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Important design aspects in air sterilising techniques and devices

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As the choice of the right air-sterilising device can significantly reduce the risk of airborne and droplet-based contamination in healthcare environments and keep caregivers and patients safe, Madhusudhan Rao, founder and MD, Oorja Cleantech reviews the various techniques used for sterilising air from contaminants and suggests some guidelines to help choose the right device

The COVID-19 pandemic has raised concerns about the role of air in health care environments. There are three ways in which respiratory viruses like SARS are spread:

  • Direct person-to-person contact
  • Via contaminated surfaces
  • The airborne route through aerosols and droplets

Social distancing and hand hygiene help in preventing the spread through person-to-person contact. Regular surface disinfection using chemical or UV light prevents the spread of infection through contaminated surfaces. Air decontamination is often the most overlooked aspect in the prevention of infection through the airborne route.

The air in the environment is an invisible mixture of matter that includes dust, microorganisms (bacteria, viruses, fungi, etc), volatile organic compounds (often emitted by cleaning products), particulate matter, lint from hospital linen and possibly pollen and animal dander from the clothes of those visiting hospitals. Air decontamination devices should clean the air from all these contaminants using a single device. An understanding of various techniques used for sterilising air from such contaminants will help in choosing the right device:

Filtration

The most used mechanical filters are High-Efficiency Particulate Air (HEPA) filters and carbon filters. HEPA filters remove 99.97 per cent of all particles, which are 0.3 microns or larger in diameter. HEPA filters cannot remove viruses like novel coronaviruses, which are smaller than 0.3 microns.Carbon filters are used to remove volatile organic compounds and odorous gases.

Some devices also use electrostatic precipitators to remove particles from the air. Electrostatic precipitators work by generating a cloud of free electrons that charge the dust particles flowing through it, making them easy to collect.

Ultraviolet Germicidal Irradiation (UVGI)

UVGI light is emitted at a wavelength of 253.7 nm usually. Exposure to ultraviolet radiation makes the pathogen harmless by damaging the pathogen’s DNA/RNA. Such pathogens cannot reproduce once they have entered their host. Two factors determine the effectiveness of UVGI: the intensity of the UVGI energy and the time span of exposure.  UVGI is being used effectively for surface disinfection and in HVAC systems in many healthcare facilities. For surface disinfection, a sufficient time span of exposure is about five minutes. However, the time span of exposure becomes a challenge when using UGVI for air sterilisation. A viable way to overcome this limitation is to increase the intensity of UVGI energy by multiple reflections of the light so that the air passing through it is completely sterilised.

Plasmacluster Ion (PCI) technology

The technology uses electrodes to generate negative (O2 – ) and positive (H+ ) ions that make the pathogen inactive by binding on their surfaces. The positive (H⁺) and negative (O₂⁻) ions of the plasmacluster ions bond on the surface of airborne viruses and other substances and change into OH- radicals. OH- radicals have very high reactivity, and, therefore, they quickly extract hydrogen (H) from the protein on the surface of viruses and other substances, thus decomposing the protein and suppressing activity. The surfaces of things like bacteria and allergens consist mainly of protein. Removing the hydrogen atom (H) from this structure inactivates the undesirable substance. Furthermore, the OH- radical bonds with the removed hydrogen atom (H) to immediately form water (H₂O), which is returned to the air. Care must be taken not to generate ozone (O3) during this process.

Photocatalytic Oxidation (PCO) technology

Photocatalytic Oxidation (PCO) is an advanced process by which viruses, bacteria, volatile organic compounds (VOCs), mould, and fungus is destroyed. PCO is achieved when UV light rays fall on a Titanium Oxide (TiO2) coated filter, thus creating hydroxyl radicals and super-oxide ions, which are highly reactive. These aggressively combine with contaminants in the air. Once bound together, the chemical reaction takes place between the super-charged ion and the pollutant, effectively “oxidising” (or burning) the pollutant. It breaks the pollutant down into harmless carbon dioxide and water molecules, making the air more purified. The advantage of this specific technology is a higher yield of free radicals, which occurs due to the exposure of catalytic compounds. PCO Technology can be used without causing any disruptions. Freestanding units can be placed in a bookshelf or on a table. They can be placed into the air duct system for even distribution in your airspace. PCO is used to kill microbial contaminants and keep your environment healthy.

An effective air steriliser should incorporate a combination of these technologies to eliminate all contaminants from the air from easier-to-filter dust particles to hard-to-remove viruses. Due to the higher concentration of dust particles in India, a pre-filter to remove higher sized dust particles should be used in combination with HEPA and carbon filters. The combination of a pre-filter, HEPA filter and carbon filter will eliminate all particulate matter up to 0.3 microns and odorous gases. Thereafter, either UVGI or PCI or PCO technology can be used to kill pathogens like viruses and bacteria.

However, any air device can also be a spreader of contamination as it draws air from different corners and circulates it. So, the contamination in one area or room can spread to the other area unless the air passing through the device is sterilised 100 per cent. This single-pass sterilisation of the air is the most important aspect in the design of an air steriliser. It can only be achieved by a combination of proper airflow control and increasing the intensity of sterilising energy (UVGI or PCI or PCO).

The choice of the right air-sterilising device based on the above guidelines can significantly reduce the risk of airborne and droplet-based contamination in healthcare environments and keep our caregivers and patients safe.

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