TABLE OF CONTENTS Sep 2003 - 1 comment

Spontaneous Combustion

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By: William M. Glenn

The blazing cover story in the Weekly World News, perhaps the cheesiest of the supermarket tabloids, was poised to tell all. There has been a rash of "nerds" spontaneously bursting into flame on university campuses across North America. Apparently, a higher level of "pyro-kinetic" energy is causing their brains to overheat to a self-immolating degree. Either that or it's the work of the saucer people. Or Saddam Hussein. The experts quoted don't seem quite sure.

You can check, but it's a safe bet that "nerds" are not listed in the National Fire Protection Association's (NFPA) most recent compilation of spontaneously combustible materials. You are much more likely to see a bucket of oily rags -- or a pile of wet hay, or a heap of soft coal, or a box of deteriorating latex gloves -- burst into flames than a myopic, pencil-necked, pocket protector-wearing geek. Nerds, or other human beings for that matter, don't possess the proper chemical proclivity to thermally self-destruct.

Unlike most fires, spontaneous combustion doesn't require a flame, a match or some other external heat source to spark the reaction. The heat needed to break those first chemical bonds and initiate the feeding frenzy of flame and light is generated by the fuel itself. Through slow chemical oxidation, or a combination of biological and chemical oxidation, the temperature of the material continues to rise until ignition takes place.

Spontaneously combustible materials tend to react rather vigorously with the oxygen in the air and, if the subsequently generated heat is not vented, you can expect a visit from your local fire department. Followed by your insurance adjuster.

It should be noted that there are other chemical reactions that generate heat. Some inorganics react violently when they come into contact with water and the heat generated can ignite nearby combustibles. Similarly, pyrophoric gases (such as arsine, diborane, phosphine and silane), liquids (hydrazine) and finely divided metals (such as magnesium, titanium, sodium, potassium, lithium, zirconium, hafnium, calcium, zinc, plutonium, uranium and thorium) ignite immediately on exposure to air. White or yellow phosphorus also burns when the air reaches 30 degrees Celsius.

Under hot, confined conditions, organic peroxides and cellulose nitrate can decompose in explosive fashion. And polymerization reactions between two or more materials can also produce a lot of heat. However, let's stick to a discussion of the kind of spontaneous fires ignited by chemical and/or biological oxidation.

Take, for example, a pile of cotton rags that have been used to soak up linseed oil. For some reason, the linseed oil story keeps flaring up again and again in fire literature, although any animal or vegetable-based oil will do.

Stuff the oily rags into an open bucket and leave them to simmer in the sun. The linseed oil slowly reacts with the oxygen in the air to form a solid skin and produce heat. Over the course of the next few days, or even a few hours on a hot day, the temperature continues to rise as the heat being generated by the chemical reaction is trapped in the swaddling rags. This further accelerates the rate of oxidation, until you hit the critical ignition point of the cotton rags (about 250 C) and... POOF've got a fire.

Take those same rags and hang them on the line to flap in the breeze and... FOOP've got dirty rags flapping in the breeze. Seal them in an air-tight can and, again, no fire.

The process is a little different in a silo of newly mown hay. Given suitable moisture and oxygen levels, the bacteria, moulds, fungi and other micro-organisms that are already present in the hay begin to chow down, again generating a significant amount of heat that remains trapped in the silo. At between 70 and 80 C, the micro-organisms die off and, under the right conditions, a chemical reaction takes over, pushing the internal temperature up into the 150 to 200 C range where self-ignition occurs.

If there is only a little available oxygen, you'll likely get a smoldering fire in pockets beneath the surface. If you break open the pile and let the air rush in, you'll get an inferno.

Once the runaway oxidation reaction has begun, spontaneous combustion is a wild ride. As the temperature in the pile of peanut husks or paint scrapings or fish guts or rubber buffings begins to rise, the reaction rate increases, and that pushes the internal temperature even higher. As a general rule (derived from the study of burning hay piles), the reaction rate doubles with every 10-degree increase in temperature. That means heat is being generated more than 30 times faster at 150 C than it was at 100 C. Once that pile starts to smoke, you've got to move quickly.

Ventilation, insulation and water content

Every fire is the result of the interaction of three factors: fuel, an available oxidizer (most often air, but more about that later) and heat. While each of these components must be present to support spontaneous combustion, the question of whether or not you're going to have a nice big blaze is reduced to essentially two variables. First, how much heat is being generated? And second, how fast is that heat being dissipated?

