A DC-9 AIRLINER was flying from Dallas to Nashville when a flight attendant notified the cockpit crew of smoke in the passenger cabin. A portion of the cabin floor was so hot and spongy that passengers had to be moved away from it. The source of the apparent fire could not be found immediately but was believed to be in the middle cargo compartment. The plane landed safely in Nashville nine minutes later and 126 people were evacuated by emergency slides.

Airport firefighters confirmed that the fire was indeed in the middle cargo compartment of the aircraft. Personnel entering the compartment to extinguish the fire observed a severely damaged fiberboard drum and some white powder that had spilled from it, along with three plastic bottles. The heat was unusually intense for the apparent magnitude of the fire, and water was not extinguishing it as effectively as expected. Also, several airline employees, passengers, and firefighters were becoming ill from the fumes and smoke; 19 of them had to receive medical treatment.

Investigators found that a 104-pound fiberboard drum containing textile treatment chemicals used to produce “stone-washed” denim cloth had been loaded into the middle cargo compartment. The drum contained five gallons of hydrogen peroxide (H2O2)solution, 25 pounds of a sodium orthosilicatebased mixture, 25 pounds of laundry “booster,” and 24 ounces of liquid “brightener.” These undeclared and improperly packaged materials, some of which are regulated under Code of Federal Regulations (CFR) 49 as hazardous materials, had mixed during the flight. The resulting exothermic reaction generated noxious fumes and, ultimately, ignited nearby combustibles.

Fifty-five gallon drums of 35 and 50 percent H2O2 were found in the shipper’s warehouse, but it had not been determined which had actually been loaded onto the aircraft. Both concentrations of H2O2 are very strong oxidizers, though, and must be labeled and shipped as such. The maximum allowable quantity of 35 percent H2O2 per package on a passenger aircraft is one quart; 50 percent H2O2 is forbidden altogether on these planes. Thus the five gallons aboard this flight violated CFR 49 regardless of concentration, and they were not properly packaged.

The orthosilicate-based mixture, a white-flaked alkaline product, primarily contained sodium hydroxide and sodium metasilicate. This product reacts exothermically with oxidizers such as H2O2. It also generates highly flammable H2 on contact with aluminum and other metals. The mixture is regulated for transportation as a corrosive, specifically as a “corrosive solid, not otherwise specified.” This shipment did not exceed the maximum net quantity of 25 pounds permitted in one package on a passenger aircraft but, again, it was not properly packaged. The laundry “brightener” and the “booster” were not regulated as hazardous materials under applicable federal laws.

This in-flight fire was a result of many violations of regulations governing the transport of hazardous materials, as well as a sequence of events that led to the incompatible chemicals being mixed and subsequently igniting.


So much hazardous-materials training assumes that a single chemical or product is involved in an incident. We drill over and over again on the eight categories of hazardous materials—or 13, if you include the five categories of “Other Regulated Materials.” We discuss explosives, flammable liquids, corrosives, compressed gases, oxidizers, and radioactive materials. Many haz-mat training courses begin by reviewing these classifications and the required placards and labels.

However, we seldom progress beyond this concept of “one chemical per incident.” We treat almost every training scenario as if just one chemical has been released or is burning. We should consider the possibility, and indeed the frequent occurrence, of incidents in which two or more chemicals are involved, in which mixing is a severe threat or already has occurred. One reason we don’t, no doubt, is the lack of reference materials addressing the subject of multichemical incidents.


Chemists recognize three possible outcomes when two or more chemicals are mixed together: The resulting produces) may be no more hazardous than the original materials, they may be less hazardous, or they may be more hazardous. The properly protected responder usually can handle resultant products with equal or lesser hazards without great concern, but it’s the third possibility—when resultant products are more hazardous than the original materials— that causes nightmares.

The hazard potential can be increased in two ways. First, the reaction products may have the same hazardous characteristics as the original chemicals, but the magnitude of the hazard may increase. For example, potassium cyanide (KCN), a white crystalline solid regulated as Poison B, is toxic primarily by ingestion. When KCN is mixed with an acid, though, deadly hydrocyanic acid (HCN) is produced. HCN is a Poison A gas that is toxic by inhalation.

Second, the product that results when chemicals are mixed may present hazards totally different from those of the original chemicals. For instance, a mixture of nitric acid and acetic acid, two corrosives, produces a shock-sensitive explosive! In this example, you at least knew that the two acids were regulated hazardous materials. How about situations in which one or more of the mixed chemicals are nonregulated, yet a severely hazardous product results? For example, powdered aluminum (a flammable solid), and sodium carbonate, the commonly used neutralizer known as “soda ash,” produce a potentially explosive mixture.

Chemicals that react with each other to produce or increase hazardous conditions are said to be “incompatible.” There are five types of reactions between incompatible chemicals:

  • The reaction can be explosive or otherwise violent, such as severe splattering or rapid pressurization of the container.
  • The reaction can be strongly exothermic.
  • Flammable products can be produced.
  • The reaction products can be toxic.
  • Corrosive materials can be produced.

