CM/Spotlight: Infection control Matter with multiple contact points is more strongly attracted to the surface and more force is required for displacement. Denser matter is less buoyant and has greater difficulty re-entering the air. Without human activity most indoor air matter originates outdoors. Gravitational settling causes larger, denser airborne substances to sink to the ground or floor faster. Outdoor sources generally are bigger and fall to the ground rapidly. Once indoors, they are deposited in the building envelope’s outer regions, like window sills, more than inside. Large-size substances, such as hair, skin scales and fibers generated by indoor activ-ities, usually settle out rapidly in carpet close to the activity area. Smaller atmospheric particles (<15 µm) most influence indoor air quality (IAQ). These particles usually are transported indoors on air currents. Particles <1 µm are a major component of atmospheric dust. If they become part of indoor air it takes time for them to settle out, if at all. pend matter from fabric than a hard surface. Less matter is transferred from an ele-vated surface when regular cleaning is conducted. In the first order matter transfer process, the amount of matter in a compartment determines how much matter transfers to another compartment, such as air. Flooring and elevated surfaces are loaded with matter. As matter is transferred from sources to air, concentrations increase based on the transfer rate. is adversely affected by a high loading (>2000 mg/m 2 ) of dusts. This is consistent with the principle that matter transfers from one compartment, such as carpet, to another, such as air, in proportion to the amount of matter in the originating compartment. Excessively loaded carpet degrades IAQ well beyond background levels. Cleaning reduces source strength. As mitigation remedies are introduced, such as managing activity, source strength is managed by steady state conditions. The IAQ also is brought to an acceptable control level. Field demonstration research indicates that cost effective cleaning technologies easily maintain the source strength of car-pet and other surfaces at levels not CM adversely affecting IAQ. Factors influencing IAQ A building’s IAQ is influenced by out-door air, effective ventilation and compart-ment (source) loading. Source loading first occurs through air deposition followed by human tracking. No activity or ventilation results in posi-tive feedback. Without ventilation particle mass accu-mulates on interior surfaces, transfers to air and degrades IAQ. This occurs in unoccupied, unventilated and ill-maintained buildings. Ventilation contributes to the possibility of achieving a steady state of air quality when faced with continuous compartment loading and infrequent cleaning. Ventilation also reduces the particle mass loading of compartments and surfaces. It also tends to keep IAQ concentrations near background levels. Removing minimal matter through cleaning brings source loading to a steady state condition in unoccupied buildings. Even removing 5 to 20 percent of particle mass produces the same condition observed in several research studies. Carpet is stronger at trapping matter than transmitting it. When there is human tracking activity in a building, carpet source loading occurs more than with other compartments. Depending on the amount of loading, air quality degrades primarily in the carpet compartment. The most significant or likely air contam-inants that can become airborne into the indoor air are particulate matter ranging from 1 to 10 µm. Field data suggest that IAQ over carpet Know your surface Particles settling out or becoming trapped, such as in carpet, are difficult to remove and re-entrain or to become airborne. Depending on their shape and the num-ber of contact points, particles settle and attach themselves to ceilings, walls, shelves, air ducts and floors with various attraction forces. Typically, an indoor room environment has four walls, a ceiling and floor. The flooring constitutes roughly 16 per-cent of the surface area. Elevated surfaces also influence IAQ but not to the same extent as flooring. Flooring, especially carpet, holds more matter. Hard, elevated surfaces, such as table-tops and shelves, are not as strong a com-partmental sink as fabrics. Vertical surfaces tend to hold matter by electrostatic forces. This matter often is smaller in size and density. It takes 10 times more energy to re-sus-www.cmmonline.com 23
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