Ammonium nitrate: What firefighters must know

Follow these 10 steps when confronted with an ammonium nitrate fire


Ammonium nitrate (AN) became infamous when convicted bomber Timothy McVeigh used it in the Oklahoma City bombing of the Alfred P. Murrah Federal Building on April 19, 1995.

Ammonium nitrate is back in our collective consciousness with the horrific Beirut, Lebanon, blast, and its reminders of the explosion at a West, Texas, fertilizer plant that claimed at least 15 lives, caused 200 injuries and destroyed more than 50 homes. The Beirut disaster is only the latest in an inglorious history between humans and ammonium nitrate.

More than a dozen other explosions involving the chemical have occurred over the past century. The deadliest was on April 16, 1947, when a series of explosions rocked the huge waterfront petrochemical complex at Texas City, just southeast of Houston.

Smoke rises from the scene of an explosion that hit the seaport of Beirut, Lebanon, Wednesday, Aug. 5, 2020. (AP Photo/Bilal Hussein)
Smoke rises from the scene of an explosion that hit the seaport of Beirut, Lebanon, Wednesday, Aug. 5, 2020. (AP Photo/Bilal Hussein)

The disaster began with an explosion on a French freighter filled with more than 2,000 tons of ammonium nitrate fertilizer. The blast and ensuing fires left at least 576 people dead and 5,000 injured.

Other disasters triggered by ammonium nitrate explosions include:

  • Roseburg, Ore. (1959): 14 killed
  • Kansas City, Mo. (1988): Six firefighters killed in a construction site explosion
  • Belgium (1942): 189 killed
  • France (1947) 29 killed
  • Toulouse, France (2001): 31 killed and more than 2,000 injured following an explosion at a hangar containing 300 tons of ammonium nitrate at a chemical and fertilizer plant
  • Romania (2004): 18 killed
  • North Korea (2004): 162 killed
  • Mexico (2007): 37 killed

What is ammonium nitrate?

Ammonium nitrate is a chemical compound whose chemical formula is NH4NO3 (CAS#: 6484-52-2). At room temperature and standard pressure, it is a white crystalline solid.

Ammonium nitrate is found as a natural mineral in the driest regions of the Atacama Desert in Chile, often as a crust on the ground or in conjunction with other nitrate, chlorate and halide minerals. Today almost 100% of the chemical used is synthetically produced.

MSDS excerpts: AN

Section 5, Fire and Explosion Data

  • Flammability of the Product: May be combustible at high temperature. 

  • Auto-Ignition Temperature: 300°C (572°F)

  • Flash Points: Higher than 93.3°C (200°F).

  • Flammable Limits: Not available.

  • Products of Combustion: Not available.

  • Fire Hazards in Presence of Various Substances: Slightly flammable to flammable in presence of heat, of combustible materials, of organic materials. Non-flammable in presence of shocks.

  • Explosion Hazards in Presence of Various Substances: Slightly explosive in presence of heat, of combustible materials, of organic materials, of metals.

  • Risks of explosion of the product in presence of mechanical impact: Not available.

  • Risks of explosion of the product in presence of static discharge: Not available.

  • Fire Fighting Media and Instructions: Oxidizing material. Do not use water jet. Use flooding quantities of water. Avoid contact with organic materials.

  • Special Remarks on Fire Hazards: Caution: Strong Oxidizer. Contact with material may cause a fire. Contact with combustible or organic materials may cause fire.

  • Special Remarks on Explosion Hazards: It is an oxidizing agent and can self-ignite/detonate when in contact with powdered metals and some organic materials such as Urea and Acetic Acid.

  • Section 14, Transportation DOT Classification: CLASS 5.1: Oxidizing material.

  • Identification: Ammonium Nitrate UNNA: 1942 PG: III Special Provisions for Transport: Marine Pollutant.

The good: Ammonium nitrate is commonly used in agriculture as a high-nitrogen fertilizer because it is more stable and does not lose nitrogen to the atmosphere compared with its cousin urea.

