Introduction to Health and Safety at Work

Chemical and biological health hazards and control
237
14.9.4 Engineering controls
The simplest and most effi cient engineering control is 
the segregation of people from the process and a chem-
ical fume cupboard is an example of this as is the hand-
ling of toxic substances in a glove box. Modifi cation of 
the process is another effective control to reduce human 
contact with hazardous substances.
More common methods, however, involve the use of 
forced ventilation – local exhaust ventilation and dilute 
ventilation.
Local exhaust ventilation
Local exhaust ventilation removes the hazardous gas, 
vapour or fume at its source before it can contaminate 
the surrounding atmosphere and harm people working 
in the vicinity. Such systems are commonly used for the 
extraction of welding fumes and dust from woodworking 
machines. All exhaust ventilation systems have the fol-
lowing fi ve basic components:
1. 
a collection hood and intake
– sometimes this is a 
nozzle-shaped point which is nearest to the work-
piece, while at other times it is simply a hood placed 
over the workstation. The speed of the air entering 
the intake nozzle is important, if it is too low then 
hazardous fume may not be removed (air speeds of 
up to 1 m/s are normally required)
2. 
ventilation ducting
– this normally acts as a con-
duit for the contaminated air and transports it to a 
fi lter and settling section. It is very important that 
this section is inspected regularly and any dust 
deposits removed. It has been known for ventilation 
ducting attached to a workshop ceiling to collapse 
under the added weight of metal dust deposits
3. 
fi lter or other air cleaning device
– normally 
located between the hood and the fan, the fi lter 
removes the contaminant from the air stream. The fi l-
ter requires regular attention to remove contaminant 
and to ensure that it continues to work effectively
4. 
fan
– this moves the air through the system. It is 
crucial that the correct type and size of fan is fi tted 
to a given system and it should only be selected by 
a competent person. It should also be positioned so 
that it can easily be maintained but does not create 
a noise hazard to nearby workers
5.