A fume hood is a ventilation unit that provides a safe working environment when working with toxic and otherwise harmful chemical fumes and airborne particles. Fume hoods are typically used in a laboratory environment and provide airflow within the hood to either exhaust harmful fumes outside or filter and reintroduce clean air back into the work space. Ducted fume hoods should be exhausted far enough away from a building to ensure the air is not pulled back into the building by air intakes. Duct work for a fume hood needs to be separate from the HVAC ducting that provides air to work spaces. Corrosion resistant ducts may be needed depending on the chemical fumes being ventilated. If a fume hood is ductless, periodic filter changes will need to take place to ensure circulated air doesn't provide a health risk.