Review the background information on each of the pollutants covered under the NAAQS and, based on the available information and considering the questions you addressed above, indicate which ambient air pollutant you believe should be a national regulatory priority for the United States and why. Support your choice by describing the types of health consequences associated with the pollutant (including any susceptible populations), its major sources, and when the standards were last reviewed and/or changed. Then, provide a brief explanation of one or more global implications for regulation of this pollutant by the United States. Be sure to cite your sources.