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1) Ozone is enriched oxygen and is nature’s own powerful generator. The many possibilities for using ozone are due to its oxidizing and germicidal properties.
2) Ozone kills micro-organisms by breaking down their protein structure. It neutralizes bacteria, viruses, moulds, fungi, yeasts, mildew, amoebae and algae, including pathogenic and non-pathogenic germs.
3) Ozone also deodorises many organic and inorganic odours, both gases and small particulate. It does this by a process of oxidation, permanently converting the odour into water vapour, and other compounds such as carbon dioxide.
4) Ozone has a very high Redox potential, being 1.5 times more reactive as an oxidizer than chlorine. It also reacts 3500 times faster than chlorine with organic material such as bacteria and viruses.
5) Ozone leaves no chemical residue as ozone rapidly decomposes back into oxygen. Since it is generated on site, the safety problems associated with liquid disinfectant storage, handling and transportation are nullified.
6) In our industrialised society we generally spend 21 out of 24 hours indoors. Each day we breathe around 20,000 times and fill our lungs with more than 10,000 litres of indoor air. This air quality is much more polluted than outside air – 30 times more for many offices and up to 1,000 times more for factories.

Indoor Air Quality

  • Unhealthy air in air-conditioned buildings and homes is an international issue that continues to worsen. Governments recognize that certain air contaminants cause wide spread discomfort, absenteeism and reduced productivity. Airborne micro-organisms have been demonstrated to be the leading cause of problems such as BRI (Building Related Illness) and SBS (Sick Building Syndrome).

  • Duct systems are not only a perfect conduit for the spread of micro-organisms, but they are also its source. Their environments are especially conducive to increasing bacteria moulds. Because they recirculate large volumes of indoor air, they can transport “system generated” micro-organisms, from room to room and therefore from person to person.

  • In the majority of cases, the effect of filtration is reduced because the proliferation of micro-organisms occurs downstream of the filters, allowing micro-organisms to travel through space before returning to the HVAC system. Also viruses anapaest bacteria are too small to be captured by the average filter. A filters, whilst alleviating IAQ problems, do not eliminate it. This is the case even if they are placed in the ceiling register, which supplies air to the space – they capture dust and some bacteria – but viruses and certain bacteria will still pass straight through them. Even the bacteria, captured on the upstream side of the HEPA filter, are not necessarily killed. Filters are ideal breeding grounds for micro-organisms and excessive growth can cause breakthrough of the filter media.

Ozone and Ventilation Ducts – Overview

Ozone gas is widely used to treat ventilation ducts using two methods: Periodic Duct cleaning and Continuous Duct dosing. It is ideal for air conditioning ducts and heating ducts, found throughout residential, commercial and industrial buildings.

• Periodic Duct Cleaning:

Ozone gas is widely used for duct cleaning and disinfecting, where it is particularly effective against mould, mildew, odours, bacteria and legionnaires disease. This is known as Periodic Duct Cleaning because it is undertaken by duct cleaning contractors whilst the building is unoccupied (at concentrations of 1 to 5 PPM). Duct cleaning may be carried out every 1 to 3 years, as required.

• Continuous Duct Dosing:

Ozone is also used for duct dosing. Ozone generators are installed as an integral part of the duct on a permanent basis. By gently injecting the correct ozone dosage, they destroy micro-organisms and odours, both in the duct system as well as in the space/room served by the duct system. This paper discusses Duct Dosing in more detail.

• Ozone has gained a rapid reputation for improving Indoor Air Quality and preventing “Building Related Illnesses”.
• Ozone Duct Dosing is suitable for residential and office buildings, hotels, clubs, hospitals, animal holding facilities, etc. It is also ideal for air conditioned food storage and preparation areas where it not only improves air quality, but can also extend the shelf life of fresh food.
• Dosing the air conditioning system will reduce odours and harmful micro-organisms in the space being ventilated, as well as preventing micro-organisms from growing and multiplying within the ductwork itself.

Application for Duct Dosing

1) Ozone Air Generators can be used to treat air and surfaces in all types of duct systems:

a) Refrigerated air conditioning units
b) Evaporated air conditioning units
c) Heating systems, fan and filter systems

2) Ozone is suitable for residential, commercial and industrial systems.

Location Guidelines for Duct Dosing

The three main guidelines for location of the Ozone Air Generators in a duct system area:

  • Each Air Handling Unit requires a separate Ozone Generator installation
    The Ozone Generator should be located just downstream of the Air Handling Unit components (filter, fan, dryer / humidifier / dehumidifier, coils / chiller / condenser). This ensures that the mild corrosion that low concentrations of ozone can cause does not affect these components.

  • The Ozone Generator should be located upstream in the supply air duct, so that as much supply duct length as possible is sterilized. The ideal position is upstream of the Plenum Chamber (and therefore upstream of the commencement of duct branches).

