The spatial and temporal distribution of ambient particulate matter pollution across the island of Puerto Rico over a 20 years period (1985 to 2004) has been analyzed. Annual and monthly analyses of particulate matter with diameters of 10 microns or less were performed using statistical and Geographic Information System (GIS) techniques applied to surface measurements obtained by the United States Environmental Protection Agency (EPA). The PM 10 data for all the counties exhibited a seasonal variability with a maximum during the months of May through August, peaking in the months of June and July and reaching a minimum during November to January. A decreasing trend in the monthly averaged PM 10 mass concentration is also observed. Significant contributions to PM 10 in Puerto Rico from volcanic activity are observed since 1995 when the Soufriere Hills volcano in the island of Montserrat started erupting.