Particles are both the oldest and newest of air pollutants. Being visible to normal human sight in the form of airborne smoke and as dirt on surfaces, they figured early in public perceptions of urban air quality (Lodge et al., 1969). Due primarily to the use of solid fuels, severe particle pollution conditions existed in the past in many cities throughout the world (Markham, 1994). Indeed, bringing them under control in large cities of the currently developed world in some cases took three quarters of a millennium (Brimblecombe, 1987). They were thus the first pollutant category to be widely monitored, initially in the form of deposited soot (Anonymous, 1912), and later as mass or simple optical measures of total particle levels collected on filters (Shaw and Owens, 1925; Meetham, 1964). Starting in the mid-20th century, a large set of urban epidemiological studies, mostly of several-day high-pollutant episodes in the UK and the US, established the scientific basis used for widespread promulgation of ambient standards in developed countries, first in the 1950s, and then, more widely, in the 1970s (Grant et al., 1999). Relying on the same evidence, the 1979 World Health Organization Air Quality Guidelines (World Health Organization and United Nations Environment Programme, 1979) in turn were used by many other countries around the world as the basis for their own standards.