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There are several types of ventilation systems, including natural ventilation, general mechanical ventilation and local exhaust ventilation.

Natural ventilation, such as simply opening doors and windows, should never be relied on to control exposure to chemicals, as several case studies in this book show.

General mechanical ventilation is suitable for only the most innocuous substances. This type of ventilation involves a fan to extract air from the building and an inlet to suck clean air back in, and merely dilutes the amount of chemicals in the workplace air. To be effective, general ventilation needs to be carefully designed, otherwise there may be areas left with no air circulation (dead zones). This was one of the problems in Brian Harris's photographic department (see p 33).

Local exhaust ventilation (LEV) attempts to extract hazardous chemicals at the point they are released, before they get into workplace atmosphere and the breathing zone of workers. There are many different types of LEV, including fume cupboards, booths, and hoods, and they too must be very carefully designed.

The exhaust air from these ventilation systems also needs to be properly treated to remove the chemicals, especially if the same air is recycled into the working environment. Like the design of other parts of the ventilation system, air treatment methods must be carefully chosen, depending on the type of chemicals involved. Ventilation systems must also be regularly checked and maintained. Regulation 9 of COSHH says, "Every employer who provides any control measure to meet the requirement of Regulation 7 shall ensure that it is maintained in an efficient state, in efficient working order and in good repair and, in the case of personal protective equipment, in a clean condition. Where engineering controls are provided to meet the requirements of Regulation 7, the employer shall ensure that thorough examinations and tests of those engineering controls are carried out - in the case of local exhaust ventilation plant, at least once every 14 months ..."

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