Since you are a math nerd you should be able to derive the air flow requirements based on the allowable concentration of whatever hazardous or toxic vapor is generated. It is a simple procedure of air in, air flow out containing the allowable concentration of the toxic vapor or fume, and the generated toxic or hazardous vapor from the waste water. The equation will be a first order D.E. having an exponential (decay) function for a solution to determining the exhaust CFM. The American Conference of Industrial Hygienist (ACGIH) or NIOSH which is OSHA enforceable regulation publish these allowable concentration for 8 hour work exposure. Normally, the allowable concentrations publish by ACGIH or NIOSH will be similar.
Another aspect that you should be aware is that the room volume that will be require in the D.E. does not necessarily mean the entire room since the air flow could be stratified and consequently the room volume could be taken within the breathing zone so for safe keeping up to 10ft. high if there is no elevated work zone.

Most of these screening rooms are Class I, Div. 2. Ask the electrican to compare the cost of Div. 1 to Div. 2.

The latest edition of NPFA states "Continuously ventilated at 12 air changes per hour" for Class I, Div. 2 applications such as you are decribing. Class 1 Division 1 is no ventilation or ventilated at less than 12 air changes per hour.

The -0.1"WC is a measure of how tight (in terms of building air leakage) the building is. You can't meet this standard with building doors open.

http://c.ymcdn.com/sites/www.ncsafewater.org/resou...

Regarding: "How can any kind of negative be pulled in a building with the gaping openings of a channel running underneath it?"

Come on man, be a little creative. You can furnish a water seal where the water must flow under a baffle wall to seal.

Some applications supply sealing covers over the channels.

Regarding: "Another aspect that you should be aware is that the room volume that will be require in the D.E. does not necessarily mean the entire room since the air flow could be stratified and consequently the room volume could be taken within the breathing zone so for safe keeping up to 10ft. high if there is no elevated work zone."

This is an incorrect statement and not included in NFPA 820.

"This is an incorrect statement and not included in NFPA 820" My advice has nothing to do with the NFPA 820 and it is an acceptable method described in various ventilation literature such as "Ventilation for the Control of the Work Environment" authored by William A Burgess, Michael J Ellenbecker and Robert D Treitman. NFPA's approach is undoubtedly conservative and even perhaps wasteful in electrical consumption and capital investment, nonetheless, the OP has options to select.

Another source of information that would be valuable come from articles written by certified industrial hygienist Jeff Burton who has written numerous article on industrial ventilation and his articles have been predominantly published in the periodical titled Occupational, Health and Safety. His approach is similar to the one I originally described and if you can do research on his work, you will have a different view point from NFPA. If you do use NFPA guideline, remember the volume to be exhausted does not necessarily mean the entire room as you can partition the points of origin of the contaminants or even duct the exhaust with hoods.

bimr hit the nail on the head. Class 1 Div 1 (Zone 1) electrical equipment is an order of magnitude more costly than Class 1 Div 2 (Zone 2) equipment. The economics of producing a Div 2 environment, by means of mechanical ventilation is usually easy to justify in most industrial situations.
GG

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

As with any modification to an existing facility, your design will be influenced by the areas outside your scope. Start with the MAU based upon the 12 x/hr. based upon gross room volume (no allowance for equipment). Class/division rating will affect the MAU election and power/controls. I have used indirect gas fired b4 with TEFC motors w/o issues. Locate MAU and control panel outside the class/division area. Utilize a remote sensor in the proper enclosure. You may be forced into steam or exp. proof electric duct heater as the heating source. Place the equipment outside the room to keep power & control wiring costs down. Dynamic FDs at walls. Second, estimate the free area of the open piping / trench connected to the “other space(s). I estimate 800-1200 fpm and the resulting CFM of the connected area should give you the DP you are looking for. I have used this in production areas when 2 spaces must be controlled to a dp between rooms. Utilize a VFD with a DP integrated control loop. Install VFD and controls outside Class/Div room to avoid wiring issues. Greenheck has GB fans with exp. resistant construction. If cannot pass with a GB fan, utilize an in-line with motor out of air with bronze and aluminum. I would look at dp between rooms and not dp to atmosphere. Your reference leg may be very negative to start.