As China tackles its air quality issues, government regulators have turned their focus to shipping, an industry that burns thousands of tons of highly polluting heavy fuel oil near densely populated coastlines every day. One way to reduce air pollution from ships is by establishing an Emission Control Area (ECA), a geographic region designated by the International Maritime Organization where more stringent emission standards apply. China has already implemented domestic emission control areas in three port clusters along China’s coastline, although they are smaller and currently have less stringent standards than an IMO ECA. What should a future IMO ECA look like? A delineation closer to shore is politically easier to achieve because China can unilaterally regulate ships in its territorial waters. However, a narrow ECA delineation may actually increase emissions if ship operators divert around the ECA to save on fuel costs, as ECA-compliant fuel is more expensive than traditional marine fuel. Therefore, China should consider how to delineate an ECA to prevent rerouting and ensure maximum emission reductions and public health benefits. In this paper, quantify the emissions reduction potential of four ECA delineation scenarios, considering the potential for ships to route around the ECA.