21 22The complexity of ambient particle composition considerably complicates pinpointing specific 23 causal associations between exposure to particles and adverse human health effects, the 24 contribution of different sources to ambient particles at different locations, and the consequent 25 formulation of policy action to most cost-effectively reduce harm caused by airborne particles. 26Nevertheless, the coupling of increasingly sophisticated measurements and models of particle 27 composition and epidemiology continue to reveal associations between particle components 28 and sources (and at lower concentrations) and a wide range of adverse health outcomes. This 29 article describes the current metrics, legislation and policies for protection of public health 30 from ambient particles, and reviews the current approaches to source apportionment of 31 particles and the latest evidence for their health effects. A particular focus is placed on particles 32 in the ultrafine fraction. The review concludes with an extended evaluation of emerging 33 challenges and future requirements in methods and metrics for understanding health outcomes, 34 and in the policy context. 35 36 For the research described here it is usually only the particle phase that is being discussed, to 44 which the word aerosol is sometimes erroneously applied (an aerosol being the combination of 45 the particles and the gas in which they are suspended). Instead, in line with correct usage, the 46 terminologies particle or particulate matter (abbreviated to PM), rather than aerosol, are used. 47
48A link between poor air quality and mortality has been recognised for centuries, becoming 49 particularly manifest as urbanisation and industrialisation intensified 3,4 . The source of air 50 pollution was formerly dominated by widespread coal and other solid-fuel burning, plus other 51 toxic emissions from largely unregulated industrial processes. In developed countries at least, 52 the extent of air pollution from such sources declined markedly with Clean Air Acts and 53 similar 'smoke control' legislation introduced from the mid-20 th century 5 , although these 54 remain important sources of air pollution in some parts of the world. From the latter part of the 55 20 th century, the coupling of increasingly sophisticated measurements of atmospheric 56 composition and epidemiological methods has continued to reveal associations between a 57 range of air pollutants (and at lower concentrations) and adverse health outcomes 4 . There is 58 recognition also of the multitude of sources and complex chemistry now contributing to poor 59 air quality, and the wider geographic scales of influence 5 . 60
61In the contemporary context, the deleterious impact of PM on air quality and health is 62 recognised by the World Health Organisation who publish advisory air quality guidelines for 63 ambient concentrations of PM (and other ground-level pollutants); see Table 1 6 . Many 64 countries or political blocs such as the European Union (EU) have developed polic...