Ammonia properties and health hazards

Before Theo was allowed into Grand Valley's engine room unescorted, Ingrid sat him down with the plant's safety data sheet for anhydrous ammonia, a personal monitor and a bottle of household ammonia cleaner. She unscrewed the cap and held it at arm's length. "That's maybe a few per cent ammonia in water," she said. "You can smell it from here, at a few parts per million in the air. The stuff in the receiver is pure, and it boils at −33 °C. If it gets out, it doesn't drip. It flashes into a cloud." This lesson describes what ammonia is, how it behaves when it escapes, what it does to the body and how exposure is measured and limited.

Physical properties

Anhydrous ammonia, NH₃, is ammonia without water. At atmospheric pressure it boils at about −33.3 °C, so in any refrigeration system at normal temperatures it is stored as a liquid under its own vapour pressure: about 430 kPa absolute at 0 °C and 1,350 kPa at 35 °C. These pressures are similar to those of R-22, so ammonia systems are not high-pressure systems in the sense that carbon dioxide systems are. Liquid ammonia is light, about 640 kg/m³ at 0 °C, roughly two-thirds the density of water.

Ammonia vapour has a molecular mass of about 17, compared with about 29 for air, so pure ammonia gas at room temperature is lighter than air and rises. That is helpful indoors, because ventilation can draw it out at the ceiling. But a release from a liquid line does not behave like pure warm gas. When liquid ammonia escapes, part of it flashes to vapour instantly, cooling the rest to its boiling point, and the remainder forms a fog of fine liquid droplets and cold vapour mixed with moist air. That cold, droplet-laden cloud can be heavier than air and can hug the ground, flow into low areas and drift outdoors for a long distance before it warms and rises. Emergency plans must assume both behaviours.

Ammonia is very soluble in water. That is why it is so irritating: it dissolves into the moisture of the eyes, nose, throat, lungs and skin and forms ammonium hydroxide, an alkali that damages tissue. It is also why water is used to knock down vapour in some emergencies and why a small leak can sometimes be found by the white fumes it forms with the moisture in the air. Water in a refrigeration system is a different matter: ammonia tolerates some water without the acid problems that fluorocarbon systems suffer, but water in the circuit raises the evaporating temperature for a given pressure and reduces capacity, so it must be monitored and removed.

Ammonia attacks copper, zinc and their alloys, including brass and bronze, especially when moisture is present. For that reason ammonia systems are built of carbon steel, stainless steel and aluminium, with steel valves and compatible gaskets, and copper or brass gauges, tools or fittings must never be used on them. Ammonia is compatible with mineral and synthetic oils that are not soluble in it, which leads to the oil management practices in lessons 4 and 8.

Flammability

Ammonia is flammable, but only within a narrow range and with difficulty. Its lower flammable limit is about 15 % by volume in air and its upper limit about 28 %. Below 15 % there is too little fuel to burn; above 28 % there is too little oxygen. It is hard to ignite, burns slowly and gives off little heat, which is why ASHRAE 34 places it in class 2L, lower flammability. But 15 % is 150,000 parts per million, a concentration that occurs only close to a large release or in an enclosed space, such as a machinery room after a major leak, and there it can ignite from an open flame, an electric arc or a hot surface. Machinery room rules on electrical equipment, emergency ventilation and shutdown, in lesson 6, are written with that in mind. In practice, the toxic hazard of ammonia is far more often the one that hurts people.

Health effects

Ammonia's effects depend on concentration and time. At low concentrations it irritates the eyes, nose and throat; most people can smell it at a few parts per million, and the smell becomes very unpleasant well before it is dangerous, which is one of ammonia's most important safety features. Higher concentrations cause coughing, tears, burning of the throat and chest, and difficulty breathing. At several hundred parts per million exposure becomes immediately dangerous to life or health: escape is still possible, but injury is likely. At concentrations of a few thousand parts per million, ammonia can cause severe injury to the airways and lungs within minutes, and higher concentrations can kill.

Liquid ammonia on skin or in the eyes causes chemical burns and, because it boils so cold, freezing injury at the same time. Eye contact is particularly serious: the alkali penetrates quickly and can cause permanent damage. That is why first aid for ammonia exposure is immediate and prolonged flushing with water, at an emergency shower or eyewash, for at least 15 minutes and usually longer, while someone calls for medical help. Contaminated clothing is removed, because it holds ammonia against the skin. Anyone who has breathed significant ammonia needs medical assessment, because lung injury can develop over hours.

One more effect matters for safe work: people get used to the smell. After some minutes in a low concentration, the nose becomes less sensitive, and a worker can stop noticing a level that would alarm someone arriving fresh. And people differ: some smell ammonia at 1 or 2 ppm, others not until 20 or more. The smell is a warning, not a measurement. Everyone working on ammonia systems carries a personal monitor, and fixed detectors guard machinery rooms and other areas.

