IntroductionSubtilisin-like protease enzymes are mass-produced and used widely in the detergent and food industries and are also used in animal feeds and leather processing. Care has to be taken in handling the raw material as it is a potential respiratory sensitising agent.1–3Due this potential risk a maximum exposure limit (MEL) of 40 ng m−3has been set.4This requires the employer to reduce exposure to a concentration that is as low as is reasonably practicable and below the MEL.5Due to the low concentrations involved, routine sampling of industrial air is usually performed with static samplers fitted with filters, such as Galley samplers, and high volume sampling at 600 l min−1or above is used. Alternatively the exposure of individual workers may be assessed using personal sampling systems attached to the clothing of the worker near their face.5These are again based on filtration but with low volume sampling at about 2 l min−1.Both approaches are limited in two respects. Firstly following sampling the filter is removed subjected to extraction and the enzyme concentration in the extract determined. Thus results are obtained hours or days after the sampling is performed. Secondly the results give a time-averaged picture of the airborne levels. This can be less than the MEL even if the MEL is exceeded transiently during an activity within the factory. It is thought that such short-term exposure could be a contributory factor in workplace sensitisation. Hence what is need is a system that can obtain airborne levels in near real-time over an extended monitoring period such as an 8 h shift.This paper describes our approach to achieving this objective. It combines a high volume air sampler based on a cyclone design with a flow injection system employing a bioreactor containing a fluorescent substrate specific for the proteolytic enzyme. Details of the design and working of the cyclone sampler are given as are the details of the mathematical model used to transform the fluorescence signals from the fluorescence immunoassay (FIA) system into airborne concentrations of enzyme. The FIA-bioreactor system was developed from two bioreactor systems for protease enzymes described previously that were based on fluorescein-labelled porcine thyroglobulin immobilised onto glass beads6or fluorescein-labelled bovine serum albumin immobilised onto a cellulose support.7In this paper we describe the development of a new substrate based on gelatin labelled with Texas Red® immobilized on cellulose and its use in a heated bioreactor attached to a fully automated flow injection analysis system. The new analytical system also employs a compact and relatively inexpensive fluorescence detection system. The combined sampling and analysis system has been positioned on a trolley and taken to two sites where proteolytic enzymes are used. The development of the analytical system and the performance of the combined system, both within the laboratory and within the factories, are described.