The evolution of particulate matter (PM) air quality policy in the European Union and in the United States between 1970 and the present has been atypical. The US government and the European Commission have mandated scheduled reviews of PM policy over the past three decades and have updated that policy to new scientific information on multiple occasions. The use of planned adaptation over such a long period and in this manner, as a means to deal with uncertainty, has not often been reproduced in air quality policy. Furthermore, particulate matter policy in the EU and US does not conform to the commonly held perception that the EU’s environmental policies are, by and large, more precautionary than the respective policies in the United States. The US decisions to adopt air quality standards for PM10 and PM2.5, in 1987 and 1997 respectively, led those in the EU by approximately nine years. An analysis of the comparative stringency of the PM standards in the US and EU shows that the PM2.5 standard the US implemented in 1997 is more stringent than the standards that have been proposed in the EU by the European Commission and the European Parliament. In September this year, the US repealed their annual standard for PM10. Prior to that, however, the annual PM10 standard the EU implemented in 1999 was more stringent than the one the US adopted in 1987. The daily PM10 standards in the EU and US are of similar stringency. In the Appendix, these comparisons in stringency are discussed in more detail. The differences between the EU and US policies are remarkable because they are based on the same science and therefore reflect dissimilar processes of interpreting that science and the uncertainties inherent in it. The two cases themselves focus on the sciencepolicy interfaces for their respective governing bodies. The EU case also looks at the science-policy interface in the Netherlands. The US case also examines policies for sulfur dioxides that relate to the PM policies. The remainder of this summary discusses how ...