Although much can be said about laboratory fume hood and pressurization controls, the real issue to focus on is containment of hazardous substances. In the case of Level 1 and 2 laboratories, hood performance is of primary concern since Level 1 and 2 laboratories do not have the stringent space pressurization control requirements of Level 3 and 4. It comes down to evaluating how well the hood does its job of containment with its various configurations, control systems, and applications. Of major concern is how containment is measured. With a smoke bomb? With a tracer gas? And under what conditions - factory or installed - static or dynamic?

The only performance standard that seems to be in use in North America is ASHRAE 110-1985R, which measures the performance of the hood under static conditions. When this test is done it is usually in the factory of the hood manufacturer. This test is rarely done once a hood is installed, because of the cost and the difficulty in duplicating the test conditions stipulated by the standard.

A revised version of the ASHRAE 110 standard has been issued for public review and acceptance of it is expected at the January 1995 ASHRAE meeting. To quote from the foreword of the standard, "There is a need for a performance test that can be used in the field to establish an as used performance rating, including the influences of the laboratory arrangement and its ventilation system." Nevertheless, the revised standard does not deal with the following important factors in the safe operation of laboratory fume hoods:

• cross-drafts
• work procedures
• clutter in the hood
• intrinsic experimental hazard
• thermal challenge
• transient response of a VAV control system to exhaust pressure changes

The ASHRAE 110 standard does not stipulate required performance levels. Rather, it describes a test procedure for determining the performance under a set of specified conditions. The standard could be part of a specification once the required performance level is determined. The types of conditions for which the standard can be used are: as manufactured (AM), as installed (AI), and as used (AU). Whether AM, AI, or AU, the method outlined comprises three tests:

• flow visualization
• face velocity measurements
• tracer gas containment

Flow visualization consists of two tests - a local challenge whereby small amounts of smoke are released at specific points inside and outside the hood and a gross challenge whereby large quantities of smoke are released inside the hood. These tests are meant to give a qualitative visual indication of hood performance, except in the case of the local challenge where the hood is deemed to fail the test if any smoke is seen to leave the hood.

Face velocity measurements are meant to give a quantitative indication of hood performance. The method for measuring face velocity is described in the standard.

Tracer gas containment tests measure the containment ability of the hood under specified conditions. This is primarily a test of the hood under static conditions, except for the one which measures the containment while the hood sash is moved according to specified parameters.

With respect to laboratory pressurization for Level 1 and 2 applications, the Health Canada Biosafety Guidelines prescribe no written requirement, except as called for by local building codes. The reason for this is that containment of hazardous substances is controlled by biological cabinets rather than by ventilation of the laboratory. This is very significant because it implies that safe containment is determined much more by the hood than it is by laboratory pressurization or ventilation rates. Nevertheless, negative pressurization is recommended by many sources, for example:

• OSHA - Federal Register
• Prudent Practices
• ASHRAE Handbook - HVAC Applications
• Handbook of Laboratory Safety
• NFPA 45

NFPA 45, which deals with fire hazards and is often included in building regulations, states (p45-12, S6-4.2), "Laboratory units and laboratory work areas in which hazardous substances are present shall be maintained at an air pressure that is negative relative to the corridors or adjacent non-laboratory areas." This good advice applies equally to biological and chemical laboratories. In principle, if there is a recognized requirement for a fume hood, this means that workers are dealing with substances too hazardous to be experimented with on an open bench top. Thus, it follows that any space with fume hoods should be negatively pressurized in conformance with NFPA 45.

As respected as it is, NFPA is not specific about transient behavior of laboratory pressurization. For example, it does not address the possibility of the negative pressurization being momentarily lost or reversed. It is left to the judgement and common sense of the designer. For Level 1 and 2 applications, this would not appear to be a problem, since containment is afforded primarily by the hood.