When you’ve been working in a particular profession for a while its easy to forget how confusing terminology can be. I find that although we take for granted what is meant by “local” and “general” ventilation, the meaning is not necessarily obvious to someone new to occupational hygiene or to non-specialists, such as managers and workers in industry.
“Local exhaust ventilation” is used to describe extraction systems that extract contaminants close to the source, thereby preventing dispersion into the workplace. Yet although most people would probably interpret “local” as meaning “close to”, I don’t think that “local ventilation” is necessarily understood to mean that capture occurs at source. I’ve seen lots of poorly designed systems, including fans located in walls a fair distance from the source classed as “local extraction”. Similarly the term “general ventilation” is rather vague and I’m not convinced that most people understand what we mean by this – i.e. the use of extraction to dilute contaminants in the ambient workplace air.
There are also some situations where the terminology breaks down. For example, in a walk in spray booth contaminated air is extracted from the whole room, not from near the source. The worker inside the booth is not fully protected as he/she is still located within the contaminant cloud. From his / her perspective this is not local extraction. Yet, at the same time, it is something more than what we would normally class as “general ventilation”.
To overcome these problems, perhaps the use of alternative terms such as source ventilation, room ventilation and dilution ventilation would be better.