The bad: It has also been used as an oxidizing agent in explosives, including improvised explosive devices. It is the main component of ANFO (Ammonium Nitrate/Fuel Oil), a popular explosive, which accounts for 80% of the explosives used in North America.

Responding when ammonium nitrate is present

The U.S. Department of Transportation classifies ammonium nitrate as an oxidizer, a Class 1.5 Insensitive Explosive, that can be an explosive by itself under certain conditions. While this is hard to initiate under normal conditions, response personnel must exercise extreme caution when responding to incidents where ammonium nitrate is present. 

Ammonium nitrate may explode, especially when subjected to confinement or high heat, but it does not readily detonate.

A critical behavior of fertilizer-grade ammonium nitrate — a strong oxidizer in concentrations above 33.5% — is that it can explode if it becomes contaminated with organic materials such as wood, paper or cardboard.

If ammonium nitrate explodes, it does so as a deflagration, a rapid auto combustion that does so at a subsonic speed of less than 1,250 feet per second.

A high explosive

Under the right conditions, ammonium nitrate may act like a high explosive changing from a solid to a gas almost instantaneously. When such a reaction occurs, an explosion results from heat or shock, it produces a detonating wave that travels at supersonic speeds.

The sudden creation of gases and the extremely rapid extension produces a devastating effect that can level structures and cause massive fire extension.

The 1947 Texas City disaster began with a fire onboard the S.S. Grandcamp, (cargo subjected to confinement and high heat) which extended to the ship's cargo of more than 2,000 tons of ammonium nitrate fertilizer unleashing a deflagration.

The resultant detonation wave from the Grandcamp set fire to an adjacent vessel, the S.S. High Flyer, docked more than 600 feet away. That fire subsequently extended to the High Flyer's cargo of 900 tons of ammonium nitrate fertilizer causing a second explosion that exacerbated the devastation.

Key tactical behaviors

The four-digit UN identification number is 1942 with an organic coating, and 2067 as the fertilizer grade. There are several other mixtures of ammonium nitrate that have four-digit numbers; they can be found in the Hazardous Materials Tables and in the DOT's North American Emergency Response Guide Book, aka the "orange book." First responders arriving at an incident where ammonium nitrate is present should:

  1. Conduct a good size-up. Gather the intel that answers the three questions: What has happened? What is happening? What will happen?
  2. Call for help. Locate facility managers on site, or get them called as soon as possible.
  3. Don't let the facility managers go anywhere — they need to get plugged into your incident action plan and organizational structure from the outset. Nobody knows more about the incident than the home team.
  4. Locate the precise location of the ammonium nitrate, which becomes the hot zone. Treat this incident as a hazmat response with a fire component from the onset and you'll likely get it right. 
  5. Evacuate an appropriate area using the DOT NAERG tables. 
  6. Isolate the area and deny entry. Employ your law enforcement and available facility security to create an effective hard perimeter — using their terminology usually goes a long way with our law enforcement brothers and sisters in gaining their cooperation.
  7. Initiate tactical operations to ensure that exposures are protected from fire. If the exposures are the fire problem, ensure that water being used for fire suppression cannot get into the hot zone.
  8. Confine the problem to the ammonium nitrate and to the hot zone. Ensure that no organic materials are or can be in contact with the ammonium nitrate. 
  9. Contain run-off water used in fire suppression or exposure activities. Ammonium nitrate is a marine pollutant.
  10. If the fire is in the hot zone or impinging on any closed containers that contain ammonium nitrate or fuel oil, immediately begin removing all personnel from the hot zone. Aggressively expand the hard perimeter beyond the facility, ensure complete evacuation of civilians within the expanded hard perimeter, and make preparations for a deflagration to minimize exposure to responders and civilians.

That last bulleted item may sound harsh — and certainly runs counter to our daily mode of operations — but in the words of George Santayana, philosopher, essayist, poet and novelist: "Those who cannot remember the past are condemned to repeat it."

This article, originally published in 2013, has been updated.

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