  • Do not locate in the Plenum Chamber itself or too close to it or the ozone will not disperse evenly through the Plenum into the branches.

  • The ideal air feedstock to the Ozone Generator should be filtered, cool and dry. To achieve this, best practice requires the use of a bypass hose, in some instances. Please note the use of a bypass hose is not essential. Good results will still be achieved by simply placing the Generator downstream of all air handling unit components.


If the Return Air is generally hot/humid, then the coils will be cooling/drying the air. The Generator is then placed in the duct, downstream of the coils, so that the air feedstock is “filtered, cool and dry”.
Alternatively the generator can be placed so that it is outside the duct. This improves maintenance access.
If the Return Air is generally cold/dry, then the coils will be heating/humidifying the air. The Generator is then placed in a position that the inlet for air feedstock is before the coils and therefore is relatively “cool and dry”. Note however that the inlet for air feedstock must be on the positive pressure side of the fan.


If the Return Air is generally hot/humid, then the coils will be cooling and the dryer will be drying the air.
If the Return Air is generally cold/dry, then the coils will be heating and the dryer/humidifier will be humidifying. The Generator is then placed in a position so as to ensure that the inlet for air feedstock is on the positive pressure side of the fan.


In such a system the return air is generally hot/dry. Then the Evaporative Cooler is cooling/humidifying.

• In some instances, the components of the air-handling unit may have the fan on the downstream side. Note if the inlet and outlet of the Generator “straddle” the fan, then air feedstock can flow backwards. To overcome this a centrifugal in-line booster fan is required, connected to the upstream side of the Ozone Generator. However, as already stated the Generator can alternatively be positioned downstream of the fan, thus avoiding the need for bypasses of boosters.
• The Ozone Generators are installed by simply fastening them to the walls of the duct (preferably the outside wall). It is useful to wire the Generator’s electrical connection to the fan circuit. If the fan is a single speed type, then the electrical circuit can be connected directly to the fan motor circuit. If the fan is 2 or 3 speed type, then a current sensing relay may be required. Ozone resistant hose is available if required.
• In smaller duct systems the Generator can be located as described above, or alternatively simply within the duct mains or each duct branch feeding into the space, or within the ceiling space with hose feeding ozone into the duct.

Sizing Guidelines for Duct Dosing

1) To calculate the necessary ozone output to achieve the desired concentration level, the first step requires the following information from the customer:

a) Layout of the space (number of rooms etc)
b) Number of air conditioning units
c) Total volume of the space or room (including the ceiling Plenum, if relevant) in m3
d) Total recirculation flow rate in m3 / hr or min
e) Total make up air flow rate in m3 / hr or min

2) The second step involves calculation of Ozone outputs, based on ideal world conditions and selected ozone half life (the rate of ozone degradation) .The following real world conditions affect the amount of ozone needed:

a) Air pollution level in the space
b) Proportion or room filled with furnishings, equipment, etc
c) Temperature, pressure and humidity in the space
d) Mixing effect of ozone in the air in the space
e) Ozone half life adjustments

Ozone Concentrations in General

• In general most countries recommend 0.1ppm as the concentration limit for ozone in the air for an occupational work environment. Some countries also recommend 0.04 or 0.05ppm as the concentration limit for air-conditioned commercial or residential environments.

In the USA for example:

  • ASHA recommends 0.3ppm (STEL) for short term 15 minute exposure for occupational workplaces.

  • OSHA recommends 0.1ppm (PEL-TWA) averaged over 8 hours per day per week for occupational workplaces.

  • ACGIH recommends 0.1ppm (ceiling) for occupational workplaces.

  • ASHRAE recommends 0.05ppm for air conditioned and ventilated spaces.

  • CSA recommend 0.04ppm for devices for household use.

For example, in Japan, Australia, Germany, France and Sweden, the concentration levels limit (variously defined) for industrial, public or occupied spaces is 0.1ppm.

Ozone Concentration for Duct Dosing

  • For the application of Duct Dosing in offices, homes, commercial premises and permanent occupied spaces, a concentration of between 0.02 and 0.05ppm (ozone in air) is recommended, measured in the space which people occupy. For duct dosing in industry and occupied work premises a concentration of between 0.02 and 0.1ppm (ozone in the air) can be used, measured in the space which people occupy.

  • Concentrations in the duct itself need to be above the 0.04ppm and 0.1ppm figures described above. The concentration in the duct dilutes and mixes as it enters the space. The required concentration needs to be achieved in the space, which is served by the duct system, in the vicinity of the breathing zone of the people in the space.

Concentrations of 0.04ppm typically destroy more than 90% of micro-organisms such as bacteria, fungi and mould. Studies show excellent micro-organisms reduction (96%) with concentrations as low as 0.02ppm.

Passive and Active Duct Dosing

  • Passive Duct Dosing involves achieving low level ozone concentrations in the space as already described (0.02 to 0.04ppm for offices and 0.02 to 0.1ppm for industry).

