Most of you perform residential carpet cleaning and some of you add duct cleaning as a diversification.

What you do, as well as how you do your job, can have a significant effect on the health of home occupants.

Several years ago, I completed a study of Boston-area homes in which I compared the characteristics of 300 randomly chosen homes to 600 homes in which occupants were experiencing respiratory problems.

I found that people living in homes with either central air conditioning or carpeted, finished basements were almost twice as likely to suffer respiratory health symptoms. What's the connection?

Breathable air particles

Every time we breathe, we inhale airborne particles along with the air.

Outdoors, depending on the season, these particles may include pollen, mold spores, soil minerals, combustion soot and even rubber tire particles.

Some of these particles can cause sneezing, itching eyes and other outdoor allergy symptoms.

Outdoor particles may be found indoors if windows are open, but generally the particles in indoor environments are different.

Believe it or not, the most commonly found particles indoors come from people.

These particles consist of microscopic skin flakes which we all shed at rates of between several hundred thousand and millions per minute, depending on our level of activity.

If pets are present, dander flakes from dogs or cats or birds may join the mix, and if mold growth is present, elevated numbers of spores may also be present in the air.

Microscopic lint fibers from clothing and paper are also found floating in air.

Eventually, most of these airborne particles, called aerosol, settle out, some in minutes and some in hours, to become what we call house dust.

Although most of the smallest particles we breathe indoors come from the outdoors, most of the larger particles in indoor air come from the aerosol that has settled out of the air into "fleecy" surfaces like carpet and furnishings.

HVAC system problems

The HVAC systems that supply heating and cooling can be additional sources of collected indoor aerosol.

Over time, these settled dust particles can be consumed, digested or turned into fecal matter by a variety of living organisms like microbes and mites.

For sensitized individuals, those who have allergies to specific substances, these transformed particles can be re-aerosolized and can cause health symptoms when inhaled.

If you are involved in the maintenance or the cleaning of dusty surfaces and systems, you should understand the correlation between health symptoms and microbial growth indoors.

Aerobic microbes, like mold and most bacteria, require three things to grow: Oxygen from the air, moisture, and nutrients, such as house dust, food spills or paper products.

Moisture can come from pipe leaks, basement floods, and even directly from water vapor in the air.

The relative humidity is a measure of how available moisture vapor is from the air.

Some mold can grow indoors when the relative humidity is in excess of 75 percent. In other words, surfaces don't even have to be wet for these molds to grow.

Most bacteria, on the other hand, require more than 95 percent relative humidity to grow.

Drying challenges

Under appropriate conditions, microbial growth can occur within 24 to 48 hours.

Thus, it goes without saying, though I find myself saying it often enough, that carpet should be completely dry within 24 hours after being cleaned.

In below-grade spaces, where air flows can be minimal and relative humidity elevated, dehumidification and additional air flow from fans may be needed to accomplish timely drying.

Opening windows in very humid weather may actually retard drying.

Dust accumulation and IAQ

Where air is flowing, dust can accumulate.

So the return ducts, blowers and coils in heating and cooling systems are prime targets for big dust build-ups.

Nearly every return duct, air-conditioning coil, condensate tray and fibrous liner that I have ever seen was covered with dust.

This build-up is not inevitable and occurs primarily because the filtration is inadequate.

In air conditioning systems, having dust present is like putting out seeds for birds — except, in this case, it is food for microbes!

Since many components in an air-conditioning system are either physically wet or exposed to high humidity, dust from almost each one of the thousands of fan coil units I've sampled was contaminated with active growth of microorganisms like bacteria, yeast and mold.

That's why, at first, the air from most of these systems presents a malodor.

The odor and other by-products of this growth can be carried on air flows into habitable areas. A contaminated air conveyance system is thus an enormous source of bio-aerosol — the aerosol resulting from microorganisms.

If the air conveyance system also supplies heating and is thus on for the majority of the year, bio-aerosol can cause health symptoms in winter as well as summer.

Improve HVAC with cleaning and maintenance

In my opinion, HVAC equipment with microbial contamination — rather than "tight buildings" or inadequate ventilation — is the cause of the majority of indoor air quality (IAQ) problems in "sick buildings" in which occupants suffer from allergies, asthma, chronic coughs, and even hypersensitivity pneumonitis (HP), to name just a few.

