Do air purifiers produce ozone, and does it matter?

Key facts

• Ozone can cause temporary decreases in lung capacity of 15 to over 20 percent in some healthy adults.

  15–20 % Established

  Technical guidelines for indoor air quality, Dubai, version 4, Appendix 2 health effects of major air contaminants.

• No United States federal agency has approved ozone-generating devices for use in occupied spaces.

  Established Time-bound claim

  US Environmental Protection Agency, Ozone Generators that are Sold as Air Cleaners.

• Ionisers and electrostatic precipitators emit ozone as a by-product of their design rather than as an intended output.

  Established

  California Air Resources Board, Hazardous Ozone-Generating Air Purifiers.

• California has capped ozone emissions from indoor air cleaning devices at 0.05 parts per million since a 2007 regulation.

  0.05 ppm Established Time-bound claim

  California Air Resources Board air cleaner certification regulation, Assembly Bill 2276.

Which air purifiers produce ozone?

Air purifiers that produce ozone fall into two groups. The first is devices that generate it intentionally and describe it as the cleaning mechanism, often using marketing terms such as activated oxygen, super oxygenated or energised oxygen. The second, and much larger, group produces it incidentally: ionisers, electrostatic precipitators, plasma and bipolar ionisation devices all use electrical charging, and that charging splits oxygen molecules as a side effect.

The regulatory position distinguishes the two but does not exempt either. The California Air Resources Board describes ionisers and electrostatic precipitators as devices that emit ozone as a by-product of their design and function, and its certification regime covers them alongside intentional generators.

Mechanical filtration produces no ozone. A high-efficiency particulate filter removes particles by physical capture, and activated carbon removes gases and odours by adsorption. Neither adds anything to the air.

What does ozone do to the airway?

Ozone is a strong oxidant, and its effects on the airway follow from that. Technical guidance for indoor air quality lists the acute effects as irritation of the eyes, respiratory tract and mucous membranes; aggravation of chronic respiratory disease including asthma; temporary decreases in lung capacity of 15 to over 20 percent in some healthy adults; inflammation of lung tissue; and impairment of the body's immune defences, increasing susceptibility to respiratory illness including bronchitis and pneumonia.

That list is notable for being drawn from a regulatory guidance document rather than from advocacy material, and for describing effects in healthy adults rather than only in sensitive groups.

The Environmental Protection Agency adds a point about efficacy: at concentrations that do not exceed public health standards, ozone is not effective at removing many odour-causing chemicals. A device operating at a safe concentration is therefore also operating below the concentration at which it would do the thing it claims.

Why does ozone rank low in the index if its effects are severe?

Ozone carries the lowest weight of any parameter in the indoor index used in Dubai, at 0.04, despite having some of the sharpest documented health effects. The reason is that index weights combine severity with prevalence and measurability rather than expressing severity alone.

Ozone is assumed to be uncommon indoors. In a building with no photocopiers, no arcing equipment and no electronic air cleaners, that assumption holds. The assumption fails specifically in homes where an occupant has bought a device that produces it — which is the situation this page describes.

How should ozone be measured indoors?

Ozone is difficult to measure meaningfully because it is short-lived. It reacts with surfaces, textiles, skin and other pollutants, and decays substantially within minutes of being produced. The consequence is that concentration is strongly dependent on distance from the source.

A measurement taken beside an operating ioniser and one taken across the same room will differ considerably, and neither is wrong. A single spot reading, taken at an arbitrary location and time, is therefore a poor test of whether a device is contributing ozone. The measurement has to be made close to the source while the source is operating, and ideally as a paired reading at two distances.

This is also why the absence of a detection on a walk-through survey should not be read as an absence of the problem.

Definitions

Electrostatic precipitator

An air cleaner that gives particles an electrical charge so they are attracted onto oppositely charged collector plates. The charging step splits oxygen molecules as a side effect, so the device emits ozone as a by-product.

Bipolar ionisation

A technique that releases positive and negative ions into the airstream to make airborne particles clump and settle. Like other charging methods it can generate ozone, and units marketed as ozone-free are certified at installation rather than across service life.

What we are sure of, and what we are not