Exposure limits

Exposure limits express how much ammonia a worker may breathe. In Ontario, the occupational exposure limits are those adopted under the Occupational Health and Safety Act, which follow the ACGIH values for ammonia: a time-weighted average of 25 ppm over an 8-hour day and a short-term exposure limit of 35 ppm over 15 minutes. The level considered immediately dangerous to life or health, the IDLH, is 300 ppm. Emergency planners also use guideline values for the public, such as the ERPG and AEGL series, which set concentrations for one-hour exposures at which people would experience discomfort, serious effects or life-threatening effects.

Concentrations of gases in air are expressed in parts per million by volume or in milligrams per cubic metre. For ammonia at room temperature, 1 ppm is about 0.70 mg/m³. A useful way to understand the numbers is to work out what a given release does to a room, assuming it mixes evenly, as the script does for Hillcrest Arena's plant room.

# Ammonia in a room: from kilograms released to parts per million (well-mixed, 25 C, 1 atm)
M = 17.03                  # g/mol, ammonia
Vm = 24.45                 # L/mol of gas at 25 C and 101.3 kPa
print(f"1 ppm = {M / Vm:.2f} mg/m3")
levels = [(5, "typical odour threshold (varies widely)"), (25, "8-hour exposure limit (TWA)"),
          (35, "15-minute exposure limit (STEL)"), (300, "immediately dangerous to life or health (IDLH)"),
          (150000, "lower flammable limit, about 15 % by volume")]
room = 15 * 10 * 6         # m3, the arena plant room (illustrative)
for kg in (0.05, 2, 45):
    gas_m3 = kg * 1000 / M * Vm / 1000
    ppm = gas_m3 / room * 1e6
    worst = max([lv for lv in levels if ppm >= lv[0]], default=None)
    note = worst[1] if worst else "below the odour threshold"
    print(f"{kg:6.2f} kg released into {room} m3 -> {ppm:9,.0f} ppm  (above: {note})")
print("Well-mixed averages hide much higher concentrations near the leak and in pockets.")

1 ppm = 0.70 mg/m3
  0.05 kg released into 900 m3 ->        80 ppm  (above: 15-minute exposure limit (STEL))
  2.00 kg released into 900 m3 ->     3,190 ppm  (above: immediately dangerous to life or health (IDLH))
 45.00 kg released into 900 m3 ->    71,785 ppm  (above: immediately dangerous to life or health (IDLH))
Well-mixed averages hide much higher concentrations near the leak and in pockets.

Even 50 grams of ammonia, a few seconds of vapour from a loose packing nut, mixed evenly through the 900 m³ plant room, gives about 80 ppm, above the short-term limit. Two kilograms, about three litres of liquid, gives over 3,000 ppm, ten times the IDLH. And 45 kg, a modest liquid release, gives about 7 % by volume, half the lower flammable limit as an average and certainly above it near the leak. Real releases do not mix evenly: near the source, in pockets and in the path of a cloud, concentrations are far higher. The calculation explains why a plant room is never entered during a release without self-contained breathing apparatus, and why the size of the room's ventilation, in lesson 6, matters so much.

Personal protective equipment

The protective equipment for ammonia work depends on the task and the possible exposure, and the plant's procedures set it out. For routine rounds in a machinery room with working detection, operators wear safety glasses and carry a personal monitor and an escape respirator. For tasks that open the system, such as draining oil or changing a gauge, procedures typically require a full-face respirator with ammonia cartridges, chemical-resistant gloves and protective clothing, with a second person standing by. Cartridge respirators protect only at low concentrations, up to the limit the manufacturer and the respiratory protection program specify, and only where there is enough oxygen; they must never be used to enter an area of unknown or high concentration.

Entering a release, or any area above the IDLH or of unknown concentration, requires self-contained breathing apparatus, SCBA, and, for liquid or high vapour concentrations, a fully encapsulating chemical protective suit. Only trained and equipped responders do that, under an incident command system, as lesson 12 describes. Every respirator user must be medically cleared, trained and fit-tested under the employer's respiratory protection program, as Ontario's regulations and CSA Z94.4 require, and beards or anything else that breaks the face seal are not allowed with tight-fitting respirators.

Emergency showers and eyewash stations are required near ammonia machinery and wherever liquid could contact workers. They must be tested regularly, kept clear and placed so that a person can reach them within seconds, typically outside the machinery room door, so that a person can escape the room and reach water at the same time. At Grand Valley, Theo's first job on his own was the weekly shower and eyewash test, with a log.

Ammonia and the people nearby

Ammonia's hazards reach beyond the plant. A large outdoor release can drift over neighbouring properties, roads and schools, and the guideline values used for the public are lower than those for trained workers, because the public includes children, older people and people with lung conditions, and because they may not recognize the smell or know what to do. That is why plants above the threshold quantities must plan for the public, as lesson 7 explains, and why even small plants such as arenas, which are often next to schools and homes, need emergency procedures that include warning neighbours and coordinating with the fire department. At Hillcrest Arena, the plant room's outside wall faced a playground, which Theo noted on his first visit and which shaped the capstone.

Workers' own health matters too. Repeated exposure to low concentrations is unpleasant and can irritate the airways, and anyone with asthma or another lung condition should discuss ammonia work with a physician. Personal monitors with logging can show a worker's exposure over a shift, and plants that log exposures can find tasks, such as oil draining or filter changes, that need better controls.