Working in a confined space brings a set of challenges and hazards far different than any other type of work. OSHA defines a confined space as any space that, “has limited or restricted means of entry or exit, is large enough for an employee to enter and perform the designed work and is not designed for continuous occupancy by the employee.”

Examples of confined spaces include underground vaults, silos, tanks, storage bins, pits and vessels. Air quality and lack of natural ventilation are major concerns when working in a confined space. Being prepared for this hazard can make a significant difference in the outcome of the project.

Air Quality and Ventilation in a Confined Space

Air quality in a confined space can quickly become compromised. Natural ventilation is not usually sufficient to maintain breathable air quality at lower elevations with limited air circulation. In addition, conditions in the confined space can change as work is performed. Performing hot work such as grinding can release particles into the air and decrease oxygen supply. The substances brought into the space for the work can also affect air quality.

It’s also important to remember air quality can vary greatly across the space depending on elevation. This is called stratification, which is the layering of different air compositions from floor to ceiling. Remember – substances will settle depending on their density. Just as oil sinks to the bottom when mixed with water, some chemical substances are lighter or heavier than air and will settle accordingly.

Once in the space, use air monitors frequently to check for stratification, however, take the time to ensure you have a plan before entering. Check the Safety Data Sheets for the density information of every substance or material you plan to use. If the densities across substances vary greatly, you can anticipate and prepare for stratification.

Once you know the air quality aspects of the space, you can properly ventilate to increase air quality. The ventilation system, fans and hoses can be set up to ensure good air quality throughout the space and maintained at that level during the course of the entry. If you have specific questions regarding permit-required confined spaces, air quality and ventilation systems, contact the FCA Safety Helpline at (866) 844-SAFE or by email at

FCA Confined Space Resources

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