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From 2011.igem.org

The ventilation hood is a workspace that helps prevent the accidental inhalation or general overexposure of the researcher to hazardous materials and vapours. The ventilation hood, also called "The Hood" can be found in most laboratories. Our Team is working in a lab at Towson University that is equipped with a hood. It is always turned on , i.e., the draw of air from the hood work area upu into its ventilation filters is never turned off. This is because some researcher using the lab may have a hazardous chemical sitting under the hood at any given time.

To use the hood, turn on the light (usually on the front or side of the hood), open the glass door just high enough that you can put all of the reagents , glassware, and measuring equipment you will need to use, then lower the glass door as far as is possible that still allows you to reach underneath to work.