On this page
In this lesson
What you'll learn
- Explain the difference between purging and continuous ventilation
- Set up supply or exhaust ventilation so clean air reaches the work area
- Estimate purge time from space volume, airflow and air changes
- Avoid common setup mistakes that pull contaminated air into the space
Ventilation dilutes and removes contaminants and replaces lost oxygen. It's required under alternate procedures and is part of almost every permit entry. But a blower dropped next to a manhole with its intake beside a running truck can make things worse. Setup matters.
Purging vs. continuous ventilation
- Purging is ventilating the space before entry to remove or dilute contaminants, until testing shows acceptable conditions.
- Continuous ventilation keeps fresh air moving throughout the entry, so contaminants released by the work or the space don't build up.
- Inerting is the opposite: displacing oxygen with an inert gas such as nitrogen to prevent fire. An inerted space is IDLH for oxygen and needs special procedures, typically supplied-air respirators.
Under alternate procedures (1910.146(c)(5)), continuous forced-air ventilation must eliminate any hazardous atmosphere before entry, be directed at the areas where workers are or will be, and continue until everyone has left. The air supply must come from a clean source and must not increase hazards.
Supply vs. exhaust
| Method | How it works | Good for |
|---|---|---|
| Supply (positive pressure) | Blower pushes fresh air into the space through ducting | Most entries; delivers fresh air right to the worker |
| Exhaust (negative pressure) | Blower pulls contaminated air out | Contaminants generated at a point, such as welding fumes or solvent work |
| Push-pull | Supply and exhaust together | Large or complex spaces, long runs |
Setting up a blower
- Place the intake upwind and away from engine exhaust, vehicles, generators and other contaminant sources.
- Run the supply duct to the work area or bottom of the space, not just into the opening, so fresh air reaches the entrant.
- Keep duct runs as short and straight as practical. Every bend and extra length reduces airflow; check the manufacturer's performance chart.
- Make sure exhaust air has a path out and doesn't discharge where people are working.
- Use intrinsically safe or explosion-proof blowers and grounded equipment where flammable atmospheres are possible.
- Never use pure oxygen to ventilate. It creates an oxygen-enriched fire hazard.
How long to purge: air changes
A common way to estimate purge time is by air changes:
Purge time (minutes) = space volume (cubic feet) × number of air changes ÷ effective airflow (cubic feet per minute)
The number of air changes depends on the contaminant and your program or manufacturer guidance; many programs use 7 or more for purging. Run your numbers in the ventilation purge calculator.
When ventilation isn't enough
Ventilation may not control:
- Sludge or residue that keeps releasing gas when disturbed.
- Large or complex spaces with dead zones.
- Concentrations at or near IDLH.
- Oxygen-deficient inerted spaces.
In these cases, entrants may need supplied-air respirators, and the space can't use alternate procedures.
Key takeaways
- Purge before entry, ventilate continuously during entry, and never use oxygen.
- Supply ventilation delivers fresh air to the worker; exhaust removes point-source contaminants.
- Intake upwind and away from exhaust; duct to the work area; short, straight runs.
- Purge time ≈ volume × air changes ÷ effective CFM, followed by testing.
- Ventilation failure means evacuation.
Knowledge check
Lesson 5 quiz: 8 questions
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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.