I think I may have melted my brain, and don't even seem to be able to get any useful Internet search answers. More coffee is obviously required. A substance has a WEL of 2mg/m3, 15 minute reference period. A person is stood with their head in this 1 cubic metre cloud with 2mg of substance in it, breathing in and out for said 15 minutes, then they must move out of said cloud when their 15 minutes is up to avoid going over the limit But it obviously makes a difference if they are just standing there or are doing strenuous work. When monitoring takes place what is the assumed flow rate to simulate someone breathing or is there a range or do we always take worst case. This is part of a larger thought process, I think I have tied my brain up in knots. I know I know this or at least used to, I probably have the info at home, but sadly I'm in work at the moment. Chris

Thanks JayPownall It is less than I was expecting, not sure why. Thanks for the info though much appreciated. Chris

Morning. Personal air samplers are set at 2.2m3 or 37 l/mim. Normal breathing rate for an adult

I don't want to throw a big spanner in the works, but if you are considering systemic effects and if the chemical can be absorbed through the skin then it is the total dose by all three routes (inhalation ingestion and dermal) that you need to consider. It is, of course, the total dose reaching the internal organ or system that is what you need to know. With many chemicals you can also trigger facial dermatitis in a previously sensitised person at levels below the standard WEL. Chris

Chris I was going to send you some thoughts on this but was told your mailbox was full. Chris

In context of inhalation, I very much doubt that one can apply the WEL Criteria to "liquid mixture aerosol" that have a proportion of a substance with a WEL and the remainder does not, unless you presume that it will become volatile and a gas at operating/ambient conditions--in which case the standard technique still applies. However, EH 40 has some guidance on mixed exposures HSE Guidance Docs to refer to are :- Monitoring strategies for toxic substances http://www.hse.gov.uk/pubns/priced/hsg173.pdf Methods for the Determination of Hazardous Substances (MDHS) http://www.hse.gov.uk/pubns/mdhs/index.htm http://www.hsl.gov.uk/re...ions/mdhs/mdhs-revisions Last, but not least, EH-40--not the WEL Valuyes, but there is a whole lot of "guidance" in it regarding calculations etc http://www.hse.gov.uk/pubns/priced/eh40.pdf

I think that there is some un-needed complication going on here. in WEL and STEL there are no mention to breathing rates. They are simply about airbourne concentrations. for a 15min STEL if the concentration of the substance is over it's 8hr WEL or, if present (where no WEL exists) then you must ensure the worker is not present for more than 15 mins. If exposure time is likeley to increase, or if concentration exceeds STEL then additional controls are required.

Just my thoughts, I like a goof theoretical scenario.... Agree with the above volume is not required nor breathing rates, but you cannot also assume 100% aerosol in air / making it 100% water... as you say impossible as inhaling liquid, but even using this: Assuming the same density as water, 0.05% of a cubic meter (1000 litres) is 0.5litres or 0.5kg/m3, 500g/m3, 500,000mg/m3. Slightly above your WEL. The trouble you have, is that realistically only a small proportion of that cubic meter is actually airbourne / aerosol. Even if you assumed that 99% of that cubic meter is air and 1% liquid your concentration has only dropped to: 1% = 10 litres, at 0.05% concentration of harmful agent, is 0.005litres or kg/m3, but still a whopping 5000mg/m3 harmful agent. I believe you can apply the WEL Criteria to "liquid mixture aerosol". But you need to know the concentration of aerosol in the air. There are some real time hand held meters which can tell you this. From here we would then need to just factor in the concentration of that aerosol which is the harmful ingredient with the WEL (0.05%). So if you measured the air and found 2000mg/m3 of aerosol, and know that the solution from which it is formed is maximum 0.05% hazardous material, the concentration the person is exposed to would be 1mg/m3. Though some of that 2000mg/m3 measured would likely be other aerosols / particles present in the air it would give you a worst case. Can you calculate the likely % aerosol in air? This depends if you know the ratio of the volume of liquid and volume of air used to generate the aerosol. This could be possible if you know the flow rates of each in the equipment you are using, for example 10litres/s of air for 100ml of fluid etc.... The best analogy I can think of for actually measuring mists / aerosols in the workplace is that for oil mists: http://www.hse.gov.uk/pubns/mdhs/pdfs/mdhs84-2.pdf But as you say unless you can demonstrate by risk assessment you are unlikely to exceed the WEL to make measurement not necessary. To factor in the effect of air replacement / ventilation or dilution would be another matter, which I could also try to explain, if I have made any sense what so ever so far?

that should be 100% fluid

My two pennorth for what its worth... Aerosols can be very strange beasts, and a lot of the behaviours associated with gases simply do not happen with aerosol... A huge variation occurs in physical and physico-chemical properties dependant on the droplet size. There are a range of factors to take into account, but perhaps the most relevant here is droplet size. If the aerosol production is from a very high energy process, then the droplets will be relatively uniform in size. Lower energy commonly produces aerosol with a wider range of diameters. An aerosol with critical droplet size could impact and deposit in a very short range of the respiratory system. The material is not distributed evenly throughout the lungs and trachea, it may only deposit in a short length or a particular part of the alveoli... Whilst the systemic load may be calculable, the direct contact range is liable to be more concentrated and do local damage which could not be predicted from the gas equivalent calculation... A fuel/air aerosol explodes far more violently than an equivalent w\w fuel air vapour mix. Think buncefield. In an aerosol droplet, the surface tension effects are greatly magnified and this may serve to concentrate or dilute the effective concentration of any substance in the liquid mix. A petrol water mix which may be considered non flammable in bulk can explode violently in aerosol form as the fuel is concentrated at the surface of the droplets... Similar anomalous issues are likely to affect toxic responses for many compounds... For corrosives and irritants such as caustic this MAY be less of an issue than it would be for eg a carcinogen, but extreme care is needed when making comparisons such as here.

Yes there is nonionic surfactant in the product I was thinking about. Yes this really is a thought experiment, though I imagine some may feel I'm just trying to get others to do an assessment for me. I use this site to help me understand things I feel others have come across and point me in the right direction ( even the wrong direction helps you eliminate or even lead to the right direction). However I don't want others to do things for me, how will I understand for the future. Providing enough info to understand is ok though. I have another product that we use that does not have this ingredient, so all good. In fact everyone where I work assumed they were the same product ( just different size containers) as my predecessors had not done an assessment. I am sure the stuff in question is ok to use I was just seeing if I could put any logic behind that "opinion". As per my post at #12 The key part being "unless the risk assessment ......Unlikely to ever exceed the WEL" Is the "Unlikely" in this just opinion or can it be backed up by some form of logic. I started my working career as an engineer and things were less fuzzy in engineering. It is like people are afraid to make a decision and clearly define things. H&S should not be surrounded by cloaked figures practicing dark arts only understood by them to produce an assessment - IMHO. Chris

Chris Have sent you yet another PM. Chris