Tag Archive | air cleaner

Should you buy an air Cleaner?

The first rule to improve your indoor air quality is to ventilate in order to recycle the fresh air. But what if the outdoor air is really bad? Or the climate extreme ? If you live in cold Norway, hot and humid Singapore or foggy Beijing, you may be considering to buy an air cleaner (also called air purifier). In Asia, air cleaners are becoming a common home appliance that people buy, like they would buy vacuum cleaners. Many technical words are used to describe working processes of air cleaners: electrostatic precipitators, HEPA filters, ionizers, sorbents, plasma, UVPCO, etc.

This post is aimed at helping you to see clearer and understanding the basic principles of these different technologies, as well as their potential risks. Be aware that there is no proper certification for air cleaners. So consumers are left with no choice but to believe the claims of air cleaner manufacturers. Indoor air pollution is not visible, which make it easy for companies to claim efficiency, even when the device provides no significant improvement. Finally, there is no requirement to test side-effects, such as ozone emissions. So consumers need to check the technical parameters to ensure a safety.

Most air cleaners combine different technologies. There are three major technologies commonly used to remove particles:

  1. Mechanical filters
  2. Electrostatic precipitators
  3. Ionizers

and three main technologies targeting gas pollutants

  1. UVPCO
  2. Sorbent
  3. Plasma System

I will  describe technologies in 2 different posts: Removing gas pollutants; let’s talk Tech!  and Removing PM: let’s talk Tech!

A good start to get a basic understanding of air cleaning technologies is the EPA summary on Residential Air Cleaners.


Removing PM; let’s talk Tech!

Air cleaners use different technologies to remove particles. What is the difference between mechanical and electrostatic filters? Are ionizers safe?

Mechanical filters 

How does it work?

Nothing very complicated or new in the basic technology of mechanical filters; particles are captured by fibrous media. But mechanical filers can also remove other types of pollutants: airborne viruses, bacteria, mould spores, allergens or gaseous pollutants, adsorbed by accumulated particles on the filter surface. There are different sizes and thicknesses of media filters using various materials and performing at various efficiencies (open-cell foams, non-woven textile cloths, paper-like mats of glass or cellulose fibres, wood fill, animal hair or synthetic fibres). High efficiency particulate air filters (often called HEPA filters) can remove 99.97% of PM of diameter below 0.3microns. Filters suffer from pressure drops and require relatively high energy compared to other alternatives.

Are there potential side effects?

Some portable air cleaners (not integrated to air handling units) with mechanical filters are pretty noisy (comparable to the noise of a washing machine, i.e. 50 to 75 dB). If equilibrium conditions are disturbed (change of airflow, temperature or relative humidity), there is a risk of desorption and release of pollutants. Gaseous pollutants may also be transformed into other pollutants. The loaded filter surface may also create a natural sink for ozone, often reacting with pollutants accumulated on the filter surface. So proper operation and maintenance of air filters is essential to make sure beneficial effects are greater than side effects. Code of practice uses 6 months or a year before changing a filter. However there is generally no way for users to know when to change the filter, as it depends of the pollution level and could be different from the lab conditions.

Electrostatic precipitator (ESP) 

How does it work?

This technology is less common than filters but is integrated in some portable air cleaners. One clear advantage of this technology is the low pressure drop  compared to mechanical filters. Thus ESP are less noisy and more energy efficient. When an ESP is operating, high voltage is applied to an air stream, providing electric charges to particles. The charged particles are then attracted to plates (or filters) of an opposite electric charge. Airborne bacteria, mould spores and allergen are also removed by a rafting process. But as PM accumulate on the plates surface, electric attraction decreases, so the Electrostatic precipitator requires regular cleaning.

Are there potential side effects?

Ozone is generated by the high voltage. Ozone is harmful by itself and can also react with other organic compounds to form harmful by-products such as formaldehyde. Ozone generation is currently not well documented because there is no legal requirement for manufacturers so if you choose to use this technology, you should choose lower voltages or systems with carbon filters (to remove VOCs).


How does it work?

Ionizers (also called ion generators) send bipolar ions into the room. The PM then becomes charged through direct contact with the ions. Charged PM accumulates and attaches to surfaces (walls, tables, floor, etc.). Ion generators may also cause charged PM to carry airborne biological pollutants. Experts have been expressing doubts on the ability of ionizers to work efficiently. Indeed, the volume of air treated may be too small, limited to the direct vicinity of the device.

Are there potential side effects?

Ionizers are generating ozone, which is harmful in itself (increasing the risk of respiratory diseases) and may also react with other organic compounds to form harmful by-products such as formaldehyde.

Removing gas pollutants; let’s talk Tech!

Three main technologies are used to remove gaseous pollutants. They have different operating principles, energy requirements, potential side effects and efficiencies. Below is a basic description of these technologies.

Photo catalytic oxidation (PCO)

How does it work?

PCO technology uses ultraviolet light (UV) on a catalytic surface (often titanium oxide TiO2). It triggers the formation of  highly reactive species (hydroxyl radicals, ions, ozone,etc. ), which enable VOCs to be decomposed into CO2 and water.

Are there potential side effects?

UVPCO generates ozone, which is harmful in itself (increasing the risk of respiratory diseases) and may also react with other organic compounds to form harmful by-products such as formaldehyde. Alcohols can also create poisoning of the catalyst and cause incomplete oxidation leading to production of aldehydes or unwanted species.


How does it work?

Solid sorbents are materials with large internal surface, enabling electrostatic interaction between the gas molecules and a surface. Some examples include activated charcoal, silica gel, activated alumina, lithium chloride, zeolites to porous clay minerals. Gaseous pollutants removed by physical sorbents include ozone, nitrogen dioxide or VOCs (except low molecular weight carbon compounds such as formaldehyde and ammonia).

For chemisorption gas filters, the working principle is similar but in addition, a bond-forming chemical reaction between the adsorbed gas pollutant molecule and the adsorbing surface occurs, involving electron transfer. One common chemisorbent is potassium permanganate, an active oxidating reagent. It  can convert formaldehyde into water and carbon dioxide. The efficiency of the sorbent, measured via the rate of adsorption, decreases with the amount of pollutants captured. Similarly to mechanical air filters, sorbents require energy to pull the air through.

Are there potential side effects?

Carbon filters have high biocompatibility and microorganisms may multiply on filters.

Plasma cluster ions 

How does it work?

One type of ion generator is called cluster generators or plasma generator (the term DBD or Dielectric Barrier Discharge may also be used). It uses AC discharges to produce clouds of positive and negative ions successively. Electrons are separated from oxygen molecules and the electrons are combined with the oxygen molecules to produce oxygen cations and anions. The oxygen anions react with water in the air and produce reactive species, which then agglomerate and form ion clusters. Ion clusters can damage the surface of airborne microbial and oxidize gaseous volatile organic compounds and PM.

Are there potential side effects?

Ion generators may generate ozone and undesired by-products.  Plasma systems tend to be very popular and displace UVPCO systems although they use a lot more energy.