Chemical Laboratory Hoods, con't.
The recommended face velocity is between 100 and 125 feet per minute averaged across the hood opening when the sash is raised 18 inches above the work surface. As you can tell from the calculation above, that's not very fast. It would be very difficult to move your hands and arm any slower than this, so your movements at the hood could bring contaminants into your breathing zone. This is why it's important to plan your work at the hood.
Remember, chemical laboratory hoods are designed to contain hazardous materials that are used or may be generated within the hood. They are not designed to capture materials outside of the hood. Make sure you work at a safe distance within the hood.
