Scrubbers, exhaust gas cleaning systems (EGCS) on seagoing vessels, are installed on about 25% of the world's merchant fleet and scrub sulphur from the exhaust gas. However, they discharge polluted discharge water into the marine environment. In the ImpEx project, discharge water samples were taken from ships and analysed in the laboratory for their harmful effects on the environment. The results show that, depending on the scrubber system (open-loop, closed-loop), the discharge water can be highly contaminated with heavy metals and polycyclic hydrocarbons (PAH). In addition, mutagenic and dioxin-like effects have been detected in ecotoxicological analyses. The overall toxicity of the discharge water samples ranges from practically non-toxic to considerably toxic in "open-loop" systems and was extremely toxic in "closed-loop" samples. From this, the authors make recommendations for action, such as the designation of zero discharge zones.