I don't normally like to sell our duct cleaning services, "with a policeman's hat on", but sometimes it is unavoidalbe. Especially in a highly regulated area like industrial occupational hygiene.
Here's an example of what I call "selling with a truncheon in your hand"
But the points are well made. Local Exhaust Ventilation (LEV, provided typically in the industrial workplace to remove hazardous airborne contaminants ranging from sawdust, through to gases and fumes) extract ductwork can get dangerously dirty. They fall, in the UK, under the Control Of Substances Hazardous to Health (COSHH) Regulations, and regular functional testing should be carried out to prove that they continue to provide the 'control measure', i.e. reliably pull out the contaminant. Similar Regulations whereby effective control measures should be in place for control of workplace contaminants exist in most countries.
Often the engineer responsible for the systems wants to clean the LEV system, because he knows that's the right thing to do, but he needs 'The Law' to convince his superiors that something should be done.
Of course LEV sytems should be designed so that the contaminant stays airborne and does not settle out on the duct surfaces..but it doesn't always work out like that. Not by a long chalk!
You can see deposits build up which will impede airflow (and so stop the system properly pulling workplace contaminants away from the worker's breathing zone). Often enough air velocity can be slowed down by a gradual accumulation of dusts, or by a larger item like a paper wrapper, piece of packaging, beer can (no really! where's a better place to hide the evidence of illicit drinking?), which then allows finer dirt to build up.
