When should we use local exhaust ventilation over general exhaust ventilation?

According to the OSHA Technical Manual, general exhaust ventilation (dilution ventilation) is appropriate when:

• Emission sources contain materials of relatively low hazard (the degree of hazard is related to toxicity, dose rate, and individual susceptibility);
• Emission sources are primarily vapors or gases, or small, respirable-size aerosols (those not likely to settle);
• Emissions occur uniformly;
• Emissions are widely dispersed;
• Moderate climatic conditions prevail;
• Heat is to be removed from the space by flushing it with outside air;
• Concentrations of vapors are to be reduced in an enclosure; and
• Portable or mobile emission sources are to be controlled

Local exhaust ventilating is appropriate when:

• Emission sources contain materials of relatively high hazard;
• Emitted materials are primarily larger-diameter particulates (likely to settle);
• Emissions vary over time;
• Emission sources consist of point sources;
• Employees work in the immediate vicinity of the emission source;
• The plant is located in a severe climate; and
• Minimizing air turnover is necessary.