All the surfaces of a soiled air conveyance system — not only the ducts but also the blower and air conditioning coil and condensate pan — must be dealt with when the system is cleaned.

If a dusty blower is downstream from the AC coil, it, too, can get damp and moldy. Just cleaning the ducts of a contaminated air conveyance system is unlikely to solve an indoor air quality problem.

Principle of filtration

How do you prevent microbial-growth problems from reoccurring?

In real estate jargon, advice to the buyer is often summarized in the terse phrase "location, location, location."

If there were comparable advice for a building owners and maintenance personnel with air conditioning systems, it would be "filtration, filtration, filtration."

Yet few — if any — building owners, maintenance personnel or even HVAC contractors are aware of the function of filtration.

Many a homeowner has expressed to me the notion that a filter will somehow keep the air he or she breathes cleaner.

There is some truth to this belief, but this is not the primary function of filtration.

And judging from the installation and maintenance inadequacies I all too often see, most technicians must feel that filtration is nothing more than an annoyance.

For example, many filters are installed into holders that aren't airtight. The filters are often too big or too small, or they get crushed or hung up on the thoughtfully placed, sheet-metal screws.

Optimizing your filtration

Optimizing filtration is the most important tool in preventing IAQ problems and health symptoms stemming from a contaminated HVAC system.

The typical fiberglass filter installed in an AC system is totally inadequate. How can a filter you can see right through collect microscopic particles?

Even worse, the presence of such a filter provides a building owner with a false sense of confidence.

The only way to keep the blower, AC coil, condensate pan and fibrous lining materials near the coil clean is with MERV (Minimum Efficiency Reporting Value) 8 filtration, such as that provided by most media filters; MERV 11 filtration is even better and should always be recommended for environments with sensitized individuals.

Filter enclosures must be airtight to the exterior, and filters installed so that no air bypasses them. If these steps require the use of more powerful blowers, so be it.

Up to now, a desire to preserve energy and energy costs has driven HVAC design.

Your goal in the industry

Now it's time to realize that failing to take steps to provide and maintain healthful IAQ can be even more expensive in terms of human health.

From the IAQ perspective, the HVAC industry has a long way to go before newly installed equipment will provide adequate filtration, and an even longer road to travel before making existing installations safe, in terms of health considerations.

No gimmicks such as UV irradiation, bypass HEPA filters (that deal with only a small fraction of the air stream), or chemically treated filters will ever substitute for the real thing: Clean fan coil surfaces, free of growing microbes, provided by efficient filtration and intelligent filter maintenance.

I encourage you to sell only pleated media filters, because electronic and electrostatic filters are never cleaned often enough.

Be sure the filter holders are airtight. If there is no access cover, use duct tape!

To keep return ducts cleaner, encourage the installation of coarse pre-filters.

Help your clients understand how HVAC systems work and the importance of regular, professional maintenance.

Since nearly every cubic foot of air in a building with an air conveyance system has been blown over some type of coil or coarsely-lined surface, and since that air will then end up in occupants' lungs, the health of every occupant in such buildings is the responsibility of building designers, engineers and maintenance personnel.

Let's hope that the maintenance community wakes up to this realization before the legal community does.

Carpet and IAQ

Carpeting has much more surface area than hard flooring does, and thus more surfaces on which dust can settle.

Carpet in rooms with water, like kitchens and bathrooms, or carpet near entrance ways where people walk with wet shoes, and in cold, exterior facing closets or in finished basements where the relative humidity tends to be higher, are prone to microbial growth.

And once there is significant growth of bacteria or mold in carpet dust, that growth can never be fully removed, whether by vacuuming or cleaning.

When a contaminated carpet is disturbed by foot traffic, allergens are aerosolized and, if inhaled, can lead to allergy symptoms in sensitized individuals.

Unfortunately, even when dead, the by-products of microbial growth can remain allergenic.

— J.M.

Jeffrey C. May is author of My House is Killing Me! (2001), The Mold Survival Guide (2004), My Office is Killing Me! (2006), and Jeff May's Healthy Home Tips (2008), all published by the Johns Hopkins University Press. He obtained an M.A. in organic chemistry from Harvard and is principal scientist of May Indoor Air Investigations LLC, Tyngsborough, MA, a company that investigates mold, moisture, odor, and indoor air quality problems in buildings. Jeff can be reached at (978)-649-1055 or jeff@mayindoorair.com. Visit his website at www.mayindoorair.com.