If your home needs air filters to remove airborne pollutants, you should consider very carefully which model you buy. It needs to be big enough, efficient enough, and also good enough to remove problem particles if a home's inhabitants are asthmatic or allergic.

• Aim for a model that can circulate 8-10 room volumes of air per hour. Your air won't be perfectly clean, but it'll be a lot better than it was.
• Asthma sufferers should target filters that remove more than 90% of particles larger than 0.3 microns in diameter.
• HEPA (high efficiency particulate accumulator) filters are five times better than standard options, removing over 99% of larger particles. They can last up to five years.
• HEPA filters cover asthma sufferers' requirements.
• An average, medium-efficiency filter is usually good enough for most asthma and allergy sufferers. You may want to use a HEPA filter in high-risk areas, such as bedrooms.
• Be careful of models which claim to be HEPAs, but which aren't.

Always look for labels from rating agencies such as ASHRAE, AHAM, and the FDA, and compare different models using their rating systems. Don't believe the manufacturers' hype without looking more deeply into any impressive claims.

Multiple Chemical Sensitivity

A troublesome, often debilitating disorder, MCS makes air filtration a real problem. You would think that sufferers would need the best possible pollution removal system they can find, and you'd be right. But there's another side to the issue: many air filters generate chemicals to which MCS sufferers are susceptible.

Fiberglass and polyester fibers used in particulate filters are a good example of this as they often contain a synthetic resin to which people can be allergic. In these and other particle filters, some manufacturers spray the material with an oil to enhance their trapping ability – and in doing so they add a substance that can cause trouble for MCS sufferers. Mold and microbes are often killed by adding yet another chemical to filters, which is yet another product that can cause a reaction.

There are ways to handle these problems. The easiest are to either bake the filter in an oven for a couple of hours at 200°F (93°C), after checking that this treatment won't ruin the filter, or to install a gas filter downstream from the particulate filter, so that it absorbs whatever substances the latter outgases.

Electrostatic precipitators and negative ion generators also have problems: they create small amounts of ozone. For most of us, that's not a problem, but for people who are sensitive to the gas, it's a nightmare.

Chemically sensitive people may also react to pollutants outgased by the plastic in electrostatic particulate filters, or even the absorption media in gas filters. In the latter case, switching to filters which use carbon created from oxidized coconut husks instead of coal may work.

If you or someone in your home suffers from MCS, talk to the filter manufacturers. Find one who is aware of the problems and discuss options – they will usually be able to set you up with a system that you can tolerate.