How long you can use it for isn't a valid question. It's not acceptable to only just achieve the limit value.  You have to reduce the exposure as far as reasonably practicable, as well as below the limit value.  And setting a time limit is far from the best way to reduce exposure - there are other methods to consider first, such as containment and ventilation.

You can only know the concentration in the air, whether in ppm or mg/m3, (to compare with the limit value given in EH40) if you get an occupational hygienist to measure it for you.

Edited by user 18 July 2023 16:54:55(UTC)  | Reason: Extended para 1

Thanks Peter and Kate, 

Peter apologies to add more context for example i use a chemical which contains ethyl acetate which is stated in the EH40 table as 400 PPM STEL. how do i know how much of this to permit on site for use ? how can i then assess suitable PPE should it be impossible to avoid using this chemical ? in brief how do use the table and what does the 400PPM figure actually mean ? 

hope you can help apologies for not explaining better in my first post. 

Simon 

Thanks Kate, 

it is starting to make more sense to me now thank you. 

so whats the difference between having a chemical with 400PPM in the table and another width 1 PPM is there a threshold, is 1PPM then safer and needs less controls than working with 400PPM or have i got this wrong ? 

It's the other way round - the more nasty a chemical is, the lower its exposure limit will be, because a smaller amount of it in the air will be enough to cause harm.  The lower the exposure limit, the lower you need to make the exposure and, other things being equal (which they aren't), the more control you will need.

Other things are not equal because how many ppm you actually get in practice will depend on how you are handling the chemical, how much you are using, how volatile the chemical is (its vapour pressure - some chemicals give off more vapour into the air than others and vapour pressure is a measure of this), the ventilation you have and so on.   So for example if you have a chemical with lower vapour pressure, less of it will get in the air and you may not need as much control.

If you are unfamiliar with ppm it might be helpful to realise they are the same idea as percentages, just on a different scale.  A million (1,000,000) parts per million is the same as a hundred parts per hundred or 100%.  Similarly 100,000 parts per million is the same as ten parts per hundred or 10%, and so on.

Originally Posted by: Taha Abdul ghani

It lists the Permissible Exposure Limits (PELs)

Sorry but the point Kate & others have been emphasising through this post is that there are NO permissible limits expressed in EH40.

EH40 is a tool to assist with COSHH compliance where the desirable exposure limit is always zero.

Even the old term MEL (Maximum Exposure Limit) has been removed from use.

Confusion is likely to worsen given EU CLP SDS contain DNEL (Derived No Effect Limit) although here there is consideration for groups such as workers or the general public by the routes of exposure.

OK time to COSHH

First you need to do a COSHH risk assessment. Like any risk assessment this is about the process not the substances. If the stuff is sitting in sealed can and unused is does not pose any(serious)  risk. As soon as someone opens it and starts to use it, it becomes a potential hazard( ie a risk). What you need to do is ( like any risk assessment) establish the level of risk and what sort of controls you need to apply to reduce the risk to something acceptable.  You start with establishing that you actually  need to use this particular substance for the process. If you can substitute  a less hazardous substance then you should.

If you decide that you must use the substances in question , then you need to establish how much of a hazard it is, ie its potential to cause harm. Most substances  look like colourless  liquids or white powers, so you need a source of information, which is where the SDS comes into the process. Note, SDS simply describe  the potential  for harm, not the risk in your workplace to your staff.

One of the trickiest routes for exposure to hazardous substances to manage is the airborne route through dusts or gases or vapours. Because this the government has set Workplace Exposure Limits(WEL) for those substance they know have an airborne route of exposure.    SDS should mention the WEL and all of these are listed in EH40.  The exposure limits are based on the average exposure over an 8 hour shift, so people may for short periods be exposed to higher than the WEL, but aim should always be to limit the exposure to as low as reasonably practicable. Some more hazardous substances, also have a Short Term Exposure Limit (STEL). This is averaged over a 15 minute period. It is typically 2 to 3 times higher than the WEL.

The only exact method of measuring the level of exposure is to get an occupational hygienist in to measure the levels in  peoples’ breathing zone while they are doing the task. Sometimes it might be possible to get a rough estimate of the level of exposure but to be sure you need it to be measured.

How to control the exposure? There is an hierarchy of controls and for volatile substances the best control is enclosing the process, then looking at LEV. If that is not possible and you can’t do anything to reduce the amount of airborne material in the atmosphere then you look to RPE, which has to be the right RPE for that substance,  fitted to the user, who is trained,  with the RPE being properly maintained stored etc.