Improvements to dust measurement techniques. Final report on CEC Contract 6253-22/8/016

The main object of the research was the development of instruments for measuring airborne dust acting on the body through the upper respiratory tract or ingestion, as well as alveolar dust. Secondary objects were the measurement of personal variations of inhaled dose of respirable dust by means of a size-selective respirator and the making of a portable spectrometer which would sort dust by falling speed and measure it gravimetrically.The main laboratory tools were a wind tunnel and a calm-air chamber in which monodisperse aerosols could be produced with particle aerodynamic diameters from about 6μm up to about 25 μm in the tunnel and 40 μm in the chamber. Dispersates were alcoholic solutions of dibutyl phthalate and di-(2-ethylhexyl)-sebacate with a fluorescent tag. Concentrations were measured by collecting on filters and weighing or assessing flourimetrically. In the calm air chamber, a wide range of probe diameters and flowrates were found to give the same concentrations, contrary to usual supposition. In the tunnel, some consideration was given to effects of turbulence and probe diameter on isokinetic sampling.Some results were also obtained using coal dusts. These were sized using a Coulter Counter Model TA, and for coal trisodium orthophosphate was found superior to saline as an electrolyte. An improved method of calibration was developed for the Coulter. It was found that the size distributions of respirable coal dusts could be characterized by the volume median diameter and the ratio of third to first quartile diameters of the volume distribution.Because sampler performance for larger particles is so wind dependent, it was necessary to determine the concentrations of particles entering the human nose and mouth under different conditions of wind and breathing, so that practical sampler behaviour could be compared with human behaviour. It was found that human head entry efficiency depended strongly on wind speed and direction, breathing rate, and particle size, but less strongly on whether the subject breathed through nose or mouth, on the size of these orifices and other facial structure, and on the angle to the wind in the vertical plane. When averages were taken for all wind directions, effects of other variables decreased, so that it is possible to specify an entry efficiency curve for a worker uniformly exposed to all wind directions, passing through about 80% at 5μm and 50% at 30 μm.Practical dust samplers were examined in the various winds to see how well they followed this curve. Special attention was given to the MRE gravimetric sampler (Cassella Type 113A), to see if its elutriator deposit could simulate upper respiratory tract dust, but the MRE was found to decrease in efficiency too quickly with increasing particle size and wind-speed. (The decrease was severe enough to suggest slight undersampling of respirable dust, but direct measurements in coal dust failed to confirm this). These and other results permitted some generalisation about the behaviour of blunt samplers in wind.Because existing samplers were found unsatisfactory, a new 47 mm filter holder was developed – the ORB sampler – which imitates the directionally-averaged human head characteristics in winds up to 2.75m/s and aerodynamic diameters up to 25 μm. It operates at 21/min, and is an area sampler, not a personal one. This sampler should permit realistic measurement of the upper-respiratory-tract dust dose.As steps towards the design of a respirator which would act as a respirable dust sampler, permitting study of individual dose variations, the size distributions of dust caught in respirator main filters and on MRE filters were compared, and measurements were made of collection efficiencies of open-foam plastic filters. An electronic breath counter for incorporation into a respirator or Wright Respirometer was made and intrinsic safety approval obtained.To facilitate the study of aerodynamic gravimetric size distributions of underground dust, a multihole cascade impactor was made, which could be powered by a Simpeds pump, and, if desired, fitted behind a Simpeds cyclone; so that the respirable fraction could be studied on its own if required. Coulter counter study of the deposits on the different stages gave information on the relations of aerodynamic and equivalent spherical diameters for coal dust. “”