Local Exhaust Hoods

What is a local exhaust hood?

A local exhaust hood is an engineering control measure designed to reduce exposure to airborne contaminants by capturing the emission at the source and transporting it to a safe emission point or to a filter/scrubber.

Classes

• Enclosing hood – Enclosing or fume hoods, are hoods surrounding the process or point where the contaminants are generated. Enclosing hoods can be completely enclosed (all sides enclosed) or partially enclosed (two or three sides enclosed). The effectiveness of an enclosing hood increases as the area of opening decreases.
• Receiving hood – Receiving hoods are designed to receive or catch the emissions from a source that has some initial velocity or movement. For example, a canopy hood receives hot rising air and gases. A rising hood must empty as quickly as it is filled and if the extraction flow rate is less than the rate of contaminated air entering the hood, leakage will occur around the perimeter.
• Capturing hood – Capturing hoods are located next to an emission source without surrounding or enclosing it. A capturing hood must generate sufficient air flow immediately around the contaminant to draw it into the hood. This is sometimes referred to as the capture zone or envelope. Capturing hoods can be fixed or moveable. Capturing hoods are limited by size and reach. Inside the capture zone, the airborne contaminant will be captured and removed by the hood. Outside of this zone the capture efficiency falls rapidly to zero and as a result, they are not suitable for energetic sources or where there are significant drafts that cannot be suppressed.

General Considerations

Hoods should be placed as close as safely possible to the contamination source. Air should travel from the source and into the hood with enough velocity to adequately capture the contaminant. The natural movement of contaminants should be taken into consideration. For example, a hood should be placed above hot processes to trap rising gas and heat.

Limitations

Any air motion outside of the hood and surrounding areas may affect how the air flows into the hood. A ventilation system will require a higher airflow speed to overcome any air disturbances. As much as possible, other sources of air motion should be minimized or eliminated. Common sources of external air movement include:

• Thermal air currents, especially from hot processes or heat generating operations;
• Motion of machinery such as grinding wheels, belt conveyor, etc.;
• Material motion such as dumping or filling;
• Movements of the operator;
• Room air currents; and
• Rapid air movement caused by spot cooling and heating equipment.

How do you know if a Local Exhaust Hood is working as designed?

Local exhaust hood performance should be evaluated using a recognized method by qualified person. Where practicable, local exhaust hoods should be equipped with a device that will provide a warning when the system is not working effectively.

Ventilation systems should be inspected, maintained and cleaned at a frequency that is sufficient to protect the health and safety of workers. Records should be kept of all inspections, maintenance and cleaning, including the individual who performed the duties. Records should be readily available to Occupational Health Committees.