November, 2011

Inspection News and Views from the American Society of Home Inspectors

Fall is Here: A Season for Indoor Air Quality Problems


Pest infestations can cause indoor air quality problems and even lead to health symptoms. A recent study1 drew a correlation between elevated indoor levels of allergens from mouse urine and exacerbated asthma symptoms. Mice can be smelly enough, but they pale in significance when compared to shrews, which eat meat and thus follow mouse delicacies indoors.

In the last few years, I've been encountering more and more complaints about the skunk-like chemical odor that shrews impart to their nests due to their body musk and the musty odors from their festering piles of damp, moldy fecal matter. One family sealed up a room in their home. Another family abandoned their finished basement.

In another home that had been unoccupied for several years, the finished basement reeked of shrews. The buyer decided to demolish the space and found dead shrews in the wall cavities. Fall is the season for mice to enter houses, and shrews may not be far behind (always check above the inner bulkhead door for openings — a favorite rodent-entry point).

Soon, heating systems will begin to run. If a furnace has the typical, inadequate fibrous "see-through" filter, there's a good chance that the blower cabinet and cooling coils are mold-contaminated. Even in new construction, an air-conveyance system can be contaminated with mold before anyone lives in the house. Why? Because the system was operated during construction and biodegradable sawdust soiled the returns, liners and cooling coils. In the cooling season, the dust on the coil, in the condensate pan and in the fiberglass liners near the coil get damp and microbial growth (first bacteria, then yeast and mold) ensues. In the fall, the heat kicks in and all the microbes dry out, making them dispersible. Even though the organisms are dead, unfortunately, they remain potentially allergenic.

What about hot water and steam systems? Baseboard convectors and radiators in older homes are often covered with biodegradable dust that can include sawdust, skin scales, pet dander, mold growth and insect fecal material.

Even for those without allergies, as a matter of simple hygiene, before moving in a new owner should vacuum these surfaces, preferably using a vacuum with a high-efficiency particulate arrestance (HEPA) filter. The cover panels of baseboard convectors should be removed so the tops and bottoms of fins can be cleaned. If there is inaccessible dust between the fins of baseboard convectors, the fins can be cleaned with detergent and water in a spray bottle.

Air conditioning isn't used this time of year. Unfortunately, though, all wall A/C units and many window A/Cs remain in place year after year, or are removed and stored in a musty basement or garage over the winter. Most such older units are full of mold growth due to inadequate filtration (just look for black spots on the blower blades or vanes of the supply louver). Before being operated, a portable A/C should be disinfected and cleaned. The filter should be cleaned and supplementary filtration material attached to the unit over the interior air intake. But frankly, given the price of portable A/Cs, it just might be easier for your client to say "no thanks" to the seller's offer to leave portable A/C units at the site and buy new ones.

As the weather turns cooler, people shut their windows and spend more and more time indoors. In the heating season, up to a third or more of the air in a house comes up from a basement or crawl space — even more, if there's a basement return present. Basements – both finished and unfinished — that have not been adequately dehumidified in the humid season can be full of non-visible mold growth. What is adequate dehumidification? Keeping the relative humidity below 50 percent in unfinished basements and 60 percent in finished basements between mid-April to mid-October.2

Again, to control the relative humidity and minimize the chances of mold growth, finished basements also must be adequately heated during the cooler months, with the thermostat set at a minimum of 58°F even when the space is not in use.

Properly ventilated crawl spaces typically are full of mold growth (see And if basements or crawl spaces contain exposed fiberglass insulation, in addition to mold and mold-eating mites, they can be infested with rodents, who love to nest in the stuff (look for finger-sized tunnels and droppings).

As a home inspector, here are some of the conditions that could suggest indoor air quality problems and that could use some further evaluation:

1. "Exposure to Mouse Allergen in U.S. Homes Associated with Asthma Symptoms"
Päivi M. Salo, Renee Jaramillo, Richard D. Cohn, Stephanie J. London, and Darryl C. Zeldin, National Institute of Environmental Health Sciences, National Institutes of Health, Department of Health and Human Services, Research Triangle Park, N.C.; Constella Group, LLC, Durham, N.C.

2. Relative humidity in basements.

This article is from the May Indoor Air Investigations Newsletter, October/November 2011. Reprinted with permission of Jeffery C. May. © 2011 Jeffrey C. May. Visit