Evaluation of grinding aerosols in terms of alveolar dose: The significance of using mass, surface area and number metrics

Aerosols generated by mechanical means are generally assumed to have low particle number and surface area concentrations compared with mass concentration. As a result, they have received little attention in the current debate over the use of number- and surface area-based metrics for low-solubility particles. However, it is plausible that some high-energy mechanical processes found in workplaces may lead to the generation of fine aerosols that are characterized by high number and surface area concentrations. A preliminary investigation has been carried out into the aerosol generated during high-speed grinding to investigate the generation of fine particles from mechanical processes. Aerosol size distribution measurements between 5 nm and 20 - 1/4m were made during grinding on steel, aluminum, polytetrafluoroethylene, granite, ceramic tile and hardwood. Distributions were weighted by alveolar deposition probability to provide an indication of potential dose against metrics of number, surface area and volume. In all cases, the number-weighted size distributions showed most particles to lie in the ultrafine particle range (diameter <100 nm). Surface area-weighted distributions show substrates susceptible to thermal aerosol formation to be dominated by ultrafine particles. Weighting measurements by particle volume led to distributions dominated by particles >1 - 1/4m, although aluminum, hardwood and steel all show substantial volume-fractions in the ultrafine region. There was evidence that the grinding tool contributed to the measured ultrafine aerosol fraction. Further work is required to isolate particle sources during similar operations.