March, 2018
Inspection News and Views from the American Society of Home Inspectors

INDOOR AIR QUALITY: Who or What Are the Real Culprits in “Sick Homes”? My 600/300Study


A number of years ago, I did a study of 600 “sick homes” (SHs) in which occupants suffered heath symptoms that could be associated with their home environments. In these homes, I took air and dust samples, and examined the samples by microscopy to determine what possible allergenic particles might be present. Over the years, I have analyzed more than 35,000 samples from homes, schools and offices.

More recently, I compared some characteristics of these SHs with a randomly selected group of 300 “control homes” (CHs) in which I had done pre-purchase ASHI home inspections for buyers. I reviewed the observations from my written reports on all 900 of the SHs and the CHs, and the results of this study are, I believe, quite significant. (An earlier version of this study was published in Johanning, E., ed. Bioaerosols, Fungi and Mycotoxins: Health Effects, Assessment, Prevention and Control. Eastern NY Occupational and Environmental Health Center: Albany, NY; 1999.)

The health symptoms that occupants suffered in the SHs included asthma (26%) and allergies or respiratory complaints, congestion and chronic fatigue (74%).

I considered the following characteristics
for the home comparisons:

basement moisture stains

wall-to-wall carpeting

finished or carpeted basements

central air conditioning (A/C)

forced hot-air heat

Basement water stains were somewhat more common in the CHs (62% versus 52%); wall-to-wall carpeting was slightly more common in the SHs (63% versus 52%). Forty-one percent of the SHs had hot-air heat, compared with only 29% of the CHs; and 44% of the SHs had finished or carpeted basements, compared with only 31% of the CHs. This led me to conclude that, in homes with hot-air heat or carpeted, finished basements, occupants were somewhat more likely to suffer health symptoms.

Air Conditioning
Thirty-seven percent of the SHs had central A/C, as compared with only 19% of the CHs—a significant finding, suggesting that in a home with central A/C, occupants are about twice as likely to have environmentally associated health symptoms.

The American College of Occupational and Environmental Medicine (ACOEM) estimates that approximately 10% of people are allergic to mold. At the time of my study, which involved homes in the Boston area, the U.S. Census reported that about 19% of homes had central A/C, in agreement with my findings. But nationwide, more than 90% of newly constructed homes have hot-air heat (that is, furnace or heat pump) with central A/C. As A/C proliferates, so too can mold allergy symptoms.

In 81% of the SHs, I found elevated levels of airborne fungal spores from three categories of mold, typically Penicillium species, Aspergillus species or both (referred to collectively as Pen/Asp spores, as they cannot be distinguished by microscopy); Cladosporium species; or a combination of these three. Correspondingly, a majority of the A/C systems in SHs were contaminated with mold growth—most often on or near cooling coils—with the same three categories of mold predominating. (In a 1996 article titled “Fungal Colonization of HVAC Fiberglass Air-Duct Liner in the U.S.A.,” in which 1,200 samples of liner from across the country were analyzed, microbiologist C. Yang found that about one-half the samples he received for analysis were colonized by species of Penicillium, Cladosporium fungi, or both, among others.)

Why Is Mold Growth So Common in A/C Systems? 
Building dust accumulates wherever there is air flow; in homes, most of this dust consists of biodegradable particles (for example, skin scales, pollen, cellulose fibers and pet dander). Biodegradable dust accumulates on cooling coils and lining materials at the interior of the A/C equipment. The air coming off a cooling coil (depending on the part of the country) is often humid and may even entrain droplets of condensed water. Depending on the level of moisture in the air, microbes (for example, bacteria, yeast or mold) begin to grow in the dust. The growth of these microbes is what accounts for the musty (or “sweat sock”) odor that occurs briefly (or persistently!) when the A/C is turned on.

Mold balls growing on A/C coil. May Indoor Air Investigations LLC.

The only means to prevent this dust accumulation in A/C equipment is the use (and maintenance) of efficient filtration. In 1987, the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) adopted a quantitative method for rating filtration efficiency called “minimum efficiency reporting value” (MERV). The scale is based on a logarithmic scale and runs from 1 to 16, so a MERV-6 filter is many times more efficient than a MERV-1 filter. 

ASHRAE recommends that all A/C equipment employ a filter with a rating of at least MERV-8. For comparison, a typical fiberglass furnace filter (the type you can see through) has a rating of about MERV-3. Most A/C systems that I’ve inspected have had MERV-3 filters; however, this is changing as more people are opting to use more expensive, pleated-media filters.

The efficiency of any filter can be compromised if unfiltered air can bypass the filter. Bypass is common in many systems because filter-access openings often are uncovered or because improperly sized filters are installed. In addition, the increasingly popular mini-split systems only come equipped with the equivalent of an insect screen for a filter, which is well below the MERV-8 ASHRAE recommendation. The filtration for window A/Cs is typically no better. Thus, the accumulation of biodegradable dust and subsequent growth of microorganisms in cooling equipment are almost inevitable. The only means of prevention is vigilant maintenance and, where feasible, efficient filtration.

In many A/C fan coils that were installed in a vertical position, the filter sits just below the cooling coil. In humid regions, when the blower is off, water may drip from the coil onto the filter, resulting in mold growth on the filter and frame. 

Mold on filter frame that was installed under an A/C coil. May Indoor Air Investigations LLC.

Manufacturers of electronic filters only recommend quarterly cleaning of the equipment; however, filter components really need monthly cleaning. When the components in filters become soiled, the efficiency drops dramatically. In most systems with electronic filters, the dust buildup in the blower cabinet (behind the filter) is equivalent to the dust buildup in the return duct (in front of the filter). 

