Overview of Air Cleaning Technology

Many attempts have been made to utilise the power of UV light in air sterilisation. The particular success of Medixair is attributable in some measure to our recognition of the degradation characteristics of UV light over relatively low distances. There are however other reasons why products being sold against Medixair fail to compete and these are noted as follows:

Low Power Products

Low UV power input via single lamps at 8-10watts is wholly inadequate in terms of the kill energy requirement detailed above for Medixair. Note – Medixair has a power input derived from 4 x 25W tubes (or a total of 100W).

Cross flow machines

Certain machines direct air across the UV lamps – as opposed to the principles of Medixair which pulls air at a controlled speed down the length of the lamp array. These “Cross Flow Machines” minimise the air exposure time to UV light – to the extent that insufficient energy is directed into the air to kill the constituent micro-organisms.

In-duct UV

It is logical to believe that the installation of UV emitters (lamps) within air conditioning and ventilation ducts would provide an ideal solution to germicidal irradiation. Whilst initially this seems a good solution, designers and installers invariably fail to recognise the core requirement for there to be sufficient “dwell time” of the air in proximity to the UV light source. In other words “in-duct” air speeds of 6-10metres/sec (compared to the air speed within Medixair of 0.3metres/sec) provides insufficient air “dwell or residence time” for the UV to have any meaningful effect.

Open UV

Open UV refers to units placed directly underneath ceilings. These allow natural air movement to circulate across the UV lamps. Such systems are particularly effective in environments where there is a high degree of air flow – say for example in an operating theatre (where 20 air changes per hour would typically be recorded).

The major disadvantages of this technology are that it requires specialist installation; cannot be used when people are present within the room (because of the dangers associated with human exposure to UV light) and must therefore be fitted within rooms sealed with electrical inter-locks. The key disadvantage of Open UV Systems is therefore that such devices may only be employed during non-working periods.

Other UV Systems – Summary

All competing technology offers either insufficient power, too little air exposure to the UV light or impracticalities associated with usage only when the room space is empty.

Medixair by comparison is highly effective, safe, portable and easy to install.

Other Technologies

There are three technologies that are positioned alongside air sterilisation – HEPA filtration, Air Ionisation and Air Ozonisation

HEPA Filtration

HEPA is the acronym for High Efficiency Particulate Arrestors. These filters, made of glass filled paper materials are capable of capturing/trapping 99.97% of particles down to 0.3microns in size. Particles of this size will include moulds, fungi, bacteria and some viruses. HEPA filtration is established as the accepted technology for clean room operation – including hospital operating theatres. In such environments there is typically a high level of air flow (20+ air changes per hour) which continuously dilutes contaminated air.

The disadvantages of such systems are the inability to trap small viral organisms, down to 0.02microns, which will pass straight through the filter. Also, all captured micro-organisms are trapped rather than killed creating the potential for continued viability. In addition, in order to remain effective HEPA Filtration systems must be regularly and scrupulously maintained and be protected from exposure to humidity which will cause them to rupture.

Air Ozonisers

Ozonisation technology is constructed on the basis of a high voltage charge which is used to break open adjacent oxygen atoms – thereby allowing positively charged oxygen ions to form into the three atom isotope of oxygen (ozone O3) – through a bonding process with surrounding O2 molecules.

The ozone molecules being in a highly reactive state they circulate and thereby come into contact with other airborne particles – such as pathogenic micro-organisms. Within this attachment there is a reaction between the oxygen atom and the micro-organism which effectively changes the nature of the micro-organism and renders it harmless. The US Environmental Protection Agency has however stated:

  • Whether pure or mixed with other chemicals, ozone can be harmful to health.
  • Available scientific evidence indicates that at concentrations not exceeding public health standards, ozone has little potential to remove indoor air contaminants.

The following group of products all use high voltage and have the same disadvantage

Air Ionisers

Air Ionization is provided through an electrical device which emits streams of negative ions into the surrounding air via electrodes.

The negatively charged ions become attracted to positively charged airborne micro-organisms which then become heavier than air and fall to the ground.
Air Ionization thereby keeps the air cleaner than it would normally be – by reducing the particles suspended in the air. It also works on aromatic molecules – thereby creating a deodorizing effect.

However, ionisers do not generally kill micro-organisms – rather causing them to precipitate out of the air where there is then a requirement to destroy them through traditional disinfection. As many organisms remain viable in the atmosphere for extended periods of time such precipitated organisms will remain as potential sources of contact infection – or may be moved by secondary air currents – until rendered harmless through disinfection.

Ionisers will create ozone and free radicals.

Cold Plasma Discharge

A recent technology which has just appeared. This again uses high voltage to create a cold plasma, which it is claimed will break open the cell wall of viruses. The mechanism for bacteria is less clear. Cold plasma is cheap to produce and should not be confused with high energy gas plasmas which are highly expensive, specialised and need high security to manage.
It has been reported that light flashes can be seen as dust particles react with the plasma. This proved disconcerting to users. Importantly as with ionisers the use of high voltage to generate the plasma will create ozone (see below) and free radicals (see below).

Titanium Dioxide

These devices us long wavelength UV (not the germicidal form). When the light shines on the titanium dioxide, electrons are released at its surface. The electrons interact with water molecules (H2O) in The process also produce ozone (O3),

Research on photocatalytic viral disinfections for airborne viruses is in its infancy. The most important issue is the lack of any research about heterogeneous photocatalytic disinfection of novel viruses such as SARS-CoV and COVID-19.

A key fact is that where UV-based inactivation procedures have been introduced for viral disinfections, these techniques often need long illumination periods or use of the recirculating mode due to their low photon energy. 

Discussion of Ozone and Free Radicles

Free radicals are highly charged hydroxyl ions (-OH) formed from atmospheric moisture. They are highly reactive. These react with pathogens but also with humans having a potential harmful effect.

The hydroxyl radicals attack bigger organic (carbon-based) pollutant molecules, breaking apart their chemical bonds and turning them into oxidation products. Hydroxyl radicals occur naturally in the atmosphere, they can themselves pose dangers by virtue of cytotoxicity.

Ozone a chemical tristate (O3) variant of normal atmospheric oxygen (O2). It is in itself, a toxic air pollutant. Purifier makers claim the amounts of ozone produced are within the guideline limit (0.05 parts per million) suggested by the US FDA but, even so, this is something to bear in mind.