PS: VAV is not for most. VAV and Low-flow was devised as a means to reduce energy costs in "fume-hood-heavy educational facilities. One to three hoods does not usually consider VAV or low-flow hood. Low flow may go the way of the add-air, the disadvantages which are discussed in a technical report at ashrae.org. Goal of VAV to reduce annual operating costs, stating that added cost of the hood is made up for by energy savings, if flow monitoring accuracy and accurate hood alarms, sophisticated air-conditioning and heating controls are in place, and sophisticated staff is available as well as the willingness to contract for building control maintenance. Running low-flow or VAV at common operating velocities of 60 fpm is not safe based on the way most users work in hoods. The hood is a piece of safety equipment, and trying to save annual cost is important, but is it worth the work liability and risk? All low-flow or VAV hoods create more time for contaminants to roll around inside the hood before they exit, leading to buildup in the hoods. Equipment and fixtures in the hood will show the the results in shorter life. The design of VAV has improved; however cross-drafts and operator movement is an influence. Some states and cities have regulations not allowing low flow or VAV hoods. Rather than spending resources to change code or regulations manufacturers and users need to remember look at the hood as a piece of safety equipment, not HVAC products. Like other reasearcher vs. administration preferences low-flow or VAV hoods like the "open-lab plan" leaves a lot to consider. Satisfactory performance under all situations is important with safety equipment. Limiting size and higher or low air-flow is an impact on performance, creating new computer controlled sash and air-handling may not perform when challenged with day to day work and various situational changes. Just as filtered hoods can not handle all of the demands of most chemical hoods needs, the VAV is more costly and may not be the hood for your situation.
Remember a hood is a piece of safety equipment first, not part of a ventilation system.

PS. Fume Hood exhausting dialogues starting as more effort is being made to contain and capture rather than just diluting exhaust. More hood users are asking, what harm is there in what is exhausting to the outdoor? If our children play outside and the wind blows exhausted fume hood air in their direction, we are concerned. It was said for years, the solution to pollution is dilution, now that may have been delusion. Diluted or concentrated chemical exposure is part of current chemical exposure limits language. We describe a situation for potential for evaporation; as potential concentration of chemical. There is the accummulation; there by mixed chemistry potentials, it would have been better if all previous science and technology had not relied so on dilution. Solutions are only discovered when there is an understanding of or an expressed problem. Current technologies and more knownlege with new ability to monitor, measure in terms of ppm, and databases helps us understand impacts of some chemicals. In the future, we will make more informed decisions on what is acceptable practices. Exhausting chemical hoods will soon be given more attention. Filtering is not necessary in all situations. No regulatory mandates filtering or scrubbing, but that is coming as there are more acceptable ways to deal with fume hood exhaust and as commercial and industrials scrub and mitigate pollution all others will be soon required to do the same. When making the decision to purchase a hood, remember to review established TLVs for health and safety within the lab, again this is where your list of chemicals is needed. I have a book and often look up Total Limit Values, or you can contact (ACGH) American Conference of Government Industrial Hygienists, Cincinnati, Ohio, to purchase one. The best responsible practice may be to include a scrubber or get a filter system with the hood, or plan to add one. Some states provide tax incentives for pollution control equipment. Recirculate and cleaning, then exhausting is going to become more and more important. Ignorance is no excuse, like so many things limiting ones liability is critical and wise. The real solution to pollution is to find ways to not create or to turn exhausted or byproducts into a more useful substances as industry is beginning to do or contained and dispose of captured pollutants appropriately.