May, 2017
Inspection News and Views from the American Society of Home Inspectors

Discovering Pests and Housing Pets


ou may have heard of Jeff May, a former home inspector; perhaps you’ve listened to one of his presentations at InspectionWorld® over the years. If so, you’ve been exposed to his passion for maintaining healthy levels of indoor air quality.

Or perhaps you’ve read or shared one of the books he’s written for homeowners, including My House Is Killing Me! The Home Guide for Families with Allergies and Asthma and Jeff May’s Healthy Home Tips. These excellent resources offer guidance about maintaining a healthy indoor environment, even when dealing with the pets and pests that can be a part of home life. Both books are available on

Jeff also maintains a website ( and publishes newsletters for home inspectors (send a request from the site: The following article is an adapted compilation of two such recent articles (“Pets and Pests,” ©2014, and “IAQ and Pests,” ©2017). The topics relate to discovering pests in the home or during a home inspection, and managing the downside of having pets when striving to achieve healthy indoor air quality.

Some aspects of what Jeff describes may make your skin crawl a bit, but as home inspectors, you can no doubt relate to the reality of making unpleasant discoveries of pests in a home. Pointing out these “realities” to clients is important. As Jeff explains, “Pests that are found in people’s homes can have a negative effect on the indoor environment.” Read this article to learn from Jeff’s extensive experience in the area of maintaining a healthy indoor environment and remind yourself of the importance of bringing these issues to the attention of your clients.

As you know, the presence of carpenter ants may indicate excess moisture and rot somewhere in the building. You’ve also seen the damage that squirrels and raccoons can do indoors. And most of you can spot the redstained urine trails of mouse runways along basement pipes or foundation walls.

But what you may not know is that pests can cause health symptoms. For example, all insects and arachnids (like mites)—in other words, “bugs”—produce fecal matter that can be allergenic.

What are some pests commonly found in houses that can have a negative impact on indoor environmental conditions? There’s a small food chain in the dust in buildings, including in homes. This chain contains mites, moths and other small (if not microscopic) creatures. One of these is a book louse, which is small but still visible to the naked eye.


Mice are well-known indoor pests. A fairly recent medical study concluded that elevated levels of mouse urine indoors can lead to increased asthma symptoms. Eighty-two percent of 800 homes tested had detectable levels of mouse allergens—it was highest in kitchen dust. Sensitized occupants in houses with elevated levels of mouse allergens had double the risk of developing asthma symptoms.

These unpleasant animals are about the same size as mice, but they have claws and longer snouts, and they eat meat instead of grains and seeds (you trap them in mouse snap traps with freeze-dried grubs or beef jerky rather than with peanut butter). Some species even have poisonous bites that paralyze their prey. Shrews are beginning to invade homes, perhaps because they are on the hunt for mice.

I found out about these unpleasant animals when I investigated a building odor in a condo complex in Vermont. The odor saturated one room and an adjacent hallway in a lower-level condo. Air flows carried the odor up into the unit above, where the owners had placed boxes of baking soda on every stair tread in the hopes of reducing the smell. It hadn’t worked. I found some suspicious-looking (and odoriferous) fecal matter on the hallway carpet in the lower unit and I sent it to the State of Vermont for identification. They confirmed that it was “shrew scat.” I found the entry hole at the exterior, outside the smelly room in the lower-level condo unit.

Shrews defecate and urinate in the same place, and their fecal matter (as well as glands on their bodies) emits an odor so powerful that people have abandoned rooms in their homes.

Another example: I investigated a school in which two kindergarten classrooms had been abandoned due to an odor. The culprits again were shrews, which had set up their domicile above the drop ceiling. When I looked above the ceiling tiles, I saw piles of shrew droppings on top of the poly vapor barrier—there was the source of the smell!

Asian ladybugs
I live in a fairly new house. The first fall after we moved in, these bugs were crawling all over the siding. Every time I opened a door or window, I would find a few crawling around in the track. It was like some Hitchcock horror movie. They may look cute, but they leave red stains and smell terrible when crushed, so if you find them in your own house, resist the urge to bash them. Vacuum them up instead. In addition, there is cross-reactivity between Asian ladybugs and cockroaches, so someone exposed to the droppings of only one of these pests can become allergic to both.

It’s common knowledge that dust mites, which live primarily in beds and often-used cushioned furniture, cause allergies and asthma symptoms. I’ve even found dustmite infestations on fish tank covers (all that moisture and those delicious fish food flakes!).

But what about all the other mites, like mold-eating mites and predator (miteeating) mites, which also live in buildings? People living in a house with a basement full of mold may be exposed to mold as well as mite allergens, and consequently, they may have symptoms due to this exposure. Allergists only test for two different types of house-dust mites, so someone could test negative to mites and yet be allergic to mold-eating mites.

Why should home inspectors care if wool moths are nibbling on a sweater or rug? The house will be emptied out anyway, prior to sale, right? Agreed. But what if the seller’s wool wall-to-wall carpeting is infested? Then the moths can infest the new occupants’ wool clothing. And moth larvae produce allergenic fecal material.

What if the occupant has battled moths by using mothballs? Mothballs are semi-volatile pesticides. In my opinion, no pesticides, including bee and wasp sprays, should be used indoors.

Just as shrews are above mice on their food chain, spiders are at the top of a smaller food chain. Spiders only eat live prey, so if there are a lot of spiders in a building, there is an insect problem on site. If you find spider webs hanging like netting from joists in an unfinished basement ceiling with exposed fiberglass insulation, it most likely means that there are mites present and the spiders are waiting. And what are the mites eating? They may be dining on invisible mold, growing in the biodegradable dust captured in the fiberglass fibers.

