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.

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