Glove boxes generally operate under negative pressure, so that any air leakage is into the box. If the material being used is sufficiently toxic to require the use of an isolation system, the exhaust air will require special treatment before release into the regular exhaust system. These small units have a low airflow; therefore, scrubbing or adsorption (or both) can be easily accomplished.
Some glove boxes operate under positive pressure. These boxes are commonly used for experiments for which protection from atmospheric moisture or oxygen is desired. If positive-pressure glove boxes are used with highly toxic materials, they should be thoroughly tested for leaks before each use. Also, a method to monitor the integrity of the system (such as a shutoff valve or a pressure gauge designed into it) is required.
Clean rooms are special laboratories or work spaces in which large volumes of air are supplied through HEPA filters to reduce the particulates present in the room. Several classifications of clean rooms are commonly used. Clean room classifications refer to the number of particles larger than 0.5 microns in size per cubic foot of volume. Unfiltered ambient air has approximately 500,000 to 1,000,000 particles per cubic foot. Certain pharmaceutical, microbiological, optical, and microelectronic facilities require clean rooms of differing classifications from Class 10,000 to Class 10 or lower. Special construction materials and techniques, air handling equipment, filters, garments, and procedures are required, depending on the cleanliness level of the facility. A laboratory consultant or expert in clean room operation should be consulted before a clean room is worked in or built.
Environmental rooms, either refrigeration cold rooms or warm rooms, for growth of organisms and cells, are designed and built to be closed air circulation systems. Thus, the release of any toxic substance into these rooms poses potential dangers. Their contained atmosphere creates significant potential for the formation of aerosols and for cross-contamination of research projects. These problems should be controlled by preventing the release of aerosols or gases into the room. Special ventilation systems can be designed, but they will almost always degrade the temperature and humidity stability of the room. Special environmentally controlled cabinets are available to condition or store smaller quantities of materials at a much lower cost than in an environmental room.
Because environmental rooms have contained atmospheres, people who work inside them must be able to escape rapidly. Doors for these rooms should have magnetic latches (preferable) or breakaway handles to allow easy escape. These rooms should have emergency lighting so that a person will not be confined in the dark if the main power fails.
As is the case for other refrigerators, volatile flammable solvents should not be used in cold rooms (see Chapter 6, section 6.C.3.1). The exposed motors for the circulation fans can serve as a source of ignition and initiate an explosion. The use of volatile acids should also be avoided in these rooms, because such acids can corrode the cooling coils in the refrigeration system, which can lead to the development of leaks of refrigerants. Other asphyxiants such as nitrogen gas should also be avoided in enclosed spaces. Oxygen monitors and flammable gas detectors are recommended when the possibility of a low oxygen or flammable atmosphere in the room exists.
Biological safety cabinets (BSCs) are common containment and protection devices used in laboratories working with biological agents. BSCs and other facilities in which viable organisms are handled require special construction and operating procedures to protect workers and the environment. Conventional laboratory fume hoods should never be used to contain biological hazards. Biosafety in Microbiological and Biomedical Laboratories (U.S. DHHS, 1993) and Biosafety in the Laboratory: Prudent Practices for the Handling and Disposal of Infectious Materials (NRC, 1989) give detailed information on this subject.
Even the best-engineered and most carefully installed ventilation system requires routine maintenance. Blocked or plugged air intakes and exhausts, as well as control system calibration and operation, can alter the performance of the total ventilation system. Filters become loaded, belts loosen, bearings require lubrication, motors need attention, ducts corrode, and minor components fail. These malfunctions, individually or collectively, can affect overall ventilation performance.
Facility-related environmental controls and safety systems, including fume hoods and room pressure con-