Filtering Indoor Air

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Filtering Indoor Air

The Washington Post

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Byline: Mike McClintock

7-29-99

When a single household occupied the land that thousands crowd

onto today, smoke from the fireplace and the smell from last night's

fish dinner disappeared into a sea of pristine air. There were pockets

of pollution and noxious odors around lumber mills and

slaughterhouses, but the air that people breathed day to day was

undeniably fresh.

Go far enough into the countryside, where there is no city glow

on the horizon and only an occasional car, and you can get a whiff of

the way things once were, although even that air is laced with

byproducts from faraway factories. Blame it on the combustion engine,

industry or progress in general.

Whatever the reasons, and despite increasingly stringent

air-quality standards, summer forecasts now routinely include

air-quality alerts--as though you should breathe less on those days.

Some people walk the streets in respirator masks every day,

fending off the constant attack of air. But many of us are not far

behind, trying to isolate a few roomfuls inside our homes.

Buildings are almost hermetically sealed, and we dash into them

for relief, relying on filters and fans to provide an oasis of

mechanically laundered air. (Recirculating as it does in tight houses,

with every dash of aerosol spray along for the ride, you can't really

say the air is fresh.)

Filter hype

To beat the bad odors and take a crack at the mysterious things

that have no odor, such as mold spores and chlorofluorocarbons, some

consumers opt for expensive filters that are supposedly dense enough

to catch almost everything. Except they don't. And the denser they

are, the less conditioned air you get through them.

Stuff a few pillows down the ducts and hardly a speck of

pollution--or air--will get through, and your fan will start to pull

an unconditioned supply through cracks around windows and doors.

That's why people in old houses with undersized furnaces

sometimes remove the dust filters during a cold snap. They get more

dust temporarily, but more heat flow too.

Save your money and discount the ringing personal endorsements

in some filter advertising. There is no such thing as an almost

microscopically fine air filter that won't get in the way of the air

and reduce your comfort level unless you boost the thermostat and

install a larger blower. All filters get in the way; that's what they

are designed to do. But some offer reasonable screening of pollutants

and dust for only a small loss of air flow, while filters at the

extremes of high and low density generally do not. Too dense and they

stifle the supply; too porous and most pollutants breeze right

through.

Clean ducts

Before you go shopping for a filter, consider these three steps

to support the work of whatever filter you buy. First, clean the

supply ducts so you don't push sanitized air through a dirty delivery

system. Second, reduce the number of pollutants produced inside the

house from caustic cleaners, sealers, adhesives and solvents. Read the

label cautions and you'll know which ones to do without. Third, supply

the house with an adequate flow of fresh air. Crack a few windows even

if it adds a few minutes of running time to the air conditioner, or

install an air exchanger that transfers temperature from conditioned

exhaust air to an unconditioned intake from outside.

In most older houses, you don't have to worry about the air

exchange rate; there are enough leaks even with the windows closed.

But most fans and filters can't cope with dead air in a house so

tightly sealed that it exchanges the indoor air once or twice a day

instead of every few hours.

The goal of efficiency

When you look at filtering systems, check the manufacturer's

literature for three key pieces of information. First, look for the

percentages of different types of pollutants removed from an air

stream, particularly if there is one culprit that causes allergic

reactions in your household.

Second, check the amount of air (in cubic feet per minute,

CFMs) the unit handles, and match that capacity to your space--with a

central system or a portable. For example, a portable system that

removes a high percentage of particles but handles a low volume of air

is not a good choice for a large area. If the unit cleans 10 cubic

feet per minute, it would take more than 2 1/2 hours to deal with

pollutants in a typical 12-by-16-foot room.

Finally, check the section on maintenance and decide if the

manufacturer's recommendation of once-a-day or once-a-month cleaning

is a job you will remember to do. If you don't, system efficiency can

drop dramatically, which is a good reason to invest in a

large-capacity, mainly self-cleaning system such as an electronic air

cleaner with a built-in filter-washer. These machines offer the least

resistance to the air supply because they push pollutants against a

row of electrically charged metal fins instead of through fine mesh.

Most central systems remove more than 90 percent of airborne

pollen and more than 80 percent of smoke particles and many other

irritants and pollutants you can't see. The only drawback is a

bug-zapping sound when larger particles make contact with the

fins--something you'll notice if the main blower is near a bedroom.