Purging and Ventilating a Confined Space

If a confined space contains an atmosphere that is flammable or considered immediately dangerous to life or health (IDLH), the area will require purging before employees can enter as the natural ventilation (natural air currents) is usually not reliable and not sufficient to maintain the air quality. Continual forced mechanical ventilation (blowers, fans) may be required to keep some areas safe during entry.  Purging/ventilation of a space is only required when atmospheric testing determines the presence of a dangerous atmosphere or there is a known atmospheric hazard present in the confined space.

What is the difference between "purging" and "ventilating"?

"Purging" involves removing contaminants inside the confined space by displacement with air to achieve acceptable atmospheric levels. For example, if a confined space originally contained a toxic gas, air would be blown into the space to reduce the concentration of the toxic gas to below the appropriate atmospheric exposure level.

After the contaminants have been removed ("purged"), the confined space may be ventilated.

"Ventilation" means the continuous provision of fresh air into the confined space by mechanical means to maintain acceptable atmospheric levels. It must be continued while work is being carried out within the space to maintain an acceptable oxygen concentration; to provide protection in case of accidental release of chemicals; to remove contaminants generated by the work performed; or to cool the enclosure.

Ventilation involves displacing air and diluting it through the introduction of fresh air (forced-air) or the continuous removal of contaminants by local exhaust ventilation for point sources. To ensure adequate ventilation, the points of air supply and exhaust should be separated as far as possible. Openings must be provided for the entry of clean replacement air or to allow the exhaust of air. Pure oxygen must not be used to ventilate a confined space.

Mechanical Ventilation Equipment:

Forced mechanical or supplied ventilation introduces fresh air into the confined space through the use of a mechanical air moving device such as a blower. The constant supply of fresh air in sufficient quantity will help to maintain the level of oxygen in the space within the safe range, as well as dilute the level of contaminants released in the confined space to an acceptable level.

It is important to ensure that air moving device is placed where the air is drawn into the confined space from a contaminant-free source. For example, it is not appropriate to place the air moving device behind a diesel generator where the exhaust gas of the generator could be drawn into the confined space.

A push/pull ventilation system usually provides more effective ventilation of a confined space than using any other ventilation system alone, and is recommended for use whenever practicable. The push-pull system introduces fresh air into the space at one end while removing contaminants by exhausting them at another end.

If you require ventilation equipment due to contractor/trade work being conducted in your schools crawl space, please contact the safety officer.