Now, the answers to those two questions revolve, to a great extent, around ventilation. You need just the right amount of air. Pack your spontaneously combustible materials too tightly together and there's just not enough air to support the oxidation... that supplies the heat... that causes the fire... that burns down the house that Jack built.

Too much ventilation, essentially too much air, and most of that heat escapes out of the pile. If you want to prevent a fire, you've got to get the heat out.

Wood chips, wet hay, oily rags: these materials are all good insulators and even better ones as they dry out. In most cases, the heat generated in the middle of the pile dissipates only slowly to the air outside. A bigger pile represents a bigger risk because there's less of that cooling air circulating through the centre and a thicker layer of insulation around the hot core.

As a pile grows, the ratio of surface area to volume decreases, and the tendency to retain heat increases. There's a mathematical equation that explains the dynamic: the surface area (which radiates the heat away from the pile) increases as a function of the square of a pile's radius, while its volume (which represents the heat-generating capacity) increases as a function of the cube of the radius. The experts talk about the "critical radius" or the "critical diameter". But simply put: little piles, like little kids, cool off quicker. Big piles, like big, scary guys, have a tendency to overheat.

The ideal pile size depends on a number of factors: the size and compaction of the particles, the moisture content, the air flow through it, the turn-over rate, the presence of any impurities, and so on. As an illustration, the Ontario Fire Marshal has issued guidelines on the optimum size for piles of wet wood chips. If you plan to store the chips for more than three months, the pile should be no more than four metres high. Less than three months, and you can go to 7.5 metres high. Larger dry chips can be stored in bigger piles, for longer periods of time, than smaller wet chips. And chips can be heaped in larger piles than sawdust. Overall, the general rule for pile size is "the smaller, the better."

Similarly, particle size plays an important role. Finely divided particles have a larger total surface area than a big chunk of the same weight. That means that there's more surface area to react -- powdered aluminum and magnesium burn much more readily that the solid metal -- while the finer dusts form a much denser pile that doesn't conduct heat.

Moisture content is also critical. Spontaneous combustion in hay piles thrives at moisture levels between 20 and 45 per cent. Less than 20 per cent, and there isn't enough water for the micro-organisms feeding in the pile to survive. No micro-organisms, no biological oxidation.

Above 45 per cent and any heat generated is likely conducted to the outside of the pile and dissipated through the process of evaporation. That's one reason that silage and compost is kept nice n' wet (with a 50 to 65 per cent moisture content), while hay is stored quite dry (12 to 16 per cent). Either too wet or too dry and there's no risk of fire. It's all a matter of balance, and the ideal moisture content varies for each material.

Ambient air temperature is important for two reasons. First, the rate of oxidation speeds up under hotter conditions. And second, the ability of a pile to radiate heat off into the environment is severely limited on a hot day. Put a pile of oil-soaked rags under a tree in the shade, and you've dramatically reduced the combustion risk. Place them next to a blasting space heater or dryer, and they are a time bomb waiting to explode.

Basics drive prevention

Preventing spontaneous combustion incidents is primarily a matter of putting bigger chunks of potentially combustible material in smaller piles, then keeping those piles cool, well-ventilated and within the prescribed moisture limits. A couple of years back, pallets of boxes containing powder-free, chlorinated latex gloves were spontaneously bursting into flame in hospital storage rooms. The problem was solved by implementing a series of precautions, based on the simple rules for preventing spontaneous combustion:

- Avoid maintaining a large inventory of powder-free latex gloves.

- Remove the shrink wrapping holding the boxes tightly together on the pallet.

- Reposition the cartons into smaller piles so that the air circulates around the boxes.

- Don't allow the ambient temperature in the storage area to become too hot.

- Implement a "first in, first out" policy so that stock doesn't sit too long in storage.

- Periodically check the stock, looking for warning signs -- such as brittleness, tackiness, or an acrid chemical odour -- that could indicate oxidation.

- If any gloves start to show signs of deterioration, break apart the stack to dissipate any heat, remove the suspect gloves from storage, and isolate them immediately.

The same principles for preventing spontaneous combustion -- limiting the combustible material, reducing the heat, maintaining properly ventilated piles, and minimizing the risk of spontaneous combustion -- will work just as well for hospitals as for other workplaces, including commercial and institutional laundries. Every year, there are hundreds of laundry fires reported in hotels, nursing homes, prisons and other facilities. Most are small, but a few have proved very costly.

