While different RPE may be needed in the short-term, your friends need better ventilation (remember the hierarchy of control). As eye-watering occurs, full-face RPE is required to protect the eyes as well as the airways. Activated charcoal should do the trick. The eye watering could be due to volatile oils (mustard, chilli, etc.) as well as, perhaps, the vinegar. Lemon juice contains citric acid, by the way, not acetic acid. LB

When you get effective extraction installed - a face mask is not a satisfactory answer - you need to work about what is going up the vent and out through the doors/windows. If anyone living or working in the vicinity, or just passing by, experiences the same or similar symptoms then you must expect trouble, a visit from EHO and a stop order, and quite possibly a substantial fine. That opens yet more cans of worms as your 'cottage industry' will face additional questions about food hygiene, liability and other insurance issues, as well as possible invalidation of existing insurance, extra tax problems..... And if you do think that a face mask is a satisfactory solution, think hard about other people. Your chef might suffer, but others might sue!

Having done some research into 'Mustard Gas', I have come up with Allyl Isothiocyanate. Might this be the active ingredient causing such discomfort? If so, I think I'd better advise them to give up this recipe before any damage is done. But I am open to expert opinion as to how this hazard can be controlled.

Rons suggestion to "do it cold" is good one - however if memory serves, the point of heating during the production of pickles and the like is to produce a reduction which is too drive off the volatile stuff and leave the flavour. Obviously doing this cold will not achieve this result. You may need to look at what it is you are trying to achieve flavour wise and try alternate methods of achieving this eg by using essences and oils rather than dry mustard, liquid vinegar etc. By the way - back in my earlier days in a production kitchen, there was no quicker way to clear the kitchen than by chucking a handful of white pepper on the range! Oh those were the days (must have been crackers).

Townshend I think mustard gas is an entirely different issue; it was so called because it smells like mustard, not because it is generated from it. LB