What is local exhaust ventilation (LEV), and which metric is commonly used to assess its effectiveness at the source?

• A. LEV captures contaminant at or near the emission source to prevent spread. Capture velocity (face velocity) is a common metric to assess effectiveness.
• B. LEV is a general room ventilation system; it is measured by ACH.
• C. LEV is a filter that removes contaminants with high efficiency; measured by removal efficiency.
• D. LEV is a device for humidification; measured by humidity.

Answer: (A)

Local exhaust ventilation is a system designed to pull contaminants away right at the emission source, using a hood and ductwork so the pollutants don’t disperse into the breathing zone or the workspace. The metric most commonly used to judge how well it works at that source is capture velocity (also called face velocity) at the hood opening. This air speed indicates how effectively the system is pulling contaminants into the capture inlet; if the capture velocity is too low, contaminants can escape the hood and exposure occurs.

In contrast, measuring general room ventilation by air changes per hour describes how quickly the entire room’s air is replaced, not how well the source is captured. A filtration efficiency metric would apply to filters after capture, not to the source capture performance. Humidity isn’t a measure of LEV effectiveness.