Particulate matter is a complex mixture of particles of various sizes and chemical composition originating from combustion and other sources. Over the past decades, research has focused on the question which particles may be more toxic than others, but it remains difficult to disentangle the effects of different types of particles fromeach other or from other pollutants.
HEI has conducted extensive research in this area, most recently with the completion of the National Particle Component Toxicity (NPACT) program (see HEI reports by Lippmann and Vedal). These studies have made a major contribution to the policy arena by showing that none of the particle components could be definitively excluded as not having an effect on public health, thus supporting the current regulatory approach to target the entire particulate matter mixture.
While fine particulate matter (particles less than 2.5 micrometer in aerodynamic diameter or PM2.5) has been the focus of a large number of recent studies, the effects of ultrafine particles (less than 100 nanometer) and larger, coarse particles (between 2.5 and 10 micrometer) from brake and tire wear and road dust cannot be discounted. In 2013, HEI published a comprehensive review of what is known about emissions, toxicity, and health effects of ultrafine particles.