State of Epidemiological Evidence for the Health Impacts of Ultrafine Particles

There has been increasing interest in the effect of ultrafine particles (UFP) on human cardiovascular and respiratory health. The adverse health impacts due to particle exposure are currently attributed to the mass concentration or the chemical composition of particles smaller than 10 μm (PM10) or 2.5 μm (PM2.5) in diameter. However, it has been hypothesised that it is actually UFP (< 0.1 μm) measured in terms of number concentration, as opposed to mass concentration, that might be responsible for the observed health effects. This paper presents the results of a critical literature review aimed at analysing the current state of epidemiological evidence for the effects of UFP on human health. In summary, the array of epidemiological studies conducted thus far suggests that UFP exposure is associated with human mortality, and respiratory and cardiovascular morbidity. This holds true despite the considerable gaps in knowledge that remain, and despite the inconsistencies found between some studies, resulting from some deficiencies in the study designs. The limited number of epidemiological studies conducted thus far indicates that there are comparable health effects of fine and ultrafine particles, which appear to be independent of each other. Fine particles show more immediate effects whilst ultrafine particles show more delayed effects on mortality. However, at present the database is too limited (in terms of both number of studies and number of subjects) and geographically restricted, to allow clear conclusions on the mode of action and/or generalisation to other settings. Consequently, it is recommended that further, better-designed studies be initiated to improve the understanding of health impacts of UFP.