Although there is a burgeoning literature on the effects of international trade on the environment, relatively little work has been done on where trade most directly effects the environment: the transportation sector. This article shows how international trade is affecting criteria air pollution emissions in the United States' shipping sector. Recent work has shown that cargo ships have been long overlooked regarding their contribution to air pollution. Indeed, ship emissions have recently been deemed "the last unregulated source of traditional air pollutants." Air pollution from ships has a number of significant local, national, and global environmental effects. Building on past studies, we examine the economic costs of this increasing and unregulated form of environmental damage. We find that total emissions from ships are largely increasing due to the increase in foreign commerce (or international trade). We analyze the period 1993 to 2001 and find that the economic costs of SO2 pollution during the period are estimated to be $1.1 billion or $126 million per year. For NOx emissions the costs are $3.7 billion over the entire period or $412 million per year. Because foreign trade is driving the growth in U.S. shipping, we also estimate the effect of the Uruguay Round on emissions. Separating out the effects of global trade agreements reveals that the trade agreement-led emissions amount to $460 million for SO2 between 1993 and 2001, or $51 million per year. For NOx they are $1.2 billion for the whole period or $144 million per year. Without adequate policy responses, we predict that these trends and costs will continue to rise with trade flows into the future.