Some molds can grow in places in which the relative humidity (RH) is over 80%. Below-grade and partially below-grade spaces are naturally cool; as air cools, its RH rises, so such spaces are prone to developing conditions of high RH that can lead to mold (mildew) growth.

The fiberglass insulation may look as clean as it did on the day it was installed, but there’s a food chain there, with spiders at the top.

Prevention and Treatment
Whether or not you do pest inspections, I encourage you to recommend that your clients prevent mouse and shrew infestations by sealing up gaps and openings at the exterior and in the garage, and by being sure that the crawl space and roof vents have intact screens (or buy hardware cloth and screening), rather than putting traps and poison all over the place.

In the humid season (in New England, for example, generally mid-April through mid-October), the RH in below-grade and partially below-grade spaces should be kept at or below 50% in unfinished spaces and 60% in finished spaces. In addition, finished basement spaces must be heated adequately in winter (with the thermostat set consistently at a minimum of 58БЛF, whether the space is being used or not)—again, this strategy can help control the RH and minimize pest infestations. (Even if a basement has never experienced water intrusion or leaks, it can be extensively infested with mold and critters due to elevated RH.)

There’s growing concern, and rightfully so, about using chemicals to combat pests. People should dry-clean wool clothing and store items in plastic rather than contaminate a closet or clothing chest with mothballs. Carpets and rugs can be treated with steam vapor from a steam-vapor machine.


Cats and Dogs
Cats and dogs can be problematic for families with allergies, asthma or both. Even if no one in the family has pet allergies, such animals are living dust mops that can carry pollen and other allergenic material into the house. If a cat or dog goes into a moldy basement, the animal also can carry mold spores upstairs.

I did one investigation in a house in which I found Penicillium  mold in the basement. I found the same mold spores on a bed pillow in the master bedroom. The homeowner kept the kitty-litter box in her musty basement and the cat used to cuddle next to her on the bed. That cat offered her a lot more than affection, that’s for sure! If you have pet allergies, it’s best not to own a cat or dog. If you have a beloved pet, however, keep the animal out of moldy spaces and don’t let the animal sleep in your bedroom.

This brings me to the topic of dog beds. Some people buy expensive, cushioned pet beds for their dogs and then don’t clean the beds or replace the beds for a decade or more. Such a bed can become infested with dust mites, and then the dog carries dust-mite allergens in its fur or hair—this is not healthy for people and not healthy for dogs (dogs can get asthma from exposure to the mite allergens). Clean the dog bed as directed, if not more frequently. Consider using a blanket for a dog bed and wash the blanket on the same schedule as you wash your own bedding.

Some parents of children with allergies or asthma choose to have fish tanks as a safer alternative to a pet cat or dog. Think again.

A retired surgeon hired me to investigate conditions in his home because whenever he spent time in his den, his eyes watered and his nose ran. “I couldn’t have operated on patients,” he said to me, “if I’d had this problem back then.” He had two fish tanks in the room. The tanks were full of beautiful tropical fish. He spent a lot of time looking after the fish and the tanks, but when he fed the fish, some of the fish food spilled out over the tank covers. Fish food flakes are protein—welcome food for dust mites. And all that warmth and moisture! Dust mites were crawling on the covers and whenever the homeowner lifted the cover to feed the fish, he was exposed to dust-mite allergens. He cleaned the tanks and the room dust, and his runny nose disappeared.

I also don’t like to see fish tanks in children’s bedrooms. If a fish tank is kept in a bedroom (or even in a classroom), however, someone should maintain the tank’s exterior to be sure it stays scrupulously clean.

The Ghosts of Pets Past
What if a family moves into a new home and someone in the family begins to experience allergy or asthma symptoms? The family member may have pet allergies, but the family has never owned a pet. Or, someone might have dust-mite allergies, but the family always has been careful about having dust-mite covers on their mattresses and bed pillows. What might be the problem?

If the previous homeowner had a cat, dog or bird, pet dander most likely is still in the house—in the dust on radiators, on baseboard heating convectors, in heating and cooling ducts or in wall-towall carpeting.

Before moving in, a new homeowner should clean thoroughly all surfaces in the house. Radiators should be vacuumed with a high-efficiency particulate air (HEPA) vacuum and treated with steam vapor (this is not the same as steam cleaning) from a steam vapor machine. Baseboard heating convectors should be cleaned similarly; this process will necessitate removing and cleaning the covers. Metal ducts should be cleaned professionally; flexible ducts (if very dusty) may have to be replaced. Any wallto-wall carpeting should be removed. If there is only subfloor beneath, the plywood should be vacuumed and painted before new carpeting or other flooring is installed. Don’t leave carpeting in hallways or on stairs (including stairs leading from the first floor to the basement); this carpeting, too, might be contaminated with dust-mite and pet allergens.

A side note of caution: Remember that people can carry pet and dust-mite allergens on their clothing and hair, so if you have allergies and find yourself starting to cough while sitting in a public place next to someone you don’t know, you might want to find another seat.

The issues of pests and pets may be a rather “creepy” subject related to home inspection, but it is an important subject nonetheless. When conducting home inspections or indoor air quality investigations, it’s crucial to notice the small “details” left behind by pests and pets because they often reveal a potential problem. Visible evidence of pets and pests in a home or a building should be of interest to home inspectors and air quality professionals alike, as well as prospective homeowners.

Jeff May is the principal scientist, May Indoor Air Investigations LLC, Tyngsborough, MA. Contact Jeff at 978-649-1055,,