Ductless Hood: Use Only With Non-Hazardous Chemicals

2007-03-08 10:33:40

( Automobiles )



You normally see chemical laboratories fitted with fume hoods to protect workers from hazardous chemical fumes. There are two basic types of fume hoods, ducted fume hood and ductless hood, also called a re-circulating fume hood.

Fume hoods are ventilated enclosures; the fan draws air from the front of the cabinet, moves it through the escape hood, and expels it. Ducted fume hoods expel the air outside the building. In ductless hood systems, you make the air safe through filtration and return it to the lab room.

Most laboratories would prefer ducted fume hoods, since they are less risky. But in situations where you cannot fit external ductwork to your laboratory, you'll have to resort to ductless hoods. The filtration system becomes very important.

You should have a pre-filtration phase, generally using open cell foam to block out large particles. This filter is cheap, and will last for half a year on normal usage. After this phase, the fan sucks the fumes through a layer of activated charcoal.

Activated charcoal removes majority of the chemicals, however, some (like ammonia) can penetrate carbon filters. You will need to add specific filtration methods to remove such chemicals before re-circulating the air. Depending on use, your main activated charcoal filter should last around two years. Recurring expenses on filters may run to several hundred dollars per year.

Ductless hoods may not fully protect workers from all chemicals, since secondary filters target specific chemicals. It may be too cumbersome to keep replacing them as you switch to another chemical. Prudent laboratory procedures dictate, therefore, that you use ductless hoods only with small amounts of chemicals with low levels of toxicity.

To prevent workers from getting a false sense of security, you should put signs near ductless hood cabinets warning that they may not give sufficient protection against hazardous chemicals.


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