This section provides an overview of ventilation systems and how to ensure confined spaces in the workplace are appropriately ventilated to ensure employee safety.  

When to Ventilate

Ventilate a confined space whenever its atmosphere is hazardous in any of the following ways:

• The air contains too little oxygen.
• The air contains too much oxygen.
• The atmosphere is flammable.
• The air is toxic.

Before Entering

• Before entering a confined space, ventilate far enough in advance so that the air will be safe before anyone enters the space.
• Before entering a confined space, test the atmosphere to make sure the ventilation has been working long enough.

After Entering

• After entering a confined space, continue ventilating for as long as anyone is in the space. Continue ventilating at least until the oxygen level and the concentrations of hazardous materials are within safe limits.
• If work inside the space can make the air unsafe, such as hot work, painting, coating using solvents, and sandblasting, ventilation must continue for as long as the work is in progress.
• The confined space entry permit will tell you if continuous ventilation is needed. There are some atmospheric hazards that ventilation alone can’t reduce to safer levels. Atmospheric testing is needed to tell you when ventilation hasn’t worked sufficiently. Always test the air in a confined space before anyone goes in.
• After entering a confined space, regularly retest or continuously monitor to make sure the air stays safe.

Types of Mechanical Ventilation

There are two (2) kinds of mechanical ventilation:

1. Local exhaust ventilation.
2. General ventilation (also known as dilution ventilation).

Local Exhaust Ventilation

• Local exhaust ventilation captures contaminants at their point of origin and removes them.
• Local exhaust ventilation is the best way to control flammable and toxic materials produced at a single point.
• Whenever possible, use local exhaust ventilation during hot work and cleaning with solvents.
• Keep the exhaust intake close to your work.
• Local exhaust ventilation doesn’t work well when contaminants are widely dispersed, and sometimes the location or shape of a confined space can make local exhaust ventilation hard to use. In these cases, use general ventilation instead.

General Ventilation

• General ventilation flushes the atmosphere by supplying and exhausting large volumes of air.
• General ventilation does not reduce the amount of contaminants released, so there are limits on when general ventilation can be used.
• For general ventilation to work well, workers should not be too close to the contaminant source. 
• The contaminants must not be highly toxic.
• The contaminants must be produced at a fairly uniform rate.
• The best uses of general ventilation are the following:
  • To provide atmospheric oxygen.
  • To control low concentrations of materials that is not highly toxic.
• Be extra careful if you use general ventilation during certain hazardous work, such as hot work.
• Re-treat the air often or monitor it continuously.
• You may need a respirator along with continuous ventilation.
• There are two types of general ventilation systems:
  1. Exhaust ventilation draws contaminated air out of an area.
  2. Supply ventilation blows fresh air in.
• Drawing air out is better when the atmosphere could be flammable or toxic. Blowing air in can spread contaminants.

 Ventilation Safety

• With either general or local ventilation, always ventilate with fresh air—never with pure oxygen.
• All electrical ventilation equipment should be grounded.
• Ventilation equipment should be electrically bonded to the confined space.
• Explosion-proof equipment can give added protection.
• Place the intake for your air supply far from any flammable or toxic materials.
• Ventilation systems may produce contaminated exhaust.
• Locate the exhaust outlet so that contaminants won’t be drawn back into the confined space.
• Place the outlet where air currents will disperse the exhaust quickly, without endangering nearby people.
• The exhaust may have to be filtered, to prevent air pollution.
• If the exhaust could be flammable, remove all ignition sources from the area.

Ventilation must provide constant circulation of fresh air through all areas of a confined space. 

There are two (2) main problems to watch out for.

1. Recirculation of contaminated exhaust back into the space.
2. Short-circuiting the airflow. This happens when fresh air moves directly from the inlet to the exhaust outlet, without reaching the other areas of the space. 

You can avoid these problems in the following ways:

• Use equipment that has enough power.
• Your equipment must be able to throw air far enough to ventilate the whole space.
• Your equipment must also be able to capture and carry away any contaminants.
• A series of fans may be needed to move air long distances or to ventilate a large area
• Locate your fresh air inlets and exhaust outlets properly. If possible, the incoming air and the exhaust air should move through separate openings, located far apart.

Duct Work

Use it effectively as it lets you direct the air flow to all areas of the space.

• Place your ducts where they won’t be damaged by your work.
• Keep your ducts as short and straight as they can be and still reach the areas they must get to.
• Do not make any sharp bends in your ducts.
• Be sure all connections are tight.

Let’s see how you can use these guidelines to ventilate various confined spaces effectively. Start by asking yourself what hazards your ventilation system will have to control.

• Is ventilation needed only to provide oxygen, or will ventilation also have to remove flammable or toxic contaminants?