There is increasing evidence that health effects occur at very low levels of PM, and without an apparent threshold. This evidence arises from studies, initially in the US, but which have more recently been carried out in Europe and elsewhere with similar conclusions. These studies have, in general, used different measures of particles, but the majority have used PM10 and the Group felt that this was the most appropriate measure for a limit value for particles at the present time. The health effect studies to date have not identified a clear no-effects threshold, and on this basis the extent of the adverse effects over.
