Ozone is a highly reactive gas that may be generated inside a building by electrical or ultraviolet equipment rather than released from a conventional liquid or solid product. Its significance indoors lies not only in its own concentration, but also in the reactions it causes with chemicals already present in the air.
Ozone contains three oxygen atoms and is much less stable than the oxygen normally present in air. That instability makes it a powerful oxidising agent. Indoors, it can react quickly with furnishings, finishes, dust, skin oils and airborne organic compounds, so its concentration may change rapidly as sources operate and reactive surfaces remove it.
A measured concentration represents the balance between generation, air exchange, chemical reaction and surface loss. A device can emit ozone continuously while a nearby instrument records only a moderate level because the gas is being consumed. That does not make the source insignificant, because reaction products may remain after much of the original ozone has disappeared.
Ozone can irritate the eyes and respiratory tract. The relevance of a result depends on concentration, duration, occupancy pattern and individual susceptibility. A person experiencing persistent respiratory or eye symptoms should seek advice from a doctor or occupational health practitioner rather than relying on an indoor-air measurement as a diagnosis.
Photocopiers and laser printers use high-voltage electrical processes to charge drums, transfer toner and control paper handling. Electrical discharge within these systems can convert some oxygen into ozone. Output varies with machine design, condition, duty cycle, maintenance and the effectiveness of internal ozone-removal components.
Ionisers and electrostatic air-cleaning devices are another important source category. These products charge particles so that they settle onto surfaces or are collected on plates. The same electrical field can generate ozone, either as an intended feature or as an unwanted by-product. The label "air purifier" therefore does not establish that a device reduces chemical contamination; its operating principle must be examined.
Some ultraviolet equipment can also generate ozone where the lamp spectrum includes ozone-forming wavelengths. This may occur in certain disinfection units, specialist treatment devices or poorly specified products. Ultraviolet equipment should not be assumed to be ozone-free merely because its primary purpose is microbial control or surface treatment.
Outdoor ozone can also enter a building through its ventilation air.
Terpenes are a broad family of volatile organic compounds used in fragranced cleaning products, air fresheners, perfumes and scented consumer products. Common fragrance ingredients can react readily with ozone, particularly where a product has recently been sprayed or applied over a large surface.
The reaction does not simply remove ozone. It breaks organic molecules into smaller and often more oxidised compounds. Formaldehyde can be produced, along with other carbonyls, organic acids, peroxides and substances not present in the original product. The exact mixture depends on the terpene, ozone level, surrounding surfaces, temperature and other gases.
Ozone-terpene chemistry can also create ultrafine particles through secondary organic aerosol formation. These particles form in the air from reaction products that condense or combine, rather than being emitted directly as dust. A room can therefore show a particle increase after fragranced cleaning even when no visible aerosol remains.
This chemistry explains why testing only for the labelled ingredients of a cleaning product may miss part of the chemical mixture. The fragrance, ozone source and reaction products form a connected system, and each needs to be considered when interpreting the room.
A device that intentionally generates ozone cannot be treated as a neutral control measure. It adds a reactive chemical to occupied air. Even where a manufacturer describes the process as oxidation, deodorisation or molecular destruction, the immediate action is the release or formation of ozone within the treated space.
Claims that ozone "removes odours" can also mislead. Some odorous compounds may be altered, but the reactions can create new compounds with their own odours and toxicological properties. A reduction in one smell is not evidence that the air has become chemically cleaner, and odour transformation is not equivalent to contaminant removal.
The distinction is particularly important in spaces using fragranced products. An ozone generator placed near cleaning chemicals, air fresheners or scented furnishings may accelerate the production of formaldehyde and secondary particles. The device is then both a source and a driver of a wider reaction mixture.
A genuine control should reduce contaminant generation, concentration or persistence without introducing a new reactive pollutant. On that basis, an ozone-generating "air purifier" is a contaminant source rather than an indoor-air control. An assessment should identify the device, its operating schedule and the products used around it before interpreting measurements.
Ozone measurement requires attention to timing because many indoor sources are intermittent. A printer room may show its highest level during sustained printing, while an ioniser may operate continuously or only when a room is unoccupied. A short reading taken while equipment is idle may not represent normal use.
Location also matters. A measurement close to a discharge point can identify source strength, while an occupied-zone measurement better represents the concentration across the room. Reactive loss on walls, furniture and building components can create sharp differences over short distances. The instrument, sampling duration and possible interference from other oxidising gases should be recorded.
