Purging dilutes and removes contaminants before entry. The usual way to estimate it is by air changes: how many times the volume of the space is replaced with fresh air.
The formula
Purge time (minutes) = volume (cubic feet) × air changes ÷ effective airflow (CFM)
- Volume: cylinder = 3.14 × radius² × height; rectangle = length × width × height.
- Effective airflow: the blower's rating minus losses from duct length, bends and elevation. Manufacturers publish charts; long runs with several bends can cut airflow by a third or more.
- Air changes: set by your program or the manufacturer. Many programs use 7 or more air changes to purge.
Setup matters more than the math
- Intake upwind, away from engine exhaust.
- Supply duct reaching the work area or bottom so air sweeps the whole space.
- Never ventilate with oxygen.
- Keep ventilating during entry, and evacuate if the blower stops.
Learn the full method in the free ventilation lesson.
Sources and official references
Last reviewed: October 25, 2026 · Free general education based on OSHA 29 CFR 1910.146 and 29 CFR 1926 Subpart AA. It does not replace the site-specific training, written program and hands-on evaluation your employer must provide, and it does not issue a certificate. Nothing you enter on this page is saved or sent.