Tag Archive | PM

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.

Type of air pollutants

Air pollutants can basically be divided in two categories:

  1. particles
  2. gas pollutants.

What are Particles? 

Many people are concerned by inanimate Particulate Matter (often called PM), but most of us are unclear of what PM actually is. PM is in fact a mixture of small solid and liquid droplets from acids (such as nitrates and sulphates), organic chemicals, metals, soil and dust particles. PM can be both inanimate and animate (such as microbiological pollutants).

Where do they come from?

Inanimate particles come from anthropogenic sources (like construction sites, agricultural activities, manufacturing, landfills, wood-burning stoves, fireplaces, vehicles, etc.) or natural sources (sandstorms, windblown dust, volcanic activity, forest fires and biogenic sources, e.g. pollen and plant wax).

What is the difference between PM10 and PM2.5?

PM10 is composed of particles of diameter less than 10 micrometers, while particles in PM2.5 have a diameter inferior to 2.5 micrometers. It is important to differentiate between PM2.5 and PM10 because the health impact is different. In particular, the particles PM10 can reach the upper part of the airways and lung, while smaller particles PM2.5 can penetrate more deeply into the lungs and alveolar region.

What about smaller particles ?

There is also a growing concern about ultrafine and nano-particles. They could in fact have more severe effects than larger particles (PM10 and PM2.5). Health impacts of particles listed by EPA (Environmental Protection Agency) include premature death in people with heart or lung diseases, aggravated asthma, decreased lung function and increased respiratory symptoms, such as airways irritation.

What kind of gas pollutants should be tracked?

WHO (World Health Organization) published air quality Guidelines in 2005. It includes a list and to description of major gas pollutants creating a risk for health. Major gas pollutants (excluding VOCs), are:

  • Carbon Monoxide (CO), interferes with oxygen-carrying capacity of blood.
  • Nitrogen dioxide (NO2), irritates the lungs, lower resistance to respiratory infections and increases the risk of miscarriage. NO2 is key precursor of nitrate particles forming important share of PM2.5.
  • Sulphur dioxide (SO2) dissolves in water vapour and form sulphuric acid. Interacting with other gases, it is then forming sulphates. Sulphate particles are one element of PM2.5.
  • Ozone (O3) formed naturally & necessary to protect from UV radiation in the stratosphere. But ground level ozone (formed from chemical reaction of oxides of nitrogen and volatile organic coumpounds (VOCs) in the presence of light and heat) can irritate the airways of the lungs & increase asthma symptoms, and cardiopulmonary problems.

Common sources of air pollution gas SO2, NO2 or CO2 are fossil fuels combustion (road transport, shipping, electricity generation, heating plants, industrial processes, cooling and gas stoves) and refining processes of coal, oil, metal-containing ores (SO2, CO2). Main sources of O3 include printers, copiers, UV bulbs, ozone generators.

What are VOCs? Why should we care?

Volatile Organic Compounds (commonly called VOCs) are more and more discussed amoung experts. They are chemical compounds containing one or more carbon atoms, which can evaporate at room temperature and normal atmospheric pressure. More than 900 different VOCs were identified by the U.S. Environmental Protection Agency in indoor environment (1989) at concentrations higher than 1 ppbv (concentration units of parts per billion by volume). The complexity to study VOCs’ impact on health comes from the fact that VOCs are harmful but typical not acutely toxic. Instead, they can have long-term and chronic health effect at low concentration. These pollutants are mainly emitted by indoor sources (building materials, cleaning agents, cosmetics, waxes, carpets, furnishings, laser printers, photocopiers, adhesives, paints, etc.). Concern about VOCs is growing but more research needs to be done on targeted VOCs. VOCs include benzene, formaldehyde, toluene, limonene, xylene, etc.

What are SVOCs?

Recently, more research has been focused on Semi Volatile Organic Compounds (SVOCs), which are particularly difficult to handle with ventilation. They are used in many products (e.g. flame retardants) and may have hormones disrupting effect. For more information on Endocrine Disrupting Chemicals, I recommend reading the WHO report of 2012.