Microbial growth in carpets usually stinks. If inhaled, the organisms that cause these odors - bacteria, yeast and/or mold - can also cause allergy and/or asthma symptoms.
Microorganisms require oxygen, a nutrient source (biodegradable dust, spilled foods, etc.), and moisture to grow. All carpets contain trapped dust, and when moisture is introduced into the dust collected between carpet fibers, microbial growth can start within hours. Under the right conditions, within a day or two, microbial growth, particularly mold (since bacteria and yeast require wetter conditions), can spread throughout the trapped dust. Schools with carpets are therefore at risk for the development of mold growth in carpets.
Where does the moisture that mold requires come from in school buildings?
Moisture in the liquid state (water): Carpets installed in or near entranceways can be dampened by foot traffic, and in classrooms, children can spill liquids onto the carpets. Water from roof or plumbing leaks can also wet carpets. If there is no moisture barrier between a carpet and a concrete slab in contact with soil, water can move through the concrete, dampening the carpet. In addition, it’s common for school carpets to be washed in the summer, when the building is not in use. The carpets often remain damp for extended periods of time, due to hot, humid weather, or a lack of ventilation. In one elementary school in Massachusetts, the carpets were washed when the relative humidity (RH) outdoors was over 80%. The classroom windows and doors remained closed, and the rooms began to smell musty as mold grew in the carpets. When the school system’s superintendent was informed of the problem, he had the carpets removed.
Moisture in the vapor state: The presence of visible, liquid water is not always a necessary prerequisite for mold growth, because some mold species can grow when the RH is in excess of 80%; the dust takes the moisture from the air the way salt does when it gets sticky in humid weather. Since the temperature of the soil near the surface of the Earth is 55°F, crawl spaces and below-grade spaces are naturally cool. A concrete floor slab or foundation in contact with the soil might be 65°F in the summer. In many parts of the country, the dew point on humid days can be as high as 70°F, so air in or near carpets laid on a concrete slab below-grade or even on concrete above a crawl space can experience an RH over 80% (or condensation may even occur). Then the carpets can become moldy. In addition, school buildings that are closed in the summer are often neither dehumidified nor air-conditioned, so humid conditions can develop indoors.
Unfortunately, most school budgets are inadequate, so maintenance repairs are often postponed. For example, carpets and pads that have been soaked by flooding are all too often dried in place with fans. Since drying only with fans can be very slow, microbial growth is provided with the conditions to proceed. Allergens from mold growth (and other microbial growth) in carpet dust remain in the carpets after the fibers have dried. These allergens can continue to be aerosolized for years by foot traffic. (The mold may be dead, but the spores and hyphal fragments remain allergenic.) Repeated washing and vacuuming may never remove all of the allergens. Carpets are also frequently vacuumed with inefficient vacuum cleaners (vacuums without HEPA filters) that emit carpet allergens in the exhaust.
Some people make the argument that carpet fibers trap particles from the air (“carpet acts as a filter”), and this is correct, but carpets capture biodegradable dust that can never be completely removed, even with thorough washing or vacuuming. Then foot traffic or vacuuming with poorly filtered equipment aerosolizes any allergens that are present. I have measured the particle emissions above carpets after slightly tapping the fibers; generally the particle counts (for particles larger than 3 microns) go up from about 3,000 per cubic foot of air (the ambient count) to 30,000 or more per cubic foot of air. At least one published study confirms this observation. Young children who crawl on a contaminated carpet thus have much greater exposure risks than adults.
Some experts deny that carpets can be a source of bioaerosol. According to Alan Hedge, a design and environmental analysis professor at Cornell University, "Concerns that carpeting in schools is contributing to an increase in respiratory problems, allergies and asthma in schools are unfounded." This view is supported by at least one study, in which airborne particulate matter was measured in both a carpeted gym room and an uncarpeted gym room. The results suggested that some cleaning practices (sweeping with push brooms without using sweeping compound, and using high-speed burnishing devices on VCT flooring), rather than carpets, are responsible for elevated levels of aerosolized particulate matter. In another study, however, particulate mass concentrations were measured both outdoors and indoors in a limited number of carpeted and uncarpeted classrooms. The results stated that "carpeted floor coverings may present an increased exposure risk to children from particulate matter harbored on the flooring material as compared to hard-surfaced flooring." Like many other studies, these two studies looked only at the amount of particulate matter (dust) and not what the particulate matter consisted of, so there is no way to know whether the dust was allergenic; thus more specific research would be helpful in finally determining the potential health risks from carpets in schools.
I recognize that carpets add soundproofing and provide a pleasant ambiance, but I also believe that carpet use in school buildings should be very limited. After all, common sense tells us that under normal usage and maintenance practices, hard-surfaced floors can never contain as much dust as carpets contain. This dust is potentially biodegradable, so why take a chance by installing carpets in an environment that is so heavily trafficked, and in which our children spend a great deal of their time?
(Excerpted with modifications from My Office is Killing Me! The Sick Building Survival Guide, Jeffrey C. May, Baltimore: MD, The Johns Hopkins University Press, 2006)
©2011 Jeffrey C. May
(The opinions of the author expressed herein do not necessarily state or reflect those of HFI: its principals, executives, board members, advisors or affiliates.)
