A large-scale spill of hydrogen-derived ammonia in rivers or at sea would be more toxic to fish and some marine mammals and birds than a comparable spill of marine gas oil (MGO), according to analysis from campaign group Environmental Defense Fund (EDF).

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Nevertheless, a catastrophic spill of MGO, a shipping fuel used widely today, would be worse on almost all other parameters, the group pointed out in its new report Ammonia at Sea.

Coastal waters, estuaries, mangroves, coral reefs and wetlands would be particularly vulnerable to a large-scale ammonia spill, as the effects would be magnified by temperature, pH and salinity, EDF said. The impacts would also be significant in rivers to a lesser degree.

And the impact would be most acute amongst fish, which would experience physiological damage and mortality when the ammonia reached toxic levels, as well as decimating habit and potential prey.

Birds, reptiles and marine mammals would also be badly affected.

When ammonia dissolves in water it forms ammonium hydroxide, which is highly toxic to marine life. And when it hits surface water, for example as part of a collision between ships that causes the tank to rupture, it reacts with condensation in the air to form a heavy, toxic mist that is very difficult to clear.

When inhaled by humans, marine mammals or birds, it can cause burns to the lungs, eyes, mouth or skin. At high concentrations it can kill.

But the effects of an ammonia spill would not be so severe in deep sea locations or in the polar regions, EDF said.

By comparison, an MGO or crude oil spill would be more devastating to marine life in almost all locations except deep sea, and to almost all marine life except fish. The effects of an MGO spill would have the highest impact on birds and invertebrates, such as squid and jellyfish.

Ammonia, derived from hydrogen, is one of the low carbon fuels that the shipping industry is keen to use to decarbonise its operations, despite concerns about safety.

The International Renewable Energy Agency (IRENA) believes that the shipping industry will drive growth for ammonia in a 1.5 degree pathway, consuming around 197 million tonnes (29%) of all ammonia demand by 2050.

But although ammonia has been shipped around the world as a cargo for decades, using it as a bunker fuel increases a risk of leaks and spills.

As a result, EDF called for regulation and spill management measures to be put in place to mitigate the ecological impacts of a large-scale leak.

“The use of ammonia as a shipping fuel could impact on aquatic environments as associated ecological receptors if a spill were to occur without mitigation measures and solid spill management practices,” the report said. “Therefore a robust regulatory framework establishing suitable mitigation measures needs to be developed for ammonia to be a viable low-carbon alternative to shipping.”

The International Maritime Organisation (IMO) is currently updating its safety codes for ships using ammonia as a fuel, and Norwegian standards agency DNV and the Cyprus Marine and Maritime Institute both believe that a rigorous regulatory regime will successfully mitigate the risk.

But critics of ammonia fuel argue that regulation at sea is notoriously lax, which would mean that the risk of a catastrophic ammonia spill is heightened.