UV Air Purifiers: The Twilight Zone

UV air purifier technologies focus on disease organisms. In air purifires, ultraviolet light is generally used in combination with other technologies.

It is used in photo-catalytic air purifiers (next article), or used to convert an existing HEPA air purifier into a germ-killing uv air purifier.

In this article we discuss the portable room ultraviolet air purifier, not the many whole house models which offer in-duct air stream and A/C coil irradiation.

Studied since the 1930s, ultraviolet "light" is electromagnetic radiation with wavelengths shorter than that of the visible region.

Since ultraviolet is not visible, it is technically not light.

The range of ultraviolet wavelengths is often subdivided into three groups; air light

UV-A (380–315 nanometers), is called Long Wave or "blacklight"

UV-B (315–280 nm), Medium Wave,

UV-C (< 280 nm), Short Wave or "germicidal".

Ultraviolet air purifier systems employ only “safe” UV-C.

Air purifiers with UV-C light use a relatively short wavelength of 254 nanometers, a time proven method of disinfection.

The bug’s DNA is damaged, making reproduction difficult.

Germs cannot develop immunity to this physical assault, and viruses are very vulnerable due to their lack of protective cell walls.

UV-C lamps are used in hospital operating theaters and in sterile manufacturing processes.

Ultraviolet has an extensive and successful history in water and waste water treatment, but the ultraviolet air purifier has been harder to implement.

Viruses are too small to be trapped by a HEPA air purifier, which will capture most bacteria.

Where moisture and suitable food, such as dead insects, are present in air purifier filter media, “bugs” can build up, causing the filter to reproduce microorganisms.

An air purifier can accumulate several fungal species.

Attempts to solve this problem using anti-microbial air filter media have mixed results.

A medical grade uv light, in the air purifier, might be the answer.

The UV light, directed only inside the air purifier, is claimed to kill the bugs, including viruses, bacteria and dust mites.

A larger issue is whether an ultraviolet air purifier can effectively sterilize the air in a room or entire home.

Some studies suggest uv air purifiers, installed in office building air duct systems, could reduce respiratory symptoms reported by workers.

Sterilization is defined as the complete destruction of all microbial species.

Disinfection, meaning reduction of microbial populations, is the correct term for air stream and air cleaning applications.

Laboratory exposure to ultraviolet light with intensity 10,000 times higher than of sunlight (dosage), for enough exposure ("dwell") time, will result in sterilization.

When taken out of the controlled environment of the laboratory, and placed in a residential air purification situation with high foot traffic, visitors, and pets, “sterilization” seems pretty ambitious.

Microbial growth can occur in crevices, surface colonies, or in protected areas such as insulation, where organisms do not circulate through the uv air purifier.

UV Dosage: Here Comes the Sun

Up to 40,000 Microwatts (per second/cm square) is deployed in ultraviolet air purifier systems.

Dewll time, lamp wattage and distance from area lit, as well as chamber design and reflectivity, will affect dosage.

Lamps will loose power when dust covered, so the lamp should follow any particulate collection media.

Compromise: Dosage vs. Air Movement

UV-C can sterilize any still environment given enough time, but air moving through a uv air purifier's filter system must pause under the UV light.

Every machine has a different ability to irradiate microorganisms. Light intensity, chamber design, and exposure time will vary.

Microbes must be exposed to UV light long enough for their internal structure (DNA and RNA) to be damaged.

A conflict is created between the need for higher fan speeds, to clean the air of other particulates, and effective air sterilization.

Either the light is less effective or not enough air is purified.

Filter sterilization Only

Rather than design air purifiers as room disinfectors, many manufacturers opt for light that just protects the HEPA air filter.

Continuous radiation may shorten the life of the HEPA filter.

The HEPA material can become brittle and develop cracks, shortening replacement periods.

UV Purifier Additional Costs

The AllerAir 5000 is about $500 without the optional lamp, which adds $150 to the price and displaces some carbon.

The effective life of the bulb is one year. The quality of the germicidal light deteriorates, even though the bulb still produces light.

Considerable price gouging on bulb replacements is the industry norm.

Biozone recommends one-year bulb replacement for all models, around $30.

AllerAir’s 10 watt bulb costs $132.

NQ Clarifier bulbs in sets of two go for about $70.

Air Purifier UV: Safety

UV-C has low skin penetrating capability. It will irritate the eye, but is believed not to cause cataract or cancer.

UV-C does not pass through eye glasses or clothing.

Exposure to UV-C can cause temporary skin redness.

Nobody should look at the bulb and the power should always be turned off before servicing or replacing the lamp.


Many of these UVC light systems produce "photolytic" ozone.

Larger lamps that create greater dosage, can also produce additional ozone.

Often a germicidal uv air purifier is used where there is an illness, to avoid infection.

Or as a preventative for infants, the elderly, or immune suppressed individuals that are susceptible to infection.

Ozone is particularly bad for these vulnerable consumers.

Ozone emissions must be eliminated before I can advocate this technology.

NQ Clarifier is a leader in UV air purification technology, and claim to have solved the ozone emissions problem with special quartz bulbs.

AllerAir also advertises an ozone free machine.


Fluorescent lamps produce UVC radiation by emissions from low-pressure mercury gas.

Low pressure mercury-vapor lamps emit about 86% of their light at 254 nanometers (nm) which matches the best germ killing wavelength.

Many ultra violet systems incorporate a glass mercury bulb without sufficient protection from breakage. It is important that shielding protects any bulb that could release mercury.

Does It Work?

There is no reliable evidence of complete ultraviolet effectiveness in air purifier applications. Ultraviolet systems can reduce incidences of inhaled microorganisms in indoor environments.

The Centers for Disease Control (CDC) says ultraviolet light in hospitals does not add to the effectiveness of HEPA air purifying equipment.

I think biological contamination will be a critical area in the future.

I believe the 21st century will be marked by epidemic disease and biological warfare, and hope for rapid development of this sector of the market.

I believe this technology is best spun off to dedicated air sterilizers.

Therefore my favorite purifiers do not have this technology, though some may have the option.

For example, AllerAir Exec models with the ultraviolet option have a 10 watt bulb, with 20 watts optional.

Those with serious illness or suppressed immune systems may want to consider a separate air sterilizer.

Ultra violet air purification is a possible option, provided a no ozone model is selected.

You can see UV air purifiers

UV air purifiers at Amazon.com.

”Unlock this door with the key of imagination…”


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