The level of airflow will depend on all sorts of things. There are all sorts of LEV about including for example biosafety cabinets. Those are subject to an EN standard. For other materials it is really down to your risk assessment and taking into account the WEL for the substances you are dealing with. But remember for some substances there is a duty to reduce exposure to an absolute minimum practicable which would mean that the LEV would be expected to be running at full capacity. One question is how might reducing the airflow in your LEV save you money. The reduction in electric bills would be marginal. Is it more to do with reducing heat loss?

The airflow of the LEV system will (should have) been set so that the most unfavourable extraction location will have sufficient flow to ensure that the contaminant is not only removed but remains airborne and transported all the way to the filters, with a sufficient margin to ensure that the system remains effective during its working life. The original design and installation specification should provide sufficient information for the engineers to make a judgement on whether you will be able to make any changes now, assuming they are competent in this area. Otherwise a specialist ventilation engineer's opinion should be sought.

Please Note EH40: Dust 44 The COSHH definition of a substance hazardous to health includes dust of any kind when present at a concentration in air equal to or greater than 10 mg.m-3 8-hour TWA of inhalable dust or 4 mg.m-3 8-hour TWA of respirable dust. This means that any dust will be subject to COSHH if people are exposed above these levels.

One factor to keep in mind is that it isn't just the flow rate that is important. Unless the design of the capture arrangement is correct the system simply will not work effectively, irrespective of the flow rate. I have seen systems where by redesigning the capture arrangement we were able to reduce the fan rating from 18 kW to 2.4 kW and still achieve a higher level of capture than was being achieved before. I always remember what the person who taught me what I know about LEV once said to me. "Chris, I don't make my money designing new LEV systems. I make it putting right other's systems that could never work!" I have had situations where the client has had the LEV systems tested and they are all achieving the flow rate specified, i.e. performance was to spec. However, a simple smoke test showed that they were not effective at removing the contamination. Performance OK, effectiveness not! If you wish to modify the flow rate, get yourself a simple smoke tube system with which you can very simply check effectiveness. Chris

i totally agree with Chris Packam surprised no one has mentioned controlling airborne contaminants at work http://www.hse.gov.uk/pubns/books/hsg258.htm

The smoke test is a good idea, but may not be relevant to chemicals heavier or lighter than smoke. You need to ensure the correct velocity at the edge of the LEV if it is a large hood.