Dave, it's a pretty broad topic and would be helpful to know what your existing level of knowledge is. Basically, you design the space around the use. The classification should be determined based on the type of work and the cleanliness requirements of the space. Is there an elevated biosafety risk? For a standard BSL-2 wet lab, such as when doing tissue culture, blood work, with various chemicals, etc., air is exchanged per ASHRAE and AIA at a rate no less than six air changes per hour (ACH). ACH is (cfm * 60 / room volume), room volume in ft3. Generally it is preferred not to recirculate air, but to have a one pass system. ASHRAE says if some air is recirculated, there should still be at least two air changes of outside air entering the lab.
If chemical fume hoods are used, you generally exceed the minimum ASHRAE and AIA ACH recommendations because of the high volume of air leaving the space. Many applications use a see-saw balance between the hood exhaust and the general exhaust of the space (if general exhaust is used). As the hood sash closes, hood exhaust drops and general exhaust modulates to maintain the programmed room cfm offset (often ~200 cfm) to maintain room pressure.
Variable volume supply air systems with reheat (very common) will throttle back the volume of supply air when the room gets too cold, then turn on the reheat to add heat. When the supply air reduces, the general exhaust will again track the supply to maintain the room at its programmed offset, for stable room pressure.
This is a common control scenario (this example is generally how Siemens lab control works), but there are many other control schemes, other BSL levels, and other lab applications that have different needs, such as positive lab pressurization. There are bypass fume hoods whose exhaust volume doesn't vary; there are constant volume systems, etc.
Lab pressure is usually a result of controlling air volume, not a parameter to use to control the HVAC systems. Pressure is at the mercy of the net open area of all the room's cracks and crevices, as well as pressures in adjacent spaces. I wouldn't recommend controlling lab air volumes based on pressure. Resultant pressure with respect to surrounding spaces (generally negative in lab applications) based on a design offset between supply and exhaust volume is often in the range of -.003 to .03, give or take...
I hope some of this helps. If you have access to the ASHRAE manuals, there should be some good information there. Good luck, -Chas.
