Stack heights

When designing and testing local exhaust ventilation systems we need to pay particular attention to the design of the extraction hoods - where the contaminant enters the system. If this isn't right then the system is unlikely to be effective at controlling contaminants. However, this doesn't mean that we shouldn't ensure that other aspects of the system are properly designed.

In many cases the system will exhaust outdoors and its then important to ensure that any contaminants remaining in the airstream are dispersed effectively so that they do not re-enter the building. This means that they shouldn't be located too close to any air intakes, vents or windows. It is also particularly important that the stack is high enough. A good "rule of thumb" to follow is that the stack should be at leas one third the height of the building (i.e. it should release at a height 1.33 times the building height). The stacks on the laboratory building shown in the picture above meet this criterion. There are plenty of others out there that don't! Here's a few.

Terminolgy

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.

Plants aren't always good for you!

I spotted this article in the New Scientist.

I've often come across the view that using plants is a good way of dealing with pollutants in the indoor environment (see my previous post here). Although there is some merit in this, it isn't always true that plants are beneficial and the research findings described in the New Scientist article shows how plants can actually increase pollution.

All living organisms process, produce and emit chemicals. Sometimes these are beneficial and sometimes they are harmful. Just because something is "natural" doesn't mean that it is harmless. Botulinum toxin is a "natural" chemical - and its extremely toxic.