Ammonia as a fuel for ships: A review of hazards and environmental dynamics during bunkering and shipping

• Ammonia hazards driven by chemical and physical dynamics are identified • Temperature, pH, and storage form govern ammonia risk in water • Human and ecological safety occasionally conflict • Human error and equipment failure are key initiators of accidental ammonia release • Mitigation measures displace ammonia risks across air–water interfaces Ammonia has been proposed as a maritime fuel because of its carbon-free nature and high energy density. However, its application introduces environmental risks through unintended releases into aquatic environments. Unlike previous reviews that focused on human risk or industrial handling, this review synthesises the multifaceted ecological hazards associated with aquatic spills. We identified three tiers of hazards: (1) primary and secondary hazards involving direct toxicity, corrosiveness, explosiveness, and particulate matter (PM 2.5 ), whose effects are more widespread than those of toxic vapour clouds; (2) physicochemically modulated hazards, where ammonia toxicity varies across near- to far-field gradients driven by pH and temperature shifts and species-specific NH₃/NH₄⁺ sensitivities, with risk potentially underestimated when relying on generalised ambient-condition criteria; and (3) indirect and cascading hazards, including the accumulation of reactive intermediates and stimulation of opportunistic bacteria and algae. Ammonia-induced toxins, including those influenced by high-ammonia-load effects, may occur under specific conditions, although they are poorly understood. Our evaluation reveals that although refrigerated ammonia reduces atmospheric dispersion and the associated human inhalation risk, it potentially increases the risk of aquatic contamination, highlighting trade-offs that current mitigation strategies have not fully addressed. Spills are likely to originate from human error and equipment failure, implying that hazard risks can be minimised through improved operator expertise and equipment design. We recommend integrated, risk- and context-specific mitigation strategies that incorporate absorptive and precipitation-based technologies to reduce the displacement of risk between environmental compartments. Finally, crucial gaps in the safe application of ammonia as a fuel have been identified.