Choose active rather than passive air purification technology, says Andrew Hobbs.
Most air purification technologies are passive – they only have an effect when the air containing the pollutant comes into close proximity or passes through the air purification unit. These include filtration, UV-C, and PCO and ionisation technologies. In other words, certain operational conditions must be met for them to be effective.
Active technology is not limited in this way. It is effective in every cubic centimetre of indoor air and surface space. Pollutants are treated wherever they are in the indoor space, which is particularly significant in the context of Covid-19 transmission.
Active technology tackles the virus immediately, while passive technology must wait for the virus to pass near or through the unit. Some technologies claim to be effective against SARS-CoV-2 but very few have been tested against the aerosolised form of the virus in real-world conditions.
How does active air purification work?
The atmosphere contains minute concentrations of hydroperoxide molecules that are constantly being created from UV light energy from the sun, water vapour and oxygen. When these molecules come into contact with organic matter, they break the matter down through oxidation. This causes the cell to die and the hydroperoxide to revert to water vapour and oxygen – a natural and sustainable process.
Active air purification products create and maintain an equilibrium of these naturally occurring ionised hydroperoxide molecules in similar concentrations throughout an indoor air and surface space.
When people start to mix, the virus spreads mainly via the airborne route or on people’s hair, clothing, skin or on surfaces because it is not removed or destroyed at the point it is transmitted.
Advanced oxidation using ionised hydroperoxide equilibrium addresses this issue. It is safe, effective, and destroys Covid and other viral and bacteria at the point of transmission, while improving IAQ..
Can I retrofit active air purification technology?
In-duct products are designed for quick and easy retrofit into existing HVAC infrastructures. As the technology that produces the ionised hydroperoxides must be located inside the airstream the products are designed to:
- Be fitted inside main AHUs; and/or
- Be inserted through the ductwork and into the airstream on the main ducts and floor duct branches.
The precise specification is dependent on the overall size and performance of the HVAC infrastructure and the organic loading of the space being treated.
As power requirements for products are generally minimal and units can be powered by and cycle on/off with HVAC systems this is another contributing factor to quick and easy retrofit. Similarly standalone units are specified according to the floor area or volume with minimal power requirements.
What constitutes best performance?
There is one patented process called Photohydroionisation (PHI) which claims to produce one quadrillion molecules of ionised hydrogen peroxide, superoxide and hydroxyl ions every second, resulting in equilibrium concentrations between 0.01-0.04ppm. This is similar to the outside air and safe to breathe.
Compared to other technologies this is significantly richer – hundreds to thousands of times more than other hydroxyl generator processes and two to three million times more than bipolar ionisation processes. Also the hydroperoxide particles of PHI last longer than other hydroxyl generators, some of which claim up to 10 minutes (even though radicals decompose in nanoseconds) and bipolar ionisation which claims up to 300 seconds. PHI particles remain active for eight hours to 20 days.
What is the cost?
Cost per unit ranges between £300 and £1,600+VAT depending on the specification. Consumers should be wary of the cost and resource overheads of ongoing maintenance.
Some active technologies come with frequent maintenance requirements, which adds significantly to the cost. For example, some bipolar ionisers must be cleaned every 12-24 hours and certain hydroperoxide products require cell changes every three months. The most cost-effective active air treatment products require cell changes every two to four years and cleaning once or twice a year at the most.
How many units are required?
Ionised hydroperoxide specifications are the most scalable in terms of the amount of technology required to treat the entire indoor environment.
In-duct requirements are a function of the overall airflows and the organic loading in a space. Standalone requirements are a function of the overall air volume and organic loading.
To give an idea, approximately 35 units were required for an 11-storey, 130,000 square foot building, and 155 units were required for a 15-storey 450,000 square foot building.
Andrew Hobbs is CEO of air quality and HVAC specialists Better Indoors
