How Much Ventilation Does Your Confined Space Need?

Forced air ventilation may be needed when occupying hazardous confined spaces, and you can calculate the required amount based on square footage and air exchanges per hour standards

How Much Ventilation Does Your Confined Space Need?

Forced air ventilation is required in confined spaces with a known hazard, including the possibility of a flash fire or explosion or the spread of toxic gases and debris, both of which can lead to oxygen depletion. 

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Confined spaces with poor ventilation trap dangerous gases and particulate matter in the atmosphere, exposing workers to potential respiratory infection, illness or even death.

If the area is known to be hazardous, workers must use a forced air blower to replace the air and reduce the risk of asphyxiation. However, many safety managers and cleaning crews struggle to properly ventilate spaces by failing to calculate the air exchange rate per cubic foot. 

No two confined spaces are exactly alike. Some don’t have enough ventilation to create a safe atmosphere, while others have none, making them unfit for prolonged human occupation. Variations in size, shape and the overall threat level determine how much circulation is needed to comply with specific regulations. 

Currently, OSHA doesn’t regulate air exchange requirements in confined spaces, leaving state occupational health and safety organizations to set and implement these standards, which vary based on region and industry. 

Workers face unique hazards when occupying these spaces. They are often asked to venture near or inside areas that rarely see the light of day. Toxic chemicals and gases floating through the air along with the lack of oxygen can put workers’ health at risk when working in basements, crawl spaces, sewers, tunnels and other confined spaces.

Here’s a guide to selecting the right ventilation equipment for the job.


Forced air ventilation is required in confined spaces with a known hazard, including the possibility of a flash fire or explosion or the spread of toxic gases and debris, both of which can lead to oxygen depletion. 

If the space is hazardous, the safety manager should refer to the state’s confined space requirements for more information, specifically the required number of air exchanges per hour, notated as ACH. This figure represents the number of times the air needs to be replaced inside the confined space every 60 minutes. If the ACH is five in your state and industry, the air should be exchanged five times per hour.

Use this formula to calculate how much ventilation is needed in a confined space: cfm= (Cubic Volume x ACH) / 60

Measure the work site to calculate the cubic volume of air in the space. Multiply the volume by the ACH. Divide by 60 to calculate the total cfm ventilation requirement.

For example, if the ACH is six, this means the air must be exchanged six times per hour. If the space in question measures 5,000 cubic feet, you would multiply this by six to get 30,000 cfm. Then you would divide that by 60 to get 500 cfm.


The safety manager needs to ensure the forced air blower circulates the air at or above the required cfm. The device should list this figure in the user manual and specifications. 

The blower should be from a reputable seller or manufacturer, which will offer a seal of approval from Underwriter’s Laboratories or the Canadian Standards Association. 

A crew should consider other factors affecting air circulation when using this equipment and before entering the work site. If hazardous air is being removed, the blower exhaust should lead out of the space without blocking the exit or entry. Large ventilation equipment can overwhelm the local breaker panel. The safety manager should ensure the outlet and electrical system can produce the necessary voltage while accounting for other projects connected to the grid.

Radial fans blow air in a circular motion with the exhaust 90 degrees from the inlet. Axial fans blow in a straight line. Axial fans are larger and consume more power but provide higher cfms than radial fans. A crew may opt to use an engine-powered radial fan if an outlet can’t be found. Axial units can’t accommodate engines and generally rely on electric power.

Depending on the layout, the blower may not distribute the incoming air evenly throughout the space. Turns, corners, dead ends, and hard-to-reach areas may have higher concentrations of hazardous gases or reduced oxygen. It may also be necessary to add ductwork to circulate the air evenly to achieve the required ACH. Ducts with 90-degree bends reduce airflow, thus lowering the estimated cfm. It’s important to see how the blower was rated in order to determine whether these variances were considered. 

Choosing a blower that exceeds the necessary cfm when using ducts and working in irregularly shaped confined spaces is best.

Once the blower is up and running, the crew should test the air using a gas monitor to see if the hazard has been remediated.

About the Author

Rick Pedley, PK Safety’s president and CEO, joined the family business in 1979. PK Safety, a supplier of occupational safety and personal protective equipment, has been operating since 1947. Visit


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