AIR PURIFIERS

An average of 138 litres of air passes through the lungs of an adult every day.

Together with the much needed oxygen, this inhaled air brings with it submicron particles, germs, allergens, chemical gases and other contaminants that penetrate deep into the aereoles of our lungs.

It is often assumed that indoor air is cleaner than outdoor air. This may be true in certain situations, like if the individual is exposed to busy roads at ground level, or if the urban air is generally polluted.

In the majority of instances, though, buildings tend to be airtight due to air-conditioning, to keep the heat out. This has the effect of causing Sick Building Syndrome, in which people fall ill due to poor Indoor Air Quality (IAQ).

This is not unlike fishes in a tank where the water is not changed - the water gets increasingly contaminated and recirculated. Likewise, the air-conditioning in most buildings, including homes, simply recirculate the air. Contaminants are continually being released into the indoor environment, which recirculates them.

In the case of the fish tank, the best solution is to change the water, and to maintain a good water filter. In indoor environments, the same is true... a good air purifier or air cleaner is essential. But first, we need to understand what contaminants there are.

Airborne contaminants

There are three major classes of airborne contaminants that are harmful to humans. They are:

• particulate matter or submicron microns
• bioaerosols like germs
• gases

These contaminants may be allergens (i.e. things that cause an allergic reaction in certain individuals), or not. Even if they are not allergenic, they could still cause severe harm to humans - for example cancer and infections.

Particulate matter or submicron particles

Minute particles are continually being released into the air by people, clothes, paper, boxes, and myriad other materials found indoors, as a result of natural exfoliation or friction. People and other indoor organisms (e.g. insects, lizards, cockroaches) constantly shed material (skin flakes etc.) into the environment. Dust mites multiply and release copious amounts of fecal material into the environment. As these material break down and go airborne, they become potent allergens for sensitive individuals. Tobacco smoke is also a major source of particulate (and gaseous) contaminants.

The particles that can successfully navigate its way into such minute passageways in our lungs are themselves very small and invisible to the naked eye. These are termed as Respirable-Sized Particles (RSPs), and generally refer to particulates in the region of 10 microns or smaller (1 micron = 1/1000^th of 1 mm). The smaller the particles, the more intrusive they are.

For convenience however, environmentalists use particulate matter at 10 microns as a gauge of particle contamination of outdoor air to measure pollution. Cleanroom and operating theatre specialists however, use 0.3 microns as the standard.

Bioaerosols or microbes

Bioaerosols simply refer to microbes or living things (i.e. bio) that are small enough to go airborne (i.e. aerosol). These include bacteria, viruses, pollens that have entered the indoor environment by piggybacking on people and materials. Pollen is an aeroallergen (i.e. airborne allergen), and is a common cause of allergic rhinitis (hay fever or running nose).

Ventilation systems (air condition ducting and draining systems) are also a good source of fungal growth. Fungal spores can cause lung infections like pulmonary aspergillosis, and the potentially terminal Legionnaire's Disease.

Gases and Volatile Organic Compounds (V.O.C.)

The third class of contaminants is equally harmful - especially volatile organic compounds. These include formaldehyde, a suspected carcinogen (i.e. cancer-causing), which is commonly offgassed from particle board walls, furniture, adhesives for carpeting, etc. Other V.O.C.s are generated from cleaning compounds, paints and solvents (including nail polish removers).

Ozone in the stratosphere is a good thing, preventing excessive UV and other solar radiation from harming living things. However, ozone at ground level is not a good thing, as they break down cells. This property which is useful for killing bacteria is also bad for humans. Ozone is produced by copying machines and laserjet printers.

Air purifiers or air cleaners

Because of the artificial build-up of contaminants in an indoor environment, it is advisable to use air purifiers or air cleaners to purify the indoor air. There are various technologies in the market, and some are better than others. However, with recent advances, medical-grade air purifiers have been made available at affordable prices. One has to be careful to select an air purifier that can remove airborne particulate contaminants, bioaerosols, as well as gases or VOCs. At the same time, one has to be careful of some air purifiers that are effective at removing some of these contaminants, but do so by introducing harmful gases like ozone or OH radicals.