CFR 49 dictates that incompatible chemicals must always be packed and shipped in such a way that they cannot accidentally come in contact with each other. The Environmental Protection Agency has similar regulations pertaining to the disposal of incompatible materials. Incompatible chemicals sometimes do get mixed, however, whether through ignorance, carelessness, or criminal intent. When this happens, emergency responders are expected to mitigate the problem.

In one incident, several gallons of hydrochloric (muriatic) acid, a corrosive, were accidentally poured into a 40-pound container of calcium hypochlorite (swimming pool chlorine), an oxidizer, in a large resort hotel. The reaction quickly filled the building with gaseous chlorine, a result that could not be predicted merely by reading the legally mandated labels on the containers.

In another case, a small can of sodium chlorate, an herbicide and oxidizer, fell from a garage shelf into a open drum of fungicide (a Poison B material) on the floor below. The ensuing reaction forced firefighters to evacuate the neighborhood until the source of the irritating air-borne sulfur particles could be found and contained.

Let’s look at specific examples of incompatible chemical mixtures that can lead to each of the five outcomes listed above, as well as some guidelines that will help you recognize situations involving potential incompatibility.

Mixtures that can react violently and often explosively. Any of the following mixtures are potentially explosive: ammonia + any halogen, chlorine + any alcohol, decolorizing carbon + any oxidizer, diethyl ether + chlorine, ethanol + calcium hypochlorite or ethanol + silver nitrate, hydrogen peroxide above 30 percent + any organic material, and sodium hypochlorite + any amine, to name a few. A mixture of sodium nitrate + sodium thiosulfate, both dry oxysalts, can explode.

This list barely scratches the surface of potentially explosive chemical combinations. However, note that there is a common theme: One of the chemicals in each combination is a regulated oxidizer—the halogens; oxysalts such as the hypochlorites, thiosulfate, and nitrate; and hydrogen peroxide. This should alert you to the possibility of explosive reactions whenever an oxidizer is spilled or released. Only in a highly controlled environment can you know for certain that the oxidizer is not going to mix with an incompatible material. Unfortunately, the fire service does not operate under such ideal conditions.

Oxidizers do not have to be present for an explosive reaction to occur. Incompatible mixtures in which one or both of the chemicals are corrosives also can explode—for example, sodium hydroxide (caustic soda) + acetic acid, nitric acid (40 percent or less) + acetic adic, sulfuric acid + ammonium hydroxide, and filming sulfuric acid (oleum) + any of the numerous ethers, alcohols, esters, and ketones.

There are myriad other potentially explosive combinations in which neither an oxidizer nor a corrosive is involved. Examples include acetylene + copper metal or any copper salt, chloroform or carbon tetrachloride + powdered aluminum or magnesium, sodium metal + any chlorinated hydrocarbon, sodium + sulfur, and ammonia + chlorine or another halogen.

You must consider the possibility of explosive or otherwise violently reactive chemical mixtures at any haz-mat incident. However, the likelihood increases significantly when oxidizers and corrosives are involved.

Just how sensitive can these explosive mixtures be? How much energy would it take to detonate them? Very little, actually. Some will explode instantaneously on contact with each other. Others react to form products that are sensitive to friction, shock (or percussion), light, or heat—the opening of a lid on a bottle, a footstep or water flowing from a straight stream, sunlight, or a rise in temperature of just a few degrees.

The strongly exothermic reaction. These reactions often lead to ignition of nearby combustibles. The in-flight fire on the airliner mentioned earlier is an example of such a reaction. In that case, the incompatible products were the oxidizer hydrogen peroxide (at least 35 percent) + a sodium orthosilicatebased mixture of solid corrosives.

Polymerization, the general type of reaction by which polymers are formed (that is, when small molecules combine to form larger molecules containing repeating structural units of the original molecules), is frequently a strongly exothermic process. When polymerization occurs in an uncontrolled situation, such as at the site of a train derailment and chemical release, ignition of the products and other combustibles may occur. For example, a small amount of acetic acid will cause acetaldehyde to polymerize with intense heat release.

Uncontrolled exothermic polymerization and the consequent violent rupture of the container are particularly severe threats with two products commonly transported by railroad—butadiene and styrene. Contact between either product and a strong oxidizer will start the polymerization. Once it has started, there is no way to halt the process; it will continue to completion.

Other incompatible chemicals that react exothermically and frequently ignite without polymerization include ammonium nitrate + acetic acid, nitric acid (especially if it is over 40 percent) + phosphorous, turpentine + chlorine or chloride salts, and a low-molecularweight alcohol + an oxysalt.

Still other chemicals, particularly hydroxides and some other oxysalts, generate considerable heat when they dissolve in water. Water is a chemical itself—the most common one of all, in fact—so we must list as incompatible mixtures water + metal hydroxides and water + other oxysalts.

