The Variability of Particulate Atmospheres

Inhalation Toxicologists accept that there will be variations in the physical nature of test aerosols which cannot be readily attributed to any single cause. The performance of a chamber suitable for the containment of a particulate atmosphere has been assessed across a range of inlet designs and operating conditions. Different inlet designs allowed different stable ventilation patterns to be developed, however, these patterns remained constant and were not considered to be sources of aerosol instability. The largest sources of instability were associated with the methods of generation. Several methods of generation capable of dispersing either liquids or from finely divided solids have been evaluated. Many such methods are used to generate atmospheres outside their intended operating range. As a result such devices are often considered to be inefficient and subject to uncontrolled variation in output. This work has shown that it is essential that the chosen method of generation is capable of exceeding the target concentration or particle size distribution in order to achieve a stable aerosol. The need for adequate electrostatic conditioning of the test aerosols prior to entry into the chamber has been demonstrated, although the significance of electrostatic charge may not yet be fully appreciated by those engaged in industrial inhalation toxicology. When operating at mass concentrations in excess of 5000 mg.m-3 the choice of sampling duration has been shown to be critical. An acceptable range of sampling periods is defined. In order to achieve a sufficiently stable particulate atmosphere it is necessary to meet certain criteria: The performance of the chamber to be used must be adequately characterized and it ’s limitations defined; the choice of generation system must allow for the adequate generation of an aerosol in excess of that required; the physical evaluation of the aerosol should include an assessment of the electrostatic charge state of the aerosol, especially at concentrations in excess of 1000mg.m-3; the aerosol sampling practices should be appropriate to the mass concentration of the aerosol; the sampling period should not be so long as to overload the sampling device, nor should it be so short as to cause losses of efficiency due to the short periods of flow instability at the beginning and end of the sampling period.