Ammonia - Alternative Fuel Spill Response Considerations

Figure 1. Ammonia leak from a tanker truck in Oklahoma, USA (November 2025). The released ammonia formed a white vapour cloud that stayed close to the ground (left) before gradually rising as it warmed and mixed with the air (right). The incident resulted in 45 hospitalisations, including 5 individuals airlifted for treatment. Of the 14 responding police officers exposed to the vapour, 5 sustained chemical burns to their airways. No fatalities were reported. (Source: ABC News)

As the cloud warms, it becomes lighter and begins to rise, shifting from lateral spreading to upward dispersion. During early phases, lateral travel for several hundred metres is possible, especially under strong winds. Importantly, the visible plume depends on humidity, and the toxic zone may extend beyond the observed cloud (Figure 2).

Figure 2: Expected behaviour of a ship-sourced ammonia release from above the waterline

On the other hand, dissolution is rapid and exothermic: ammonia reacts with water to form ammonium hydroxide (NH₄OH), creating short-lived spikes in temperature and pH near the release point. This results in a highly corrosive solution (pH >11) nearby the release location, within seconds. Seawater’s buffering capacity limits large-scale pH change, but significant local shifts can occur close to the spill. Dispersion rate depends on mixing intensity; energetic open waters disperse faster, while sheltered ports and inland waterways disperse more slowly.

Once dissolved, ammonia dissociates into unionised ammonia (NH₃) and ionised ammonium (NH₄⁺). The unionised form is the toxic form because it crosses biological membranes readily (e.g. fish gills), causing cellular damage and interfering with metabolic and neurological functions. The ammonium form is less bioavailable. The ratio between NH₃ and NH₄⁺ depends strongly on pH, temperature and salinity: higher pH and higher temperature increase the proportion of unionised NH₃, implying regional and seasonal variability in toxicity. Unionised NH₃ can volatilise; NH₄⁺ remains dissolved or may bind to sediments. In ports and harbours with slower water movement and finer sediments, ammonium may sorb to sediments and later be released when conditions change or sediments are disturbed.

3) Impacts on human health: Toxicity and corrosivity

Ammonia poses significant health risks because it is both highly toxic and strongly corrosive. It is primarily absorbed through the respiratory tract and is hygroscopic, seeking out moisture and rapidly forming a caustic solution. Eyes, lungs and skin are therefore particularly vulnerable, with corrosive burns occurring as ammonia dissolves into moist tissues.

Exposure to high concentrations can lead to severe and potentially fatal effects, including permanent injury or death by asphyxiation. It is noteworthy that ammonia has a very low odour threshold (approximately 5–20 ppm), providing an early indication of its presence, before it reaches hazardous levels. Direct contact with liquefied ammonia can cause severe corrosive damage to skin and eyes.

Fire risk exists but is typically secondary to toxicity. Ammonia has a relatively narrow flammability range (15.5–27% v/v) (Figure 6) and relatively high ignition energy, making ignition difficult in open air without catalysts or combustible material. However, in particular conditions, ignition of spilled liquid ammonia could produce a pool fire. If an onboard fire occurs, ammonia can break down at high temperatures (around 450°C) to form hydrogen, which is highly flammable. Ammonia could also undergo BLEVE under scenarios involving tanks heating above their boiling point and the failure of gas release systems.

4) Exposure control and operational implications

Exposure control relies heavily on continuous monitoring of airborne ammonia concentrations and selecting personal protection equipment (PPE) based on actual measured concentrations. Respiratory and eye protection are essential, but higher levels of protection are time-consuming to don and physically demanding to use, reducing mobility and operational efficiency. Guidance emphasises that vapours are the key concern and responders should not enter areas containing aerosols or dense gas clouds. For smaller incidents with lower toxicity situations, lower levels of PPE could be favoured.

5) Environmental effects and persistence

Ammonia is considered non-persistent and does not bioaccumulate; it typically clears faster than petroleum hydrocarbons (hours to days in many marine scenarios). Rapid dissipation is driven by volatility, solubility (enabling dilution) and lack of persistent sediment binding. Nevertheless, environmental impact can be substantial close to the release. High ammonia levels can cause nutrient overload, reduce oxygen levels and harm marine life. Fish and corals are particularly sensitive; mangrove species may be more tolerant. In fish, elevated ammonia can damage gills, disrupt metabolism and impair neurological function, affecting the wider food chain.

However, the concentration at which these effects begin is uncertain, given that ammonia naturally occurs in both freshwater and marine systems at low levels (typically 0–0.2 mg/L⁴). Ammonia concentrations in seawater are generally lower than in freshwater, and because seawater typically has a pH near 8, most ammonia is present in the less toxic ammonium form. For these reasons, it has traditionally been assumed that ammonia in the marine environment would not reach levels high enough to cause widespread harm. However, the volumes that could be released during a ship-sourced incident involving ammonia as a marine fuel are far greater than those normally encountered, and this assumption may need to be revisited.

Overall, ammonia incidents combine rapid partitioning between air and water with severe acute toxicity risk. Significant local ecological impacts are expected, even though persistence is low.

Response considerations for releases of ammonia

Response to an ammonia release is shaped by its tendency to partition between the atmosphere and the water column, with a portion volatilising as a toxic vapour cloud while the remainder dissolves into seawater. As a result, the immediate priority is the protection of human life from inhalation exposure. Crew and responders must be evacuated from affected areas, moving laterally and upwind, while exclusion zones are established and maintained downwind. Continuous atmospheric monitoring using ammonia gas detectors, supported where possible by dispersion modelling, is essential to track vapour movement and inform safe decision-making. Remote water sprays may be used to reduce airborne concentrations, although water should not be applied directly to liquid ammonia pools.

In parallel, the potential for fire or explosion must be managed by controlling ignition sources and cooling any tanks exposed to heat or flame. Traditional containment and recovery are not feasible, and satellite imagery offers limited operational value. Vapour management therefore relies on targeted interventions, including water sprays or curtains to suppress or deflect vapour clouds, physical shielding or covering for small releases, and passive ‘monitor and evaluate’ approaches where conditions make active intervention unsafe.

Short-term measures such as tarping may be used for small, localised releases to suppress vapour generation and buy time for source control, provided personnel operate in appropriate protective equipment. Water curtains and fog systems can also reduce airborne concentrations, although their effectiveness is limited by plume size and wind conditions, and they generate corrosive runoff that must be managed. In situations where direct intervention offers little benefit, natural dispersion and dissolution may be allowed to reduce concentrations, supported by continuous monitoring.

Management of ammonia dissolved in water focuses on monitoring ammonia concentrations and pH to understand plume behaviour. Where feasible, dilution and dispersion are promoted through natural or mechanical mixing, with aeration used in enclosed or sensitive areas to reduce ecological impacts. Additional protective measures may include exclusion zones around aquaculture facilities, fisheries and critical water intakes, as well as temporary fisheries closures. Ongoing wildlife monitoring supports assessment of acute impacts and informs any further mitigation measures required.

Special thanks are extended to ITOPF for their invaluable expertise and contribution to the research and development of this article.