  • Active Duct Dosing involves using high concentrations (for example 1ppm) during hours when the space is unoccupied. An example might be active dosing of a fish market at 1ppm during night hours between 10pm and 2am, passive dosing at 0.04ppm for the rest of the time. Because ozone has a short half-life, it reverts to oxygen quickly. Therefore the higher concentrations achieved during the active dosing phase quickly reduce when the passive dosing phase commences.

  • Combined active and passive dosing can be effective in areas where professional disinfection is needed once a day, with continuous deodorization and maintained disinfection at other times.

Control Methods for Duct Dosing

  • Our Ozone Sensors can be used in conjunction with our Ozone generators to automatically maintain a predetermined concentration level, e.g. 0.04ppm or 0.1ppm. The controller acts like a thermostat to turn the generator on and off automatically. The sensor is located in the centre of the space, or at several points within a large space.

  • A hand-held ozone Monitor can be used for spot checks of ozone concentration levels.

Ozone versus Ultra Violet Lamps

• Some companies offer a continuous duct disinfecting system, which use ultra violet lamps. This is sometimes known as UVG (Ultra Violet Germicidal Irradication) or Uvc. The problem with this system is that it reduces pathogens only in the immediate treatment area therefore only in the area which has “direct line of sight” with the tubes. This is because ultra violet disinfecting lamps operate on the basis of direct exposure to the light source.
• Ozone however treats not only the local area, but also the entire lining of the duct system. It disinfects both the air in the duct and space as well as surfaces of the ducts and space.
• The ultra violet lamps do not kill micro organisms effectively if air speeds are too high or the air is too cool. This is the situation in air conditioning systems.
• Ultra violet lamps are notoriously maintenance intensive requiring constant cleaning (dust on the tubes reduces their effectiveness dramatically).
• Ultra violet lamps have a limited life span, 1 year; replacement tubes are prohibitively expensive and break easily.
• The low levels of ozone output attainable from UV allow for only small or light applications.

Duct Dosing versus “In Space Generators”

1) Ozone generators are available, which can be mounted in the space itself – to the wall, to the ceiling space and protruding into the space, or used as a portable unit.
2) Duct dosing however is preferable to mounting in the space itself – for the following reasons:
a) The ozone deodorizes and disinfects the duct and space, not just the space.
b) The ozone is dispersed into the space relatively evenly. This is based on the assumption that most air conditioning duct systems are well designed and balanced, to spread the conditioned air (cooled or heated) throughout the whole space. By injecting ozone into the same system, ozone is carried with the air and distributed evenly.
c) The ozone generator is hidden from view. Therefore when a duct system exists, duct dosing should be used as the method of choice.

Ozone Dosing versus Sanitizing Liquids

1) Some companies offer sanitising liquids, which are injected into the duct, as either large mist droplets or fine aerosols. They can be injected periodically or in some cases, continuously.

2) Sanitizing liquids have the following disadvantages:

a) They “wet” the inside of the duct. This in turn attracts and holds dust and therefore the dust builds up and lines the duct. Eventually the dust dries up and is re-entrained into the airstream, supplying a steady stream of dust into the conditioned air and into the space.
b) Because of the “wetting” effect, when dust builds up on the liquid the anti-microbial action of the liquid is impaired, as it is insulated from the airstream dust.
c) Even in dustless conditions, the anti-microbial effect is eventually “used up” and is not continuous.
d) If the liquid is applied periodically then shortly after the application its concentration in the duct is high. During this early stage, the air can entrain the liquid and deposit it in the space at reasonably high concentrations, as a mist or solvent. This is unacceptable for people working in the space.
e) The liquid clings to duct walls. Thus it does not sterilize those micro-organisms which travel down the centre line of the duct.

3) Ozone duct dosing however, does not “wet” the duct, does not attract or release dust, is continuous and steady state (assuming maintenance and control), kills micro-organisms both on duct surfaces and in the airstream, and leaves no chemical residue as it returns to oxygen after a short period.


  • Ozone gas is a very effective disinfectant / deodorant for ventilation systems.

  • As it leaves no chemical residue, it is ideal for continuous dosing of air conditioning systems in occupied buildings, where it provides the following benefits:

1) Cleaning duct systems

2) Cleaner space/rooms served by the duct system

3) Less incidence of illnesses transmitted by airborne pathogenic bacteria and viruses

4) Reduces build up of mould on ventilation system filters and on condenser coils

5) Slower food spoilage rates, less food poisoning and increased shelf life of fresh foods

6) Reduced indoor air pollution levels

7) Reduced tendency for Building Related Illness (BRI)

8) Reduced tendency for Sick Building Syndrome (SBS)

9) Improved Indoor Air Quality (IAQ)

Can reduce power consumption and system costs by 30% due to less make up air required (as cleaner recirculated air) and lower pressure drop (due to less mould on filter and condenser coils).

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