Getting to know potential indoor air contaminants, and how to minimize or remove them from your living environment is time well spent. Not only that fighting indoor air pollution is most effective when air filtration is combined with minimizing pollution at the source; it also helps you figure out what kind of air filtration system you need for effective protection.

In general, air filters are either particulate or gas filters; they can be used either separately or combined, and either in conjunction with a forced-air heating/cooling system, or as portable units.

Every air-filtration system, or device, lists the smallest particle size which it efficiently removes (for instance, "removes 95% of particulates down to 3 micron"). It is good to have at least general idea of what contaminants are efficiently filtered out by a particular air filter. Here's what the size scale of common particulate indoor air contaminants looks like:

▪ pollens are over 10 microns (1 micron=0.001mm)
▪ animal dander is in the 0.5-10 micron range
▪ mold spores are from 1 micron to several microns in size,
▪ smoke particulates range from 0.01 to 1 micron, and
▪ viruses (large) are about 0.01 micron in size.

Particulate air filters come in several forms:

4 standard furnace filters are mechanical particulate filters with very low efficiency in reducing particulate levels

4 electrostatic air filters are somewhat better, but not significantly

4 electrostatic precipitators are best of the three, but still not highly efficient, and also losing it quickly with use

4 medium-efficiency air filters vary quite a bit in their particulate removing capability; those that go down to 2-3 microns are sufficient for most residential applications

4 High Efficiency Particulate Accumulators (HEPA) filters are efficient down to sub-micron particle sizes; true HEPA filters removes 99.97% of particles down to 0.3 microns

For removal of gaseous indoor air contaminants, you need adsorption air filters. They they are most often made with activated carbon (or "charcoal"), and sometimes with activated alumina. For removal of both, particulate and gaseous contaminants, the air-filtration unit needs combined adsorption and a particulate filter.

The main difference between activated carbon on one, and alumina on the other side, is that the former works well in removing high-molecular-weight gases (benzene, tuolene, ethers) but, unless specially treated, not so well with low-molecular-weight gases, like formaldehyde, ammonia or propane. Since the efficiency of adsorption filters directly depends on their area, the more of activated carbon, the better performance.

On the other hand, activated alumina works well with low-molecular-weight gases. Also, it reacts with gaseous contaminants, changing them chemically, rather than just adsorbing them, as activated carbon does.

Your choice of air filter depends on the job you need it to do. It can be a single filter, or any combination of them. For portable units, size of air-purifier is also important factor of its efficiency. Room-sized units need to be large enough to efficiently service a room they are intended for. Desktop units, in general, have too small capacity for efficient air filtering.

Air-filtration in conjunction with a forced air heating/cooling system should have air-handler fan that runs separately, in order to have continuous, efficient filtration. R

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