The primary form of protection from overexposure by inhalation in Wittenberg science laboratories is fume hoods. Laboratories are generally equipped with at least one negative pressure fume hood that pulls vapors of hazardous chemicals away from the user. If the laboratory does not have a hood, then a negative pressure glove box should be used or an appropriate respirator.
The CHP states that whenever exposure by inhalation is likely to exceed the threshold limits described in the SDS for that particular chemical, a fume hood should be used. Therefore, before using a compound check the SDS for that compound to determine whether it should be used exclusively in a fume hood. The CHP also provides guidelines for threshold limit value (TLV) or permissible exposure level (PEL). If the TLV or PEL is less than 50 ppm or 100 mg/m3 then the chemical should be used only in a fume hood.
Before using a fume hood, check to see that it is working properly. This can be accomplished by closing the sash to within one inch of being completely closed and taking a small strip of tissue or Kimwipes and placing it near the one inch opening. If the hood is working, the strip of tissue should be drawn into the hood demonstrating negative pressure. If the strip does not show negative pressure, then inform the lab supervisor to see if the room ventilation is turned on. The hoods do not work if the room ventilation is not working. If the room ventilation is working and the hood is not, call physical plant so a maintenance worker can check to see if a fan belt is broken or a circuit breaker has been tripped.
The hood performance can be greatly affected also by these factors:
- Objects obstructing the return ducts. In particular, often people store chemical in hood. This cluttering of the hood interferes with the air being pull from the back bottom portion for the hood. Remove unnecessary chemical and equipment and return them to their proper location.
- The position of the equipment or apparatus should be at least 6 inches back from the front sash. This can improve the capture rate by 1000 times.
- Positioning the sash in the down position greatly increases the flow rate through the aperture. It is strongly recommended that the hood sash be kept down whenever possible.
- Secondary currents created by portable fans, traffic in front of the hood, and wind conditions outside the building can push fumes into the room.
- Positioning of the baffle at the rear wall of the hood determines whether the air will be drawn form the top or bottom of the hood. This baffle plate should be in the down position for fumes lighter than air and when using a burner in the hood. The position of the baffle should be up for fumes that are heavier than air. Most hoods are set at the median position to draw moderately from both points. Only the Building Supervisor and laboratory supervisors are authorized to change the baffle position.
After you have checked the SDS and find that you are not required to use a hood, the reagent may be used in the open laboratory provided the room ventilation is working.