Volatile Organic Compounds

Volatile organic compounds (VOC) are ubiquitous components not only of ambient air but also of indoor air environments, including offices, commercial and retail buildings, and homes (Shah and Singh, 1988). There are three sources/categories for VOC: (1) en-trainment of air from outside the building, (2) emissions from building materials, and (3) human activities inside buildings.

As might be expected given the nature of the sources, a very large variety of organic compounds have been identified and measured indoors (e.g., Brown et al., 1994; Crump, 1995; Kostiainen, 1995). These number in the hundreds of different compounds, with the particular species and their concentrations depending on the particular sources present as well as the air exchange rates. Table 15.2 summarizes some of the types of organics that have been measured in indoor air and typical sources (Tichenor and Mason, 1988; Crump, 1995). Because of the VOC sources present indoors, the indoor-to-outdoor concentration ratios are quite large for many compounds.

For example, Table 15.3 shows some typical ratios of indoor-to-outdoor concentrations for specific compounds found in each of the classes shown in Table 15.2, which are frequently present indoors (Brown et al., 1994). These data are based on a review of the literature and include data from a number of different countries. The ratio is for all but one compound substantially greater than one. Also shown in Table 15.3 is a typical range of concentrations expressed as the overall weighted average of the geometric mean, where the weighting was done using the number of available measurements. Some of the compounds associated with the three sources—entrainment from outdoors, emissions from building materials, and anthropogenic activities—are now briefly reviewed.

Entrainment of air from outdoor sources. Entrainment of outdoor air through ventilation systems brings with it the species found in ambient air, which have been discussed throughout this book. Some of them, such as HN03 and to a lesser extent 03, can be removed on surfaces such as those in air conditioning systems, and hence the indoor concentrations tend to be lower than those outdoors. Others such as NO tend to have similar concentrations indoors and outdoors if there are no significant combustion sources indoors (e.g., Weschler et al., 1994). In the case of hydrocarbons, the concentrations of compounds that do not have significant indoor sources tend to be about the same as the outdoor concentrations. For example, Lewis and Zweidinger (1992) measured VOC in 10 homes in winter and showed that the concentrations of ethene, benzene, 2-methylpentane, methylcyclopentane, 2,2,4-tri-methylpentane, and 2,3-dimethylbutane indoors were within experimental error of those outdoors.

TABLE 15.2 Some VOCs Measured Indoors and Their Sources"

Class of Compounds

Species

Typical Sources

Hydrocarbons

Aliphatic hydrocarbons

Paints, adhesives, gasoline, combustion products, floor waxes

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