"Electronic filtration works, but only if the equipment is vigilantly maintained." 

Because few homeowners will dismantle the equipment and run the filters through the dishwasher every month, I always recommend that they replace the metal components with a disposable MERV-11 pleated-media filter. Most manufacturers provide such filters to fit in the electronic-filter case. (I always recommend that families who deal with allergies use a filter with at least a MERV-11 filter rating. A HEPA filter has a rating of MERV-16. Higher-rated filters consist of pleated, paper-like fiberglass sheets.)

In colder climates, attic ductwork and attic fan coils for A/C-only systems can experience condensation of moisture from warm house air that flows by convection into the duct system in the winter. This can result in mold growth in the accumulated dust. Tell your clients that the supplies and returns for all such units should be closed for the winter.

The problem areas for heating systems with furnaces are typically in basements or crawl spaces where conditions can lead to high relative humidity (RH) in returns, blower cabinets (particularly those resting on concrete in basements that are not dehumidified) and return ducts. In homes in which RH is not controlled, humid air in blower cabinets can lead to mold growth in the dust. Also, crawl-space temperatures may be below the dew point of the ambient house air, leading to condensation and mold growth in crawl-space ducts. 

When central humidification systems are present, they often are the type with water reservoirs. Dust from poorly filtered air flow collects in the reservoir or on the evaporative pad, serving as a nutrient for microbial growth. On the other hand, the newer, once-through evaporative humidifiers with aluminum pads seem to work well. 

Rotating-pad humidifier tray with microorganisms. May Indoor Air Investigations LLC.

Finished Basements
I have probably taken dust samples from more than 1,000 basement carpets. I would guess that no more than a dozen were free of mold growth because most basements are not adequately dehumidified. The perimeter of the basement where the concrete floor meets the foundation walls will always be cooler than the ambient air; sometimes there will be condensation in these areas. 

I always take two samples of basement carpet and I typically take them from opposite ends of the basement near the foundation wall. Sometimes, there is mold growth in only one of these areas. (Samples from the middle may contain a few settled spores, but not evidence of actual mold growth—that is, with chains of spores, and hyphae—because the RH in the carpet at the center may not be as high as at the perimeter.)

Below is a scanning electron micrograph of a carpet fiber from a new basement carpet. I took the sample in an area in which the carpet had been dampened by leakage from an overflowing A/C condensate pump. The carpet fiber is at the bottom of the image; there are mold hyphae crossing the fiber’s surface and spores (round or oval) in the upper portion of the micrograph.

Mold growth on a carpet fiber in basement. May Indoor Air Investigations LLC. 

Many finished basements have baseboard convectors. Any rust on the sheet metal is an indication that there has been excess RH and possibly condensation. (The temperature of the boiler water in the fin tubing can be below the dew point of summer air.) I often have found significant mold growth in the dust trapped between a heating unit’s fins, so I recommend to clients that they eliminate all dust from basement baseboard convectors by removing the cover plate, HEPA- vacuuming all surfaces, placing cloths beneath the fin tubing to capture grime and blasting the fins with steam from a high-pressure, steam-vapor machine.

The paneling near the floor in finished basements often is covered with moldy dust, which cannot be seen easily by just looking at the paneling from the front. Shine a bright flashlight at a glancing angle if there are many spider-silk tracks and any colony-like dust spots; there could be mold growth present. Although most home inspectors do not report on mold in their reports, I believe they should at least advise their clients that the paneling should be cleaned of all dust. If the mold growth is extensive, the cleaning may have to be done by professionals, working under containment conditions.

Mold growth on basement paneling. May Indoor Air Investigations LLC.

Unfinished Basements
Here again, I often find mold growth due to poor control of the relative humidity (RH). Such growth can occur on joists, subfloor, foundation walls (particularly at lower few feet and in cold corners) and on stored goods, especially on surfaces facing the concrete slab or foundation walls. 

Exposed fiberglass insulation also can become moldy because the mold is subsisting on dust captured in the fiberglass fibers; the dust can include sawdust if the basement contains a shop or if wood was ever sawed in the basement. (In another study I did, I found that there was significant mold growth in almost half of the 42 samples I took from exposed-basement and crawl-space insulation—enough growth so that tens of thousands of spores could be aerosolized by simply pulling down a small section of contaminated insulation.) Mold in fiberglass insulation is rarely visible. Wear an N95 mask if you move basement insulation and keep your clients away!

Control the Relative Humidity (RH)
Homeowners rarely control the RH below grade adequately, so conditions may be favorable for mold growth, as some species of mold can grow above 80% RH. Even a finished ceiling (particularly acoustical tiles or drop ceilings) that faces the cold floor can acquire significant nonvisible mold growth (though sometimes visible as small yellow spots).

Mold growth on acoustical tile ceiling in a finished basement. May Indoor Air Investigations LLC.

In unfinished basements, the RH should be kept at no more than 50%. In finished basements (within insulated walls), the RH should be kept at no more than 60%. The RH should be measured with a thermohygrometer (in a remote corner), independent from the dehumidifier.

Final Words
It’s worth asking your clients if they or members of their families have allergies or asthma. If they do, inform them about mechanical equipment maintenance and the importance of controlling RH below grade. If you don’t want to deal with such issues, include information that provides clear exclusions regarding indoor air quality in your report.

Jeffrey May is the principal scientist, May Indoor Air Investigations LLC, Tyngsboro, MA. Contact Jeff at 978-649-1055, or visit