Posted By Gordon Hello all, I was hoping someone could explain 'in real simple terms' understanding WEL for the purpose of a COSHH assessment. So for the purpose of helping me understand: Substance A carries a maximum safe value of 200ppm / 1080 mg/m3 air. Im told manufacturers calculate WEL at an average of an 8hr working day. Would I be correct in saying I shouldn't be trying to work out how many ppm the person is exposed to just as long as the substance is used in the correct manner with the appropriate control measurements and that the exposure is less than 8 hours other wise after 8 hours they would be exceeding the 200ppm? If the person had used the substance for 4 hours, would they have been exposed to only 100ppm? Thank you for your guidance in this matter.

Posted By Paul Leadbetter Hi, Gordon The concentration is independent of exposure time, all things being equal. If the concentration is 200 ppm, then the exposure will be 200 ppm, regardless of exposure time. However, concentration x time = dose; so, if the exposure time is halved, the dose is halved. Paul

Posted By jg sayle I maybe should have justified that TWA is an abbreviation for Time-Weighted Average Exposure

Posted By Peter F. Gordon, I bet you're delighted that you asked people to simplify WEL.

Posted By garyh If you study EH40 it explains WELs and also tells you about calculating time weighted averages. My understanding is that the WEL is the limit to which you can be exposed to without any ill effects on your health. However, my experience is that if people's exposure is more than 50% (some say 30%) then this is unacceptable and you need to get exposure down. Hope this helps.

Posted By garyh See page 8 of EH40. I teach this at NEBOSH dip level. Basically the 'No-Observed Adverse Effect Level' (NOAEL) is used as the basis for a WEL.

Posted By garyh COSHH assessor......... Page 36 of EH40........."If a NOAEL (or LOAEL) can be identified, then this value is taken as a starting point for estimating the highest level of occupational exposure at which no adverse health effects would be expected to occur in workers or their progeny following exposure over a working lifetime." Granted, this is not neccessarily the only basis (eg, other factors such as toxicological, epidemiological, socio economic factors etc) used by ACT / HSE, but is usually what determines the WEL. If you know different, please expand on this.

Posted By DaGuru Ahh, Dontcha love COSHH!

Posted By Bill01 Good point Chris. If inhalation occurs then you may have other routes to consider e.g skin, mucous membranes, ingestion etc. Chemicals may well behave very differently depending on the route of entry.