Why is it important to work in the fume hood when heating up the reaction mixture?

Proper Work Practices

The level of protection provided by a fume hood is affected by the manner in which the fume hood is used. No fume hood, however well designed, can provide adequate containment unless good laboratory practices are used, as follow:

The hood user should know the normal operating configuration (NOC) of the hood and should design experiments so that this configuration can be maintained whenever hazardous materials are at risk. The NOC refers to the position of the sash established when the hood was installed and certified (i.e. how far open is the maximum safe sash position). This is where the survey sticker with arrow is located.

Before using the hood, the user should check the hood survey sticker to determine where the sash should be positioned for optimum containment for that particular unit.

The hood user should also check the magnehelic gauge or other hood performance indicator and compare its reading to the reading indicated on the hood survey sticker. If the reading differs significantly (20% or more for a magnehelic gauge) from that on the sticker the hood may not be operating properly. Contact EHS with hood location and contact information.

Never use a hood to control exposure to hazardous substances without first verifying that it is operating properly.

To optimize the performance of the fume hood, follow the practices listed below:

Mark a line with tape 6 inches behind the sash and keep all chemicals and equipment behind that line during experiments. This will help to keep materials from escaping the hood when disturbances like air currents from people walking past the hood, etc., interfere with airflow at the face of the hood.

Poor	Good	Best
Poor placement of materials.	Good placement of materials.	Best placement of materials.
Images from Kewaunee Fume Hoods

Keep the sash completely lowered any time an experiment is in progress and the hood is unattended. Note: Lowering the sash provides additional personal protection from projectiles and can also result in significant energy conservation. Keep the hood sash clean and clear.

Check area around the hood for sources of cross drafts, such as open windows, supply air grilles, fans and doors. Cross drafts may cause turbulence that can allow leaks from the hood into the lab. Also, avoid placing fans or equipment with fans in the hood in an orientation that causes the fan to blow out of the hood as the inward flow is unlikely to be strong enough to keep air and possible contaminants from escaping out of the hood. 

Extend only hands and arms into the hood and avoid leaning against it. If the hood user stands up against the face of the hood, air currents produced by turbulent airflow may transport contaminants into the experimenter's breathing zone.

Visually inspect the baffles (openings at the top and rear of the hood) to be sure that the slots are open and unobstructed. Keep baffles and other ventilation openings clean, free of accumulating dust, and unobstructed by collections of numerous containers. The work surface of a fume hood is not the place to store materials. It is meant to keep hazardous materials from being inhaled during manipulation.

For optimum performance, adjust the baffles when working with high temperature equipment and/or heavy gases or vapors. See figure below for suggested baffle positions.

Normal	Hot Work	Heavy Gases
Normal baffle positioning	Baffle position to use for hot work	Baffle position for heavy gases

Normal baffle position all slots are evenly opened.

High temperature work such as using hot plates; lower slots are minimized since heated vapors tend to rise

Heavy gasses and vapors are better captured when upper slots are minimized

Do not block slots. If large equipment must be placed in the hood, put it on blocks to raise it approximately 2 inches above the surface so that air may pass beneath it. See figure below.

Poor Placement	Good Placement
Poor large equipment placement.	Good large equipment placement.

Place large or bulky equipment near the rear of the fume hood. Large items near the face of the hood may cause excessive air turbulence and variations in face velocity.

Do not use the hood as a storage device. Keep only the materials necessary for the experiment inside of the hood. If chemicals must be stored in the hood for a period of time, install shelves on the sides of the hood, away from the baffles.

Provide secondary containment for containers that could break or spill, to minimize the spread of spilled liquids. Dishpan type containers are provided by EHS upon request. Contact EHS or use the Safety Supply Order Form (login required).

Items contaminated with odorous or hazardous materials should be removed from the hood only after decontamination or if placed in a closed outer container to avoid releasing contaminants into the laboratory air.

When using cylinders containing highly toxic or extremely odorous gases, purchase only the minimal practical quantity. Consider using a flow-restricting orifice to limit the rate of release in the event of equipment failure. In some circumstances, exhaust system control devices or emission monitoring in the exhaust stack may be appropriate.

All electrical devices should be connected to receptacles located outside the hood to eliminate the potential for electrical arcing that could ignite a flammable or reactive chemical.

DO NOT USE A HOOD FOR ANY FUNCTION FOR WHICH IT WAS NOT INTENDED. Certain chemicals or reactions require specially constructed hoods. Examples are perchloric acid or high-pressure reactions. Most special use hoods are labeled with the uses for which they are designed.

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