All hoods should be labeled with the latest face velocity measurement and when it was made. Be sure that all hoods are checked at the required intervals. Work as far inside the hood as possible. Keep exhaust slots free of obstructions Use an anemometer or strips of tissue paper to verify air flow. Avoid disabling any alarms that indicate inadequate flow.
Snorkels are most useful where the contaminant releases are localized.
The control zone may be extended somewhat by using baffles or partial enclosures. Small scattered sources of contamination are handled best by general ventilation. When the worker is directly involved or working closely with the contaminant, general ventilation is less effective. The key to use of general ventilation is to not exceed the TLV for the contaminant.