can be counterproductive:

• Saturated hydrocarbons (alkanes): These compounds, which include many common VOCs, have no double bonds for the ozone to react with. This means ozone has little to no impact on their concentration.

• Benzene series: These VOCs have a stable ring structure that is not affected by ozone.

• Urea: A key component of pet urine odor, urea will not oxidize with ozone.

• Many common indoor VOCs: In most indoor environments, ozone generators are inefficient at removing a broad range of VOCs. The reaction rates are often very slow, and high concentrations of ozone would be needed to achieve results, which is unsafe for humans. 

The Dangers of Using Ozone to Remove VOCs

Health and safety agencies, such as the U.S. Environmental Protection Agency (EPA), advise against using ozone generators for air purification due to several key dangers: 

• Creation of harmful byproducts: When ozone reacts with unsaturated VOCs, it can produce dangerous intermediates and secondary pollutants, including highly toxic fine particulates and formaldehyde.

• Ineffectiveness against many VOCs: Because ozone only works on certain types of VOCs, it fails to address a wide range of pollutants, such as saturated hydrocarbons.

• Respiratory health risks: Inhaling ozone is a health risk and can cause or worsen respiratory conditions like asthma. Any concentration of ozone high enough to effectively react with many VOCs is also high enough to be unsafe for humans. 

What byproducts are formed when ozone reacts with VOCs?

When ozone reacts with volatile organic compounds (VOCs), particularly those containing unsaturated bonds, it does not simply eliminate them but generates a variety of harmful byproducts. These include aldehydes, organic acids, and fine or ultrafine particles known as Secondary Organic Aerosols (SOA). 

Harmful byproducts

• Aldehydes and ketones: Ozone reactions often produce small, irritating carbonyl compounds like formaldehyde and acetaldehyde.

  • Formaldehyde: A known human carcinogen, formaldehyde can be produced in significant quantities during ozone-VOC reactions, especially when terpenes from essential oils or consumer products are involved.

  • Acrolein: A byproduct associated with ozone reactions, acrolein is a strong irritant that can affect the eyes and airways.

• Organic acids: Compounds like formic and acetic acid can be formed and may contribute to health issues.

• Secondary Organic Aerosols (SOA): These are fine and ultrafine particles created when oxidized VOCs condense into the aerosol phase.

  • Formation: The chemical process is complex, but key reactants are oxygenated VOCs produced by the initial ozone reaction.

  • Health effects: Because of their small size, these particles can easily penetrate the respiratory system and are associated with heart and lung problems.

• Other irritants: Various hydroperoxides and other irritants can be generated during these chemical reactions, further degrading indoor air quality. 

Specific examples of reactions

The byproducts formed depend on the specific VOC that reacts with ozone.

• Terpenes and terpenoids: Many common household products like air fresheners and cleaning agents contain terpenes (e.g., limonene), which react rapidly with ozone to create a cocktail of pollutants.

  • An ozone-limonene reaction can produce significant amounts of formaldehyde, as well as irritating particles.

• Surface reactions: Reactions can also occur on surfaces like clothing and carpets. For example, ozone can react with squalene in human skin oils, producing a number of byproducts, including acetone and 6-methyl-5-hepten-2-one. 

The net result

These reactions mean that using ozone to "clean" the air is not only ineffective against many common VOCs, but it actively replaces relatively less harmful VOCs with more dangerous and irritating ones. Government health and environmental agencies strongly advise against using ozone-generating air purifiers for this reason.