A Reuters report today describes a leak at a food processing plant in Rogers, AR that ended up sending 18 employees to a local hospital for ‘evaluation and treatment’. The chemical involved was anhydrous ammonia and it was almost certainly released from the refrigeration system used to cool/store poultry. This is not an unusual occurrence and just about every fire department of any size will have to respond to one of these leaks at some point.
Anhydrous ammonia (AA) is the second most widely used toxic gas in the United States. AA is a toxic inhalation hazard gas; it has the capability of killing people exposed to relatively small concentrations of the gas. Along with its industrial cousin chlorine gas, AA is used, in spite of its hazards, because it is such a versatile chemical. It is used as a chemical intermediate in the manufacture of a number of products including fertilizers, explosives, and pharmaceuticals. It is directly applied to the soil as a fertilizer.
There are two uses where accidental ammonia exposures are most common, during transportation to and from farms in small tank trailers towed by tractors or farm truck, and in leaks from refrigeration systems. The farm related spills are usually in isolated rural areas and rarely affect anyone beyond the farmer/drive who is typically very experienced at holding his breath and heading upwind out of the small ammonia cloud.
Refrigeration related incidents are about as common, but typically affect a larger number of employees who have a harder time getting out the ammonia cloud as it is usually contained within a building.
Fortunately, ammonia is a ‘friendly’ toxic gas. While the immediately dangerous to life and health (IDLH) limit for ammonia is 300 ppm it is easily detectable by smell at concentrations as low as 5 ppm (even lower in some people). The pungent odor tends to drive people away from leaks.
AA is used in many commercial scale refrigeration systems because of its low cost and high heat transfer capability. Most exposures in this environment are from small leaks at joints in the piping system. If the process areas where the piping is found are properly monitored, these leaks are seldom more than maintenance problems, but typically require local evacuations within the building to avoid unnecessary exposures.
More serious exposure issues are normally due to venting issues. Any gas storage system has to be protected against over pressurization. Failure to do so can result in a catastrophic loss of pressure situation where the vessel fails in what really does look like an explosion and causes significant amounts of physical damage and a large toxic cloud. To avoid that situation, engineers design relief systems that vent off (release) excess pressure to the atmosphere (or preferably to a scrubber system). Since AA is lighter than air, venting small amounts to the atmosphere is frequently not noticed.
When vent systems are not properly designed or when emergency venting takes place during line-breaks (opening the pipes that carry the AA for maintenance reasons) you can have real problems. This is particularly true when the venting takes place close enough to an air handling system intake that the gas cloud does not rise above the intake by the time it reaches that space on the roof. The typical HVAC system is not designed to remove AA before pumping the contaminated air into living and working spaces.
That is what appears to have happened in this case. During maintenance operations a line containing anhydrous ammonia was opened too close to an air-conditioning system intake. It was apparently a relatively small leak as the article does not mention anyone being seriously injured or being held overnight for observation.
I have worked around ammonia gas (from ammonium hydroxide not the more dangerous anhydrous ammonia) and even exposures to low concentrations in the air can be very irritating to eyes, nose, throat and lungs. I’m one of the unfortunate few that experience temporary blindness at relatively safe levels of ammonia exposure, so I fully understand why exposed employees are routinely transported to emergency rooms for observation and treatment. The immediate effects of moderate exposure are very unpleasant, but there is usually no long term damage.