General ventilation is the venting of air from a shop to the outside through openings in the walls or roof. A fan is often used to exhaust air but there is no connection between the fan and a specific operation within the shop. Any air exhausted by this type system must be easily replaced or it will not work. Therefore replacement air must be provided by a mechanical air supply unit or an open window or door. If your shop uses general ventilation, try to arrange your work so that the source of replacement air is close to you and the air movement, perhaps toward an exhaust fan, is away from you. Higher hazard work should be done using a local exhaust ventilation system which captures the contaminants right where they are produced.
Local Exhaust Ventilation (LEV)
A local exhaust ventilation system captures air contaminants where they are produced and moves the contaminated air to the outdoors. Some systems will have an air cleaning device, to remove the contaminants before they are exhausted outside. A spray booth and its associated ducts and fan are an example of a local exhaust ventilation system. Other local exhaust ventilation systems might be found in the paint prep and paint mixing areas of autobody shops. When working with local exhaust systems there are just a few things to remember.
- Work as close to the hood as possible. The capture distance of most hoods is short and if you are working outside the capture zone of the hood, it will not be effective.
- Don’t position yourself between the exhaust hood and your work. This will draw contaminated air back into your face.
- Make sure the hood is not blocked and any filters are not overly dirty. You will need to replace filters following the manufacturer’s instructions and when the pressure drop across the filters increases beyond the manufacturer’s recommendation.
Components of a Local Exhaust System
A typical local exhaust ventilation system is composed of five parts: hood, duct, air cleaner, fan, and stack. Local exhaust ventilation is designed to capture an emitted contaminant at or near its source, before the contaminant has a chance to disperse into the workplace air. [from OSHA Technical Manual, Section III: Chapter 3: Ventilation
Locally Exhausted (or Ventilated) Sander
Sanding, to remove paint from surfaces and to smooth body panels repaired with filler, generates high dust concentrations. This dust may contain harmful substances and exposures may exceed OSHA standards. Workers exposed to this dust can be protected by using specially designed sanders that are connected to vacuum systems. A HEPA vacuum or a central vacuum system will do. These locally exhausted sanders draw the dust through holes in the sanding pad - but be careful that the holes are not blocked! Ventilated sanders can cut total dust concentrations to one-tenth the levels produced using unventilated sanders. The use of ventilated sanders also results in a cleaner shop and shortens clean up time.