In most cases, the culprit is a faulty dryer or other overheated and malfunctioning piece of equipment. However, in about five per cent of incidents, spontaneous combustion is to blame. Typically, the scenario entails laundered clothes, towels or rags that had been soaked previously with cooking oils or the oil used in massage or physiotherapy clinics. These are washed, dried and left heaped and hot in a plastic cart overnight to be folded or ironed the next morning. But before the day shift reports for work, the oily residue begins to oxidize in its nest of warm cotton and the polyethylene cartful of laundry bursts into flame. The plastic cart is not only an excellent insulator, once you ignite it, the polyethylene will burn with the intensity of jet fuel. If you're lucky, the automatic sprinklers will contain the damage.

Preventive measures must start at the work site where the clothing or rags are initially contaminated with a combustible oil product:

- When you are done with an oil-soaked rag, hang it up to dry thoroughly.

- Don't tightly fold or roll up the used rags, rollers, or mops and leave them sitting out in the sun.

- If you don't plan on reusing an oil-soaked rag, dispose of it in a sealed metal container and store the container in a shady place.

As for the laundry facility itself, tips include the following:

- Process soiled laundry immediately. If that's not possible, store any oil-soaked material outside away from other combustibles.

- Use the dryer's cool down cycle to lower the temperature of the load to a safer level, and don't leave large loads in the dryer after the cycle ends.

- Use open-sided bins or baskets to allow air circulation. Don't leave warm laundry tightly packed or dumped in bins overnight.

- Allow clothes to cool before folding or bundling. Arrange unfolded laundry in a donut shape to let the heat dissipate.

- Never leave bundled or folded garments next to a boiler, hot water heater or other external heat source that could support the oxidation process.

- Wipe the inside of the washer and dryer with clean rags to prevent the contamination of subsequent batches.

Spontaneous combustion doesn't always involve a chemical or biological reaction with oxygen. Sometimes, another oxidizing agent steps in to play that role. If you remember your high school chemistry, an oxidizer is that solid, liquid or gas that contributes an atom of oxygen (or, in some cases, some other element) to support the combustion of another material. You can't have a fire without an oxidizing agent. It's one-half of the oxidization-reduction reaction that generates all the heat and light. While the oxidizing agent may not be combustible itself, it will dramatically increase the combustibility of any flammable material into which it comes in contact. The NFPA classifies oxidizing agents depending on the fire danger they pose, with Class 4 being the most dangerous (see chart at right).

While all oxidizers are potentially dangerous, the chemicals in Classes 2, 3 and 4 will cause the spontaneous combustion of flammable materials without the aid of an external ignition source. So preventing fires involving strong oxidizers is based, in part, on reducing the odds that a strong oxidizer will come into contact with any combustible material.

- Quantities of oxidizing materials should be kept to the absolute minimum necessary for meeting immediate needs. Bulk quantities should be stored in special facilities designed for that purpose.

- Storage of oxidizing materials should always be in accordance with material safety data sheet requirements, and the storage area must be cool and well-ventilated.

- Oxidizing materials should be stored in approved containers and kept separate from flammable, combustible or reactive materials. They must not be stored on combustible floors, platforms, pallets or shelves.

- All staff should be fully informed about the dangers posed by oxidizers and the precautions needed to prevent spontaneous combustion.

Spontaneous combustion is not an accident; it's not an act of God. It's the result of a well-understood, scientifically verifiable chain of chemical reactions. And since we know what it is, we know how to prevent it.

But does that mean we can do something about this terrible epidemic of nerd fires. Probably not. But, to be sure, it would be best to stack your nerds loosely in small piles and ensure they are well-ventilated. You can't be too careful, you know.

William M. Glenn is associate editor of hazardous materials for ohs canada.

Major sources:

National Fire Protection Association Fire Protection Handbook, which can be ordered through its website at

Canadian Building Digest, National Research Council Canada,

"Primer on Spontaneous Heating and Pyrophoricity", U.S. Department of Energy,

Canadian Centre for Occupational Health and Safety,

Ontario's Ministry of Agriculture and Food,

"Hay Mow and Silo Fires", British Columbia's Ministry of Agriculture, Food and Fisheries,

McClure Industries Inc.,

Coin Laundry Association,

U.S. Food and Drug Administration public health advisory,

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Reader Comments

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Good article on spontaneous combustion. Lots of quality information here.

Posted March 9, 2013 01:09 PM

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