One problem with all hoods, but definitely more so with chemical hoods than others in our experience, is that as a cabinet space in a lab they tend to accumulate stuff and are frequently used for storage.

Chemical fume hoods are designed to capture and exhaust contaminates resulting from working with chemicals. They are sometimes referred to as wet benches as the chemicals used (solvents, corrosives etc.) are usually liquid. Their design attempts to create a capture zone in front of the hood, draw contaminates away from the worker and into the hood where they are exhausted.

It is up to the operator to know how to adjust flows for their particular need. Some storage in the hood may not affect your use and could be left in while performing other operations. Things to check and keep in mind are: First, are there dead spots in the face velocity and are they located where capture is needed? We recommend face velocity be checked using a grid pattern and that readings not differ by more than 10%. Alternately, air current or smoke tubes could be used to detect dead or low flow zones. Second, where is capture needed? Are you working with vapors that are lighter than air or heavier? If they are heavier than air, the dampers should be adjusted to capture at the bottom of the hood (e.g. open the bottom slot and close down the upper one) and storage blocking the lower slot may hinder flow and thus hinder proper capture. One quick fix is to install a shelf above the lower baffle so reagents and chemicals stored on the shelf do not block the lower slot. If the vapors are lighter than air you may be okay with some storage in the hood. Use smoke tests to confirm this.

From: Get in the Flow! by The Safety Guys