Ultrafine particles in workplace atmospheres

Inhaled ultrafine particles are increasingly being recognized as a potentia l 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', wh ere aerosol is generated from the mechanical handling and attrition of soli d or liquid material, and are not considered to be plausible sources of ult rafine particles. However, hot processes, involving the vaporization of mat erial, and inevitable subsequent cooling, do have the potential to generate significant number concentrations of ultrafine particles. However, conside ration 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 v apour concentrations) or by agglomeration between smaller particles (as hap pens at high number concentrations). Not much is known about ultrafine part icles in actual workplaces, mainly because our view has been obscured for t he past few decades by the fact that most occupational aerosol standards ha ve 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 add ress the problem. Most current aerosol standards are expressed in terms of the mass concentra tion of particulate matter conforming to a particle size fraction, where th e latter is based on knowledge of how particle size relates to where partic les deposit in the human respiratory tract and any subsequent effects. At p resent no such basis exists for ultrafine particles, but one is needed befo re progress can be achieved towards meaningful standards for occupational u ltrafine aerosols. It is expected that, for ultrafine particles, such a sta ndard 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 b asis of the physical and chemical nature of the particle at that sire, huma n physiology and toxicology.