Interpretation should also consider secondary pollutants. Low ozone during sampling does not rule out significant ozone chemistry if the gas has already reacted with terpenes or surfaces. Where the circumstances suggest this process, formaldehyde, selected volatile organic compounds and particle behaviour may need to be considered alongside ozone.
A spot reading is therefore most useful as part of a source-led investigation. Equipment inventories, operating times, cleaning schedules and fragrance use can explain patterns that a concentration alone cannot. This helps distinguish an active ozone source from a residual chemical mixture formed earlier.
For existing buildings, Table 2 of Dubai Municipality's Technical Guidelines for Indoor Air Quality for Healthy Life, document reference DM-HSD-GU119-IAQ, gives an ozone value of 0.06 ppm, equal to 100 micrograms per cubic metre, over eight hours, together with a ceiling of 800 micrograms per cubic metre. Table 2 is expressed in mandatory terms within Dubai Municipality's technical guideline; the guideline is not statutory law, and this existing-building route is not voluntary.
No one-hour Dubai Municipality ozone value should be stated. The relevant cell in the published source is defective, so it does not provide a reliable permitted figure. Substituting a value from another organisation or reconstructing one from an apparent conversion would create a number that the guideline itself does not clearly establish.
Al Sa'fat, the Dubai Green Building System, addresses listed categories of existing buildings in section 401.07. It states that the buildings must apply the stated procedures and gives ozone below 0.06 ppm, equal to 120 micrograms per cubic metre, as an 8-hour time-weighted average. Al Sa'fat is a building-code and rating instrument rather than occupational-health law, and the figure belongs to that specific existing-building provision.
The two instruments should not be blended into a generic Dubai limit. Although both use 0.06 ppm, their stated mass concentrations differ, and they sit within documents with different scopes and functions. A report should name the instrument being applied, reproduce its status accurately and avoid presenting either figure as a universal statutory indoor-air limit for every building in the UAE.
The first control step is to remove or disable unnecessary ozone-generating equipment. Ionisers, electrostatic devices and ultraviolet units should be checked for their operating principle and manufacturer information rather than accepted on the basis of an air-cleaning claim. Printing and copying equipment should be maintained and located with attention to emission potential and room use.
Source control should be paired with control of reactive co-pollutants. Heavy use of fragranced sprays, air fresheners and terpene-rich cleaning products increases the material available for secondary chemistry when ozone is present. Substitution with less reactive and less heavily fragranced products can reduce that mixture.
Testing after intervention should represent actual operation. A low result obtained with the source switched off does not demonstrate control during use. Ozone may fall after a source is removed while formaldehyde or secondary particles persist because reaction products and surface reservoirs remain.
The broader subjects of indoor air quality testing and monitoring are covered on the mould and indoor air quality resource; an ozone investigation on this resource remains focused on chemical sources, reaction pathways and the interpretation of ozone-related contaminants.
Ozone indoors is normally generated by equipment brought into a finished building rather than by the fabric, so the Dubai Municipality figure quoted here is usually applied after an installation rather than at handover; which of that guideline's clauses are mandatory is explained in Regulation of indoor chemical contaminants in the UAE.
Dubai Municipality DM-HSD-GU119-IAQ Version 4 (11 December 2024) — clause 9-8-3 figures apply to a voluntary certificate route; clause 9-8-4 is worded as mandatory within the guideline
Not necessarily. Emissions vary with electrical design, age, condition, workload and internal control components. A small printer used occasionally may have little effect on a large room, while sustained operation of several machines in an enclosed print area may produce a clearer pattern. The assessment should be based on the actual equipment and its use.
Odour is not a dependable test. Ozone may be present below the level at which a person recognises its smell, and adaptation can reduce perception during continued exposure. Reaction with room surfaces and fragrances can also remove ozone while generating other compounds, so the absence of a noticeable smell does not establish that the device is chemically inactive.
Not always. Ozone is highly reactive and may be consumed before measurement. A low result can occur after it has formed formaldehyde, other oxidised compounds and ultrafine particles. Timing, equipment operation and the presence of terpene-containing products should be reviewed alongside the result.
Ozone may change some odorous chemicals, but it can also generate new contaminants and leave the cause unresolved. Its use in an occupied building should not be treated as routine air cleaning. Persistent odours are better investigated by identifying the source and chemical pathway, while symptomatic occupants should seek advice from a doctor or occupational health practitioner.