Ozone and Indoor Air: Indoor Risks and Applications

July 2020

New situations, such as the one we are experiencing in 2020, bring with them new opportunities. Indoor air quality Indoor air quality has become a priority, but it should not be pursued at any price. Any action taken to improve indoor spaces must be carried out with guarantees and safety.

The case of indoor sanitisation with ozone is one such situation. There is no guarantee of its efficacy against SARS-CoV-2. In fact, ozone is not recognised as a virucidal agent by the Spanish Ministry of Health and is currently under review in the European Union for its use as an environmental disinfectant.

Ozone: origin and characteristics

Ozone (O3) is a molecule composed of three oxygen atoms that is formed when the two atoms of the oxygen molecule (O2) dissociate and join with another gaseous oxygen molecule to form ozone. It is colourless at ambient temperature and pressure (although at high concentrations it can have a blue hue) and gives off a strong odour.

Ozone is found at two different atmospheric levels, with different behaviours.

On the one hand, there is stratospheric ozone, which is naturally located in the stratosphere, between 10 and 50 km from the earth's surface. This ozone is formed by the effect of solar radiation that breaks down oxygen molecules (O2), combining them into O3. It forms part of the ozone layer, filtering out most of the ultraviolet radiation and allowing life on earth.

On the other hand, we find tropospheric ozone, formed by chemical reaction between nitrogen oxides and volatile organic compounds due to natural sources and anthropogenic pollution, in the presence of sunlight. In this case, it has direct consequences for human health due to its effect on respiratory health and skin ageing in prolonged exposure.

And finally, it is also a by-product in some ionisers, printers or other office equipment, as well as specific ozonisers, where passing dry air and/or pure oxygen at low temperature through a high voltage electric field dissociates the oxygen molecules giving rise to ozone.

So why use ozone?

By its very nature, ozone is highly oxidising, which is why it has emerged as a potential indoor disinfectant and biocide to eliminate pathogenic micro-organisms such as viruses, bacterias, fungi, mould, spores or unpleasant odours. However, its main application is more guaranteed in the field of water purification and purification.

On the other hand, due to its high instability, its use differs greatly from other industrial gases, as it cannot be stored or transported, but must be generated on site.

Despite this, its oxidising power continues to generate interest due to its high penetration capacity in closed spaces, with a life of 20 to 60 minutes until its total disintegration in the form of oxygen.

So, if indoor use results in oxygen production, where are the main disadvantages?

  • It is highly reactive and corrosive. Special caution is required as it may damage certain materials in the area of application.
  • It has high inhalation toxicity (respiratory irritation, bronchial hypersensitivity, etc.). This totally disqualifies its use in occupied spaces as its efficacy lies in concentrations higher than those recommended for health. According to some studies, the concentration to inactivate viruses should be around 20 ppm (although there is no guarantee at the moment), while an ozone concentration higher than 0.6 ppm is already toxic for humans. For this reason, regulations limit the concentration of ozone in working environments from levels of 0.05 ppm.
  • May be explosive at concentrations above 240 µg/m3 (~ 0.12 ppm).
  • It is a high-cost disinfection technique.

Recommended levels:

The limits established in Spain for short-term occupational exposures (VLA-EC) are classified according to the type of work to be carried out:


La concentración recomendada por el organismo de salud Health Canada es de 240 < µg/m³ en concentraciones horarias. La OMS establece que cuando la concentración de ozono en el aire que se respira es superior a los 240 µg/m3 durante más de ocho horas, existe un claro riesgo para la salud, ya que reduce considerablemente la función pulmonar, inflama las vías respiratorias y agudiza cuadros previos de asma, además de favorecer infecciones respiratorias.

The Spanish legislation UNE 400-201-94 ozone generators - air treatment - chemical safety, establishes safe exposures below 100 µg/m3.

Risks of high ozone concentration

Apart from doubts about its disinfection efficacy, ozone is in fact harmful to the health of workers and users due to its irritant effect on the respiratory tract. In high concentrations it can cause breathing difficulties, coughing, fatigue, as well as irritation and damage to the airways and lungs, and can therefore worsen previous asthma or bronchitis and compromise the ability to fight other infections or respiratory insufficiencies.

Like many pollutants, it affects healthy people or people with previous pathologies in different ways.

The risk is higher in places of high physical activity, as increased breathing frequency leads to higher ozone inhalation, which may result in increased respiratory consequences.

Recommendations for use

As with other pollutants, ozone should not be located indoors on a continuous basis . In addition, its high reactivity leads it to react with other compounds, resulting in secondary pollution. For example, combining the presence of terpenes with ozone indoors can increase formaldehyde concentrations.

For this reason, it is important to avoid exposing hygiene and cleaning products to indoor air by ensuring that they are stored with tightly closed caps and in properly preserved and preferably ventilated places. In this way we prevent any indoor presence of ozone, whether as a by-product, due to outdoor contamination or due to the use of ozonators, from leading to a significant increase in other indoor pollutants.

If ozonators are used as a specific treatment, a joint assessment of the indoor environment must be carried out prior to use. This is to avoid secondary contamination, as well as to ensure unoccupied use and to verify safe levels when the space is to be re-occupied. However, ozone is not effective in removing gas phase pollutants (such as nitrogen dioxide) or particulate matter.

Therefore, inBiot recommends that any intervention in which ozone is to be used should be complemented with monitoring and evaluation of its concentration in an interior space.

In this way it can be ensured that there is no synergistic and cumulative exposure to different pollutants with the risk that this entails. In this way we can guarantee ozone-free occupied spaces and ensure that any strategy is always accompanied by adequate air renewal.

Because health must go hand in hand with safety. And for this, continuous monitoring and evaluation are essential.

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