Ultrafine particles in workplace atmospheres

Abstract

Inhaled ultrafine particles are increasingly being recognized as a potential threat to health. Aerosols in workplace environments may come from a wide variety of sources, depending on the type of activity and processes taking place. Some activities and processes are acknowledged as being ‘dusty’, where aerosol is generated from the mechanical handling and attrition of solid or liquid material, and are not considered to be plausible sources of ultrafine particles. However, hot processes, involving the vaporization of material, and inevitable subsequent cooling, do have the potential to generate significant number concentrations of ultrafine particles. However, consideration of the physical conditions required for the generation of particles in the range below 100 nm suggests that those conditions are not easily met in workplaces. More generally, the conditions are such that particles grow out of this range, either by continuing condensation (as happens at high vapour concentrations) or by agglomeration between smaller particles (as happens at high number concentrations). Not much is known about ultrafine particles in actual workplaces, mainly because our view has been obscured for the past few decades by the fact that most occupational aerosol standards have been based on the mass concentration of airborne particulate matter. Now that a new awareness has set in, it is expected that new research will address the problem. Most current aerosol standards are expressed in terms of the mass concentration of particulate matter conforming to a particle size fraction, where the latter is based on knowledge of how particle size relates to where particles deposit in the human respiratory tract and any subsequent effects. At present no such basis exists for ultrafine particles, but one is needed before progress can be achieved towards meaningful standards for occupational ultrafine aerosols. It is expected that, for ultrafine particles, such a standard may, in the future, be expressed in terms of the number concentration of particles less than a certain size, that size to be determined on the basis of the physical and chemical nature of the particle at that size, human physiology and toxicology.