The first distinction that matters is between general ventilation and local exhaust. General ventilation dilutes ambient heat, odor, and low-level background contaminants to keep the room comfortable. Local exhaust does a different job entirely — it captures pollutants right where they're generated, before they ever reach the breathing zone. That covers equipment such as fume hoods, flexible extraction arms, atomic absorption hoods, process instrument exhaust, and ventilated chemical storage cabinets.

Two labs with identical floor plans can need very different exhaust totals — one running light routine work, the other packed with fume hoods and dedicated exhaust devices. That's why airflow sizing has to start from the actual pollutant sources and installed equipment, not from square footage.

Once individual demands are known, the next question is whether that equipment can realistically run at the same time. If everything could switch on simultaneously, the system should generally be sized for that scenario. Where variable-air-volume control or genuine time-sharing exists, the system's peak demand can be modeled more realistically — but a "diversity factor" is a project-specific engineering judgment, not a number you can borrow from another lab.

If ten fume hoods share one exhaust system, the fan can't be sized against "the number that's usually open" unless there's real control logic behind that assumption. Without it, once actual simultaneous use exceeds the design case, total system airflow falls short — and every hood's face velocity drops together.

Every cubic meter pulled out has to be replaced, or the room's negative pressure climbs unchecked — bringing sticking doors, whistling gaps, uncontrolled corridor air infiltration, unstable fume hood face velocity, temperature and humidity drift, higher HVAC load, and pressure relationships between rooms that no longer hold.

Supply and exhaust volumes are deliberately unequal by design intent, not by accident. Rooms meant to stay negative typically exhaust more than they're supplied, using that deficit to pull air from cleaner zones toward higher-risk ones — but the exact offset depends on door and window sealing, gap dimensions, pass-through boxes, and every other leakage path in the room, so it can't be set from a fixed ratio alone.

Once total exhaust volume is set, duct size follows from airflow and target velocity:

The right design velocity depends on whether the exhaust stream carries corrosive vapor or particulate, what material the duct is made from, allowable resistance and noise, available ceiling depth, terminal connection requirements, and whether more branches might be added later.

System resistance stacks up from several sources at once: internal resistance inside fume hoods and other exhaust devices, duct friction, elbows and tees and reducers, dampers and check valves and silencers, any filtration or scrubbing media, and the outdoor discharge stack itself.

Airflow and total pressure aren't two separate maximums to check off independently — a fan's actual output shifts along its performance curve as resistance changes. The fan needs to be verified at the specific operating point defined by design airflow and calculated system resistance together.

A design that passes calculation doesn't guarantee field performance. Common culprits include:

A telltale pattern: total fan airflow looks fine, the nearest exhaust point pulls hard, but the fume hood furthest down the line is starved. That's rarely a fan that's too small — more often it's unbalanced branch resistance. Simply speeding the fan up usually just strengthens the near branch further while the far one barely improves, adding noise and energy cost for little benefit.

Design calculation establishes the target; commissioning proves whether the installed system actually hits it. A thorough commissioning check covers each fume hood's real face velocity, airflow at every exhaust point and branch, total system supply vs. exhaust volume, room-to-room pressure differentials, fan frequency/current/status, resistance across any treatment equipment, system stability as devices cycle on and off, and — for VAV systems — control response as sash positions and hood counts change.

Hitting the total design airflow at the fan outlet means nothing if one branch is over-supplied and another under-supplied. Commissioning has to measure and balance at each individual exhaust point, not just once at the fan.