A delegate has sort of rocked my foundation today. they asked if you had a substance and it was below the LTEL but above the short term limit what would you do. I went through the whole acute and chronic discussion but as EH40 had both STEL and LTEL for the said substance they were failing to see the point - how could i explain it please someone help!!!

Thanks for the quick response but i am afraid that is a bit of a fluff. because it does not directly answer the question which is STEL not exceeded but LTEL is how do you proceed. Do you A) not work at all B) Stop before the 8hrs Or i am way off the mark hear.

hi Matelot, Please read the question properly. So an appropriate response can be given thanks.

The short answer is, if you absolutely had to work in that atmosphere, would be to just work 15 minute sessions ( which I believe is the time during which a STEL is measured? Happily stand corrected on that!). However, if someone asked me to go and work in, for example 140 ppm CO (TWA something like 30 ppm, STEL something like 200 ppm - from memory so not sure if they're right...) I am pretty sure I would want some more ventilation in there! I imagine you would be rotating staff like mad, and bearing in mind how things like CO get stuck inside you, it might take a day or two before you could use folk again. I know you weren't keen on matelots answers, but in reality there would be something far wrong in a situation like that and reading between the lines (Between the WELs perhaps...), it would be clear that better controls were needed if sample results had shown the proposed scenario to be the case. HTH Andrew

matelot i am sorry for my initial question you gave the appropriate response however Toe for the second question which is the first one in reverse you are absolutely incorrect. when was the last time you checked EH40 - as a general rule the LTEL will be lower than the STEL. but thanks for the responses much appreciated.

Jez: Your delegate is describing a situation where, for short periods of time, the operator exposure is above limits, but averaged over the working day they appear to be below limits. Which means the activities are exposing people to short duration 'bursts' of the substance in question. You don't really need to get into differentiating between chronic and acute effects - you need to be able to control the exposures! Fugitive emissions, exposure when adding materials to reactors, - there are several scenarios where this might be common. The solution (beyond sampling the activity and the workers into submission!) is to identify where the exposure is coming from - and to introduce additional controls for the activity / time / location in question. If it's during the loading of materials - then perhaps increased (or additional) local ventilation, a mechanical bag slitter / pourer, a raised operator platform (to take the face out of the cloud of product.... It all depends on the actual circumstances - but there are nearly always fairly simple steps that can be taken to control this kind of high exposure short duration scenario. Additional PPE may be seen as a temporary fix, but would generally be acceptable as an interim measure only. At least that would be my take on the query...

Refer to para 64 to para 66 , Pages 37 & 38 for the subtle difference in appluication & use of STEL & LTEL of EH40/2005 Workplace exposure limits--Containing the list of workplace exposure limits for use with the Control of Substances Hazardous to Health Regulations (as amended) EH40 (Second edition, published 2011 http://www.hse.gov.uk/pubns/priced/eh40.pdf Para 64 Effects of exposure to substances hazardous to health vary considerably depending on the nature of the substance and the pattern of exposure. Some effects require prolonged or accumulated exposure. The long-term (8-hour TWA) exposure limit is intended to control such effects by restricting the total intake by inhalation over one or more workshifts, depending on the length of the shift. Other effects may be seen after brief exposures. Short-term exposure limits (usually 15 minutes) may be applied to control these effects. For those substances for which no short-term limit is specified, it is recommended that a figure of three times the long-term limit be used as a guideline for controlling short-term peaks in exposure. Some workplace activities give rise to frequent short (less than 15 minutes) periods of high exposure which, if averaged over time, do not exceed either an 8-hour TWA or a 15-minute TWA. Such exposures have the potential to cause harm and should be subject to reasonably practicable means of control unless a ‘suitable and sufficient’ risk assessment shows no risk to health from such exposures.

The setting of WELs is not a precise science! In interesting article on "OELs and the effective control of exposure to substances hazardous to health in the UK (Version 3)"by Dr Mark Piney, HM Principal Specialist Inspector (Occupational Hygiene) in October 2001. http://www.hse.gov.uk/coshh/oel.pdf That is why ultimately, you have the 8 Principles of good practice for the control of exposure to substances hazardous to health explicitly as an integral part of COSHH.

Thanks all much appreciated....

"Can you have a higher LTEL than STEL?" Im pretty sure you cant, as 15 minutes into your 8 hour average / shift you would have exceeded your STEL.

Yes, I understand all that, but the question here postulates a chemical for which the 'safe' LTEL is higher than the STEL. That is, the opposite of what you describe. So it's saying if you have an exposure for 15 minutes that's dangerous, but you can have a higher exposure for longer and that will be OK. So it supposes a chemical that eg 15 minutes limit is 50ppm, but 8 hours limit is 100 ppm. Now suppose you're actually exposed to 90ppm. How does your body know after 15 minutes of the 90 ppm that you're going to keep it up for 8 hours so it's fine after all? It still seems to me to be an oxymoron to postulate a substance with a LTEL higher than the STEL.

The exposure limit is an instantaneous value, isn't it? That is, LTEL is an instantaneous value that is safe for 8 hours exposure, and STEL is an instantaneous value that is safe for 15 minutes exposure (in both cases, assuming that outside that period the exposure is zero). What I think you're saying is that the OP originally intended to say "what if the time weighted average of the exposure is below LTEL, but there is occasional very short duration exposure above the STEL?" Is that right? So this is saying suppose a LTEL of 50ppm, a STEL of 100ppm, and an actual exposure of 200pm for five minutes. Instantaneous exposure is 200ppm, which is above STEL, but 15 minute TWA is 67ppm and 8 hour TWA is 2ppm (assuming one incident in a day), so you're below both limits TWA but instantaneously well above both. That makes much more sense to me, thank you. In that case the answer seems to be straightforward - "Some workplace activities give rise to frequent short (less than 15 minutes) periods of high exposure which, if averaged over time, do not exceed either an 8-hour TWA or a 15-minute TWA. Such exposures have the potential to cause harm and should be subject to reasonably practicable means of control unless a 'suitable and sufficient' risk assessment shows no risk to health from such exposures.", as already quoted in the thread. At least, as straightforward as any question that has the answer 'do a sufficient risk assessment'.