Control
Core concepts
Controls follow the hierarchy: elimination, substitution, engineering controls, administrative controls, then PPE as the last line. Engineering controls include local exhaust ventilation (LEV) - hood, duct, air cleaner, fan, discharge - and dilution/general ventilation, which relies on mixing and is unsuitable for highly toxic or high-generation-rate agents.
Know the LEV vocabulary: capture velocity (air speed at the contaminant source needed to pull it into the hood), transport velocity (duct speed needed to keep material suspended), hood static pressure (a measure of resistance; rising SP signals blockage or filter loading), and make-up air (without it, a building under negative pressure starves the exhaust and capture collapses).
For respirators, the assigned protection factor (APF) sets the maximum use concentration: MUC = OEL × APF. APFs do not authorize IDLH entry or oxygen-deficient atmospheres.
Traps
- Choosing PPE first because it is cheap and fast.
- Confusing capture velocity with duct transport velocity.
- Forgetting make-up air, then blaming the hood.
- Treating dilution ventilation as adequate for a carcinogen or a high-toxicity solvent.
- Using an APF to justify entry into an IDLH atmosphere.
Worked example
A 6-inch round duct must move 2,000 fpm. Area = π(0.25 ft)² = 0.196 ft². Q = V × A = 2,000 × 0.196 ≈ 392 cfm. If measured flow is far lower, look for blockage, filter loading, or fan/make-up air problems before adding respirators.
Rule of thumb
A plain open-ended hood generally needs about 100 fpm capture velocity at the source for low-toxicity, low-velocity contaminants; enclosing the operation reduces the airflow required and is almost always the better design.