ANFO is a bulk industrial explosive made from two common ingredients: ammonium nitrate and fuel oil, mixed at a ratio of 94% to 6% by weight. The name is simply an acronym for Ammonium Nitrate/Fuel Oil. It is the most widely used explosive in the mining industry today, having largely replaced dynamite due to its low cost, stability, and ease of handling.
How ANFO Works
The two components of ANFO serve distinct roles. Ammonium nitrate is the oxidizer, meaning it supplies the oxygen needed for a rapid chemical reaction. The fuel oil (typically diesel) is the energy source that combines with that oxygen. Neither ingredient is particularly dangerous on its own in normal conditions. Ammonium nitrate is a common agricultural fertilizer, and diesel fuel is obviously handled routinely. But when the two are mixed and initiated with a strong enough shock, they react almost instantaneously, releasing a massive volume of hot gas.
In an ideal detonation, the reaction produces water vapor, carbon dioxide, and nitrogen gas. In practice, the chemistry is never perfectly balanced, so blasts also generate toxic byproducts, primarily carbon monoxide and nitrogen oxides. The ratio of fuel oil matters: too much or too little shifts the chemical balance and increases the amount of toxic fumes produced.
ANFO belongs to a class called non-ideal high explosives. Unlike conventional military explosives where the fuel and oxidizer are combined within a single molecule, ANFO’s two components exist as separate phases. This makes it less powerful per unit of weight, with a typical detonation velocity around 5,270 meters per second, though that number can drop significantly depending on the diameter of the charge and the confinement conditions. For smaller charges, the detonation velocity can fall as much as 30% below the ideal value.
Why the Mining Industry Relies on ANFO
Cost is the primary reason ANFO dominates commercial blasting. At roughly $1,320 per ton (including ammonium nitrate, fuel, and drilling fees), it is cheaper than alternatives like bulk emulsion explosives, which run about $1,367 per ton for the materials alone and require more product to fill each blast hole because of their higher density. When calculated per unit of rock broken, ANFO costs around $0.45 per bank cubic meter compared to $0.64 for emulsion at standard blast patterns. For operations moving millions of tons of rock per year, that difference adds up fast.
Beyond price, ANFO is simple to use. It can be poured or pneumatically loaded directly into drill holes at the blast site. It has low sensitivity to accidental ignition from impact, friction, or heat. In testing where samples of ANFO, TNT, C-4, and Composition B were struck at velocities up to 190 to 200 meters per second, none of the materials reacted. That low sensitivity is also useful in complex blast designs that require staged detonation sequences, where you need confidence that nearby charges won’t go off prematurely.
ANFO is used across open-pit coal mining, quarrying, metal mining, and civil construction projects like road cuts, tunnels, and demolition. Essentially, any operation that needs to break large volumes of rock economically will consider ANFO as the default option.
Limitations and Weaknesses
ANFO’s biggest practical limitation is water. Ammonium nitrate dissolves readily, so in wet blast holes or rainy conditions, the mixture can degrade or fail to detonate entirely. Mines with high water tables often switch to water-resistant emulsion explosives for submerged holes while using ANFO in dry conditions above the water line.
The low detonation velocity that makes ANFO safer to handle also makes it less effective in hard, dense rock where a faster, more powerful shock wave is needed to fracture the material. In those situations, operators may blend ANFO with emulsion (a product sometimes called heavy ANFO) or use a different explosive altogether. The relatively low density of ANFO, around 0.8 grams per cubic centimeter, also means it delivers less energy per volume of blast hole compared to denser alternatives.
Storage and Regulatory Requirements
In the United States, the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) classifies ANFO as a blasting agent rather than a high explosive, because the finished mixture cannot be detonated by a standard No. 8 test blasting cap when unconfined. This classification matters for storage: blasting agents can be kept in Type 4 or Type 5 magazines, which have less stringent construction requirements than the magazines needed for high explosives like dynamite.
That said, the regulations are still significant. Magazines must be locked with specific security hardware, such as two padlocks on separate hasps or a combination of mortise lock and padlock. Vehicular magazines (trailers used for mobile delivery) need at least one steel padlock, and the vehicle’s wheels must be removed or immobilized with a kingpin lock when parked. Smoking, open flames, and spark-producing devices are banned within 50 feet of any outdoor magazine. All storage facilities must be inspected at least every seven days.
Storage locations must also comply with distance tables that dictate how far magazines need to be from inhabited buildings, public roads, and other explosives storage. The individual components, ammonium nitrate and diesel fuel, are not regulated as explosives when stored separately, which is one reason ANFO is often mixed on-site or in specialized delivery trucks rather than stored as a finished product.
Environmental Concerns
The primary environmental issue with ANFO is nitrate contamination of water. Ammonium nitrate is highly soluble, and during blasting operations, not all of the explosive detonates. Spilled or undetonated ammonium nitrate dissolves in rainwater and groundwater, breaking down into nitrate and ammonium ions that leach into the surrounding environment. Research has traced elevated nitrate levels in rivers near mining operations directly back to blasting explosives.
This contamination can push nitrate concentrations above the drinking water limit of 10 milligrams per liter and contribute to eutrophication, a process where excess nutrients trigger algae blooms that deplete oxygen in lakes and streams. Waste rock dumps are a particular concern, as undetonated explosive residues continue to release nitrate over long periods as water percolates through the rubble. The problem is widespread enough that it is now considered a routine environmental impact of large-scale mining operations that rely on nitrogen-based explosives.
Post-Blast Fumes
Workers near a blast face a different hazard: toxic gas. Even a well-designed ANFO detonation produces some carbon monoxide and nitrogen dioxide alongside the harmless water vapor, carbon dioxide, and nitrogen. Poor oxygen balance in the mix, confinement conditions, or incomplete detonation can dramatically increase these toxic gases. Nitrogen dioxide is especially dangerous because it can cause severe lung damage at relatively low concentrations, and symptoms sometimes appear hours after exposure. Standard practice at mining and construction sites is to wait for fumes to clear before allowing workers back into a blast area, with the required wait time depending on ventilation and weather conditions.