FIGURE 8.7 Typical fume hood scrubber schematic.

trations at a steady state until the upstream and downstream levels are almost identical. To prevent breakthrough, the adsorbent must be either changed or regenerated on a regular basis. Downstream monitoring to detect breakthrough or sampling of the media to determine the remaining capacity of the bed should be performed regularly.

An undesirable characteristic of these types of scrubbers is that if high concentrations of organics or hydrocarbons are carried into the bed, as would occur if a liquid were spilled inside the hood, a large exotherm will occur in the reaction zone of the bed. This exotherm may cause a fire in the scrubber. These devices and other downstream devices such as particulate filters should be located to minimize the effects of a fire, should one occur. Fires can start in these devices at surprisingly low temperatures due to the catalytic action of the adsorbent matrix. Therefore, such devices should be used and operated with care.

8.C.8.4 Particulate Filters

Air from fume hoods and biological safety cabinets in which radioactive or biologically active materials are used should be properly filtered to remove these agents so that they are not released into the atmosphere. Other hazardous particulates may require this type of treatment as well. The most popular method of accomplishing this removal is by using a HEPA filter bank. These HEPA filters trap 99.97% of all particulates greater than 3 microns in diameter. These systems must be specified, purchased, and installed so that the filters can be changed without exposing the worker or the environment to the agents trapped in the filter. Sterilizing the filter bank is prudent before changing filters that may contain etiologic agents.

The "bag-in, bag-out" method of replacing filters is a popular way to prevent worker exposure. This method separates the contaminated filter and housing from the

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