Air sterilizers work differently from air purifiers to counter micro-organisms such as viruses in the air.

Air sterilizers destroy some airborne bacteria, fungi, and viruses. They are not 100% effective and do not remove particles or chemicals from the air. The EPA also recommends that they are not used as the sole method for controlling infection.

However, there is a role for sterilizers. They can provide surface disinfection in a HVAC system and possibly as an addition to a fan/filter based air purifier. Also in a commercial setting they can be effective in increasing the sterility of a room.

Please note that when buying a machine to purify your air that when some manufacturers market their sterilizer devices, they talk about an “air purifier” and then go on to say the device “sterilizes” the air. As there is no official definition of an air purifier or air sterilizer, and as air sterilizers do purify the air, this is reasonable. It could however potentially lead to you buying a sterilizer when what you really needed was an “air purifier”. A standard HEPA air purifier usually means a fan/filter device, which also removes particles from the air. Particles in the air can be very harmful to human health.

There are 2 main types of sterilizer-UV light and thermal

Air sterilizers apply energy to the micro-organisms-airborne bacteria, fungi or viruses, in the air to destroy them. The energy used is usually ultraviolet (UV) light, but they can also use heat.

This illustration shows how a steriliser with a UV light works-

A UV sterilizer is different from a UV air purifier. The UV light air purifier also has a powerful fan and true HEPA filter. In contrast, the air sterilizer has a weaker fan or no fan at all and no filter. It may have a more powerful UV light. So a sterilizer is more targeted at airborne germs and does not produce such wide-ranging air purification as the usual portable air purifier.

Air sterilizers/cleaners using ultraviolet light are sometimes referred to as ultraviolet germicidal irradiation (UVGI) air cleaners. Their effectiveness depends on the exposure time of infectious particles going through the sterilizer and the power of the UV radiation. They can be used as standalone units or integrated into air purifiers or HVAC systems. One study investigated the effect on workers’ symptoms with UVGI switched on or off in and HVAC system. The workers had a 20% reduction in work-related symptoms and a 40% reduction in respiratory symptoms.

There needs to be sufficient exposure time and power of UV light to break the molecular bonds of DNA so killing bacteria, fungi, and to a lesser extent viruses. In a residential setting, the air is usually flowing past the UV lamp and so the exposure time is only a few seconds. Also, bacterial and fungal spores are quite resistant to inactivation. There is also the remote chance that the microorganism can repair its DNA once it has gone through the UV sterilizer-although this would only apply to fungi(mold) and bacteria, but not viruses. So UV sterilizers do not completely sterilize the air.

Wall mounted UV lights which irradiate the room constantly can reduce the bacterial and viral count in the air very effectively. The problem with conventional UV lights irradiating rooms with people in has been that they are a hazard for human eyes and skin. However far-UVC light has been developed, and because the wavelength is so short, it does not penetrate the white of the eye or into the lower layers of the skin. In one recent study, it reduced airborne microbes by 98% and was equivalent to 184 air changes per hour. With conventional air purifiers, the maximum that can be achieved is between five and 20 air changes per hour.

UVGI cleaners can also be used in surface disinfection. As the surface is continuously irradiated with UV light, this can be very effective. One study showed a 99% reduction in contamination on an exposed surface but only a 25 to 30% reduction in airborne bacteria.

They are also used in hospitals where they have been shown to reduce bacterial infection 50%. This is sometimes used by websites as a reason for having an air purifier with a UV light. The problem is that the UV sterilizers used in this hospital study were 48 inches tall by 24 inches wide!

A heat technology sterilizer will heat the air 400 degrees Fahrenheit to destroy allergens, bacteria, fungi, and viruses. The heated air will naturally rise by convection and so this type of sterilizer does not need a fan to work and so is silent in operation. Allergens that come into contact with the very high temperatures will also largely be destroyed.

Advantages of air sterilizers

The heat-based sterilizers do not interfere with the look of the room and consume only about 50 watts of energy. They can cover large rooms and do not need any maintenance. They are small the one model is 7 inches by 7 inches by 13 inches.

Modern UV sterilizers for use in residential settings should use UV C light, which does not supply enough energy for the production of ozone. So sterilizers do not act as ozone generators and neither do they produce negative ions. This is also true of air purifiers of the fan and HEPA filter design. It is important because both ozone and negative ions may affect health negatively.

Air sterilizers do not need a change of filters as air purifiers using a fan and HEPA filter do, and so are cheaper to run and in addition maintenance free. Per square foot, they are also cheaper to run and buy because one sterilizer will treat a relatively extensive area.

Not all air fan/filter purifiers will eliminate 100% of viruses, but the standard HEPA grade air filters found in most purifiers were thought to reduce infection rates during the SARS epidemic. This type of filter has also been shown to clear 99.99% of H1N1 virus from a room-the same as some manufacturers’ claim for air sterilizers. Also, there are now fan/filter air purifiers with Hyper-HEPA air filters which will remove all viruses.

Problems with air sterilizers

Using a UV lamp in air purifiers, in either HVAC systems or portable units, offer only slight infection control benefits for the processed air above those offered by filters alone. However, in HVAC units it offers surface decontamination of HVAC cooling coils and drain pans. The UV decontamination of these static elements is more effective, as there is a longer exposure time of any infectious agent on them.

Some manufacturers claim that 99.99% of microorganisms in the air passing through the sterilizer are killed and allergens altered. The catch is that often not much of the air in the room goes through the purifier per minute compared to a standard HEPA filter/fan air purifier. This is because the fans are less powerful, so their overall effectiveness is lower.

The problem of the volume of air processed per minute is an even larger drawback for their sterilizers without fans. For instance, the thermal sterilizers trickle air through the sterilizer at a relatively much slower rate. If you put your hand over a 6O watt bulb and feel the airflow, you will get some idea of how slowly the air will be processed. Obviously much slower than a fan/filter air purifier.

An air sterilizer cannot deal with particulate matter in the air which a HEPA air purifier can. So they cannot deal with airborne particles-one of the most important aspects of indoor air quality. They are unable to remove cigarette smoke, pet dander, airborne allergens, dust mite and mold spores.

They also cannot deal with volatile organic compounds as there is no carbon filter.

So streilizers do not deal effectively with the majority of types of airborne pollutant.

So should you buy one?

The main problem with all types of sterilizers is that they do not remove particles from the air. Standard fan/filter-based HEPA filtration of the air does this very effectively. Most particles in the air are small enough to get deep into the lungs and enter the bloodstream. Some of these particles have been found in the brains of people. Moreover, dementia has been linked to air pollution. As particles in the air have been linked to many damaging health outcomes it is important that you consider whether you really need an air purifier instead.

The EPA 2018 reports that “UVGI does not appear to be effective as the sole control device. When UVGI is used, it should be used in addition to-not as a replacement for-conventional particle filtration systems, because UVGI does not actually capture or remove particles”.

So you should not buy a sterilizer for an area in your home unless you already have a good air purifying system of the fan/filter design working in that area.