The number of ships using exhaust gas cleaning systems, better known as “scrubbers,” has grown from just three ships in 2008 to more than 4,300 in 2020. Although scrubbers are effective at reducing air emissions of sulfur dioxide (SO2), the sulfur and other contaminants removed from the exhaust gas—including carcinogens such as polycyclic aromatic hydrocarbons (PAHs) and heavy metals—are dumped overboard in the form of washwater, also called discharge water. Meanwhile, the International Maritime Organization’s (IMO) guidelines for scrubber discharges have not been strengthened since 2008, despite being reviewed in 2009, 2015, and 2020, and the guidelines ignore the cumulative effects of many ships operating and discharging in heavily trafficked areas. Such cumulative effects are to be expected given the rapid increase in the number of ships with scrubbers. This study estimates air and water emission factors for ships using heavy fuel oil (HFO) with scrubbers based on the available literature and the methods of the Fourth IMO Greenhouse Gas Study. Additionally, the authors compare the emissions associated with ships using scrubbers to ships without scrubbers using marine gas oil (MGO).