Exposure to physical agents is one of those concepts that sometimes just seems to elude understanding. Then, even when you feel you understand it, you try to explain it to someone and it all comes unstuck. I thought I'd share with you how we try to explain the concept on our training courses and seminars. Even if you don't need the explanation, maybe this will be of some use in explaining it to others.


Exposure applies to many areas of health and safety, though perhaps the one people struggle with most is Noise, and that's only because the numbers work in a strange way. The key point here is that all physical agents, whether that be Dust, Noise, Radiation, Vibration or EMF's work in the same way, so once you understand one, the rest should follow. The purpose of using exposures is that, almost without exception, the harm to employees comes from how long they are exposed to these things. It doesn't matter how much noise there is in a room, for example - if no one goes in there, then there can be no exposure! This means that before you enter a space where a hazard exists, your exposure is zero. This does work too, if you put zero time into the equations, you get zero exposure.

The standard day can also cause concern, because I'm not sure I know anyone who works exactly eight hours every day! It doesn't matter at all how long you work or how long you are exposed to something hazardous as long as you compare that to a standard eight hour day. This means that you are considered to have 100% of exposure time after 8 hours, so if there is 93dB of noise in a workspace and you stay in there for 8 hours, you will have 93dB of exposure. Simply enough, if you are in the hazard for 4 hours, you will have half the exposure (as noise doubles or halves every 3dB, then in our example, that would mean 90dB). It also works the other way, which is where it gets a bit more ethereal; if you are exposed for more than 8 hours, then the exposure will keep increasing. If in our example, you work for 16 hours, your exposure will have doubled, so would then be 96dB. You have to not worry about the fact that this is more than the actual measured noise level just think of the logic of it.

Analogies are always a good idea for explaining ideas, and the one I use is sunbathing (no really, I do!). if you imagine one day with beautiful sunshine for a full 8 hours (hard to do at this time of year, I know!) and then imagine you are just about to go out into it to sunbathe. Just before you step outside, your exposure will be zero (because you've not had any exposure yet). The amount of sunburn you get from that point, will be entirely dependent on the time you spend soaking up the rays, but the maximum you can have is 8 hours (100%). If you are only out there for 2 hours, your exposure would only be 25% (and hopefully, so would the sunburn). If you then had to compare that to another day, when the sun shone for 10 hours, then the maximum possible exposure on that day would actually be 125%.

There are exceptions to this, for example; when looking at some hazardous substances, where very short duration exposures are dangerous and effectively no exposure should be allowed. Also ionising radiation exposures tend to be averaged out over a 12 month period.

A little story...

I have a friend, Peter, who was in a rock band, performing 75 minute sessions, 3 nights per week and he never wore hearing protection (this was 20 years ago)! He still plays about once per week and still doesn't wear hearing protection, despite my remonstrations (just another health and safety bod trying to spoil his fun!). He has a brother, John, who has worked most of his life on construction sites driving diggers and who wore cheap, nasty hearing protection all the time. The point is that John suffers quite pronounced Noise Induced Hearing Loss (NIHL), while Peter is about normal for his age. It is highly likely that the reason Peter is ok is that the exposure time was so much less that his brother's, even though the levels were most certainly higher. It is the exposure, not the level that is critical!