Department of

Architectural Engineering

 


Outdoor Purge Air Systems

Airborne pathogens can be removed by purging with outside air, which is naturally sterilized. Airborne bacteria and viruses pathogenic for humans rarely occur in the outdoor air, and cannot survive long if they do. Spores of fungi and actinomycetes can occur in outside air but rarely occur in hazardous concentrations (Goodfellow 1984). The concentration of fungal spores in outdoor air varies, but is often as low as 100 CFU/m3 in residential areas (see Table 2.3).

The only condition in which purging with outside air is not a solution to an indoor microbial contamination problem is when microbial growth has occurred inside the air handling unit, because this may increase respiratory distress throughout the building. Therefore, under normal conditions, purging a building with outside air is an acceptable way of removing airborne pathogens, especially contagious human pathogens.

Even the cleanest of human environments is full of microbes. Table 2.1 lists the variety of airborne microorganisms that have been isolated on American and Soviet spacecraft (Nicogossian 1977 & 1993, Johnson et al 1977). The last column identifies those species that are found in outdoor air, based on several studies (Kemp 1995, Li 1992, Straja 1996). This table highlights an important distinction -- contagious human pathogens are found concentrated indoors, not outdoors (Gregory 1973), close to their main source, humans. Fungi can occur in both locations (Samson 1994, Reponen 1992).

Limits for Indoor Airborne Microbes

No standards exist for acceptable levels of indoor air contamination with microorganisms, since the infectivity of pathogens is extremely species dependent, although a number of guidelines exist for indoor spore levels, and a few exist for indoor bacterial levels (Rao 1996, Su et al 1992, Godish 1995).

Table 2.3 and Table 2.4 summarize some of the lower levels that have been suggested as limits, along with data from various sources indicating average ambient outdoor or indoor levels. These limits are by no means the only limits specified in the literature, but they are representative of the low end of all the limits or averages that have been published. The bacteria referred to are implicitly ambient, or environmental bacteria. Pathogenic bacteria and viruses, particularly contagious pathogens, are considered to have no safe limits (Rao et al 1996).

The rate of removal of airborne pathogens depends on two factors, the air change rate (ACH) and the ventilation effectiveness (or degree of air mixing). If plug flow (piston flow) were assumed, then one air change would completely remove all pathogens that were initially present in a room. This is rarely the case, except when it is by design (ASHRAE 1991).

Complete air mixing will delay the removal of airborne pathogens in an exponential manner. This represents the limiting case for normal buildings, and is a reasonable and simple model to use for evaluating the removal rate of airborne pathogens. Given the assumption of complete air mixing, the primary factor determining the removal of airborne pathogens is the air change rate.

Airborne pathogen hazards are dependent on the species of microbe. Some microbes are extremely lethal at very low doses. Some guidelines exist for levels of airborne fungi, and these are used as general indicators. The ACGIH and the AIHA define 1000 CFU/m3 as an upper limit for concentrations in indoor environments, while the CEC defines 2000 CFU/m3 as a "very high" level (Rao et al 1996). A value of 10,000 CFU/m3 of nondescript airborne microbes could therefore be considered a hazardous level for indoor environments.

The chart below illustrates the purging effect of different rates of outdoor air, in terms of ACH (air change per hour). The actual amount of outdoor air that can be economically brought in to a building can depend heavily on ambient conditions. In mild dry climates, large volumes of outdoor air can be used to purge a building continuously with little added cost. In hot, dry climates it is possible to use two stage evaporative coolers to recover a large fraction of exhaust cooling and thereby bring in outdoor air at possible high volumes for a much reduced cost. In cold climates, high efficiency air-to-air or run around heat exchangers can recover heat losses, but the problem becomes one of economics as well as system operating parameters.

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