Here, again, a common theme emerges from these examples: An exothermic reaction frequently, but not always, involves an oxidizer. Never overlook the possibility that what may seem to be spontaneous ignition of combustibles may have been caused by release of an oxidizer.

The flammable solid category includes many chemicals that are also water-reactive. Use water with caution on and around these products because some ignite on contact with water—for example, alkali metals, such as sodium, potassium, and lithium. {Note: The letter “W” with a bar through it does not mean that water cannot be used on these products. It simply means to usewater cautiously because there is the potential for adverse reactions.)

Flammable products produced. There is a thin and sometimes indistinguishable line between mixtures that explode or react violently, those that are strongly exothermic, and those that produce flammable products. However, the reactions that we shall discuss here are those that produce flammable products only. They are not exothermic, so autoignition usually is not a problem — ignition occurs generally from an external source. Also, the reactions discussed here produce materials that burn, not ones that are apt to detonate.

Two types of salts that present this problem are the hydrides and carbides. These salts are composed of a metal and either hydrogen or carbon. Each reacts with water. A metal hydride + water generates highly flammable hydrogen gas, and an alkaline earth metal carbide + water produces acetylene. Thus, it is essential that each of these products be kept dry.

Be sure you know which type of carbide you are handling. Carbides of transition metals such as silver, copper, and mercury are explosives whereas it is the alkaline earth metal carbides that release acetylene when wet. The reaction between calcium carbide and water, for instance, is used in the commercial manufacture of acetylene.

Hydrogen gas is evolved when some metals, such as zinc, are in contact with acidic corrosives, water, or even damp air.

When powdered carbon such as lampblack or decolorizing carbon is mixed with an oil of animal or plant origin (linseed, cottonseed, sunflower, etc.), this product is highly flammable and has a very low flash point.

Fluorine, a halogen, is one of the strongest oxidizers known and thus presents special cases of flammable products resulting from mixtures. Virtually anything will burn in an atmosphere of fluorine—not just ordinary combustibles but materials such as asphalt, crushed limestone, soil rich in organic matter, and common metals such as iron, copper, and stainless steel. These reactions technically require an external ignition source but not in the usual sense. Any act that produces the slightest friction will usually suffice.

Toxic products formed. Cyanide compounds such as sodium cyanide (NaCN) and potassium cyanide (KCN) are prime examples. They are solid materials regulated as Poison B that are hazardous mainly if ingested. However, when mixed with an acidic corrosive, hydrocyanic acid (HCN) gas is formed, a Poison A gas. It is also highly flammable and capable of violent polymerization under some poorly defined conditions.

Phosphides are binary salts ( a metal + elemental phosphorus) that decompose to highly toxic and spontaneously flammable phosphine on contact with acids or water. Nitrides are similar to phosphides except nitrogen is substituted for phosphorus and the decomposition product is ammonia. Another group of salts, the sulfides (a metal + sulfur), are decomposed by acids, with release of hydrogen sulfide, another toxic and highly flammable gas.

Gaseous halogens are commonly formed by mixtures of incompatible products. Chlorine, for example, results when ammonia and household bleach (sodium or calcium hypochlorite) are mixed. In fact, a halogen gas is likely to be released whenever any halogen-containing compound is mixed with a corrosive, either an acidic or an alkaline material.

Many pesticides are incompatible with each other, a particularly serious problem because the reaction products are apt to be highly toxic. Also, there is a strong likelihood that incompatible pesticides will be stored and transported together simply because of the huge quantities and extreme diversity of these products that are on today’s market. Examples of incompatible pesticides include parathion + endrin, both insecticides, which react violently with each other; soldum chlorate, a herbicide and very strong oxidizer, which reacts violently with many chemicals, including charcoal, metal oxides, sulfuric acid, metal sulfides, and some organic acids; and methyl bromide, a soil fumigant, which forms explosive mixtures with dimethyl sulfoxide.

The formation of corrosive products. We have already mentioned some of these in conjunction with other hazards. Corrosivity is just one of several hazards that some products have. Halogen gases, for example, are highly corrosive as well as toxic.

When some haz-mat personnel respond to a corrosive release, the first and foremost thought in their minds is “neutralization,” which they equate with bringing the pH to 7.0. To do this, though, a strong base must be added to a spilled acid and vice versa—that is, you must handle a second hazardous material to render the original one less harmful. This in itself is very risky. Even riskier, perhaps, is the possibility of adding too much neutralizing agent. You can quickly convert an acidic spill into an alkaline one, and the latter is much more difficult to clean up! The reverse is also possible—excess acidic neutralizer added to a basic material will create an acidic product. These reactions arc typically exothermic, and the evolution of some type of gas must also be anticipated.

Several incidents—some explosive, others accompanied by violent frothing, etc.—have occurred during neutralization efforts, such as when trying to neutralize potassium hydroxide with an acid, sulfuric acid + diethylamine and sulfuric acid + methylpyridine, sodium carbonate (sodium ash) + aromatic amines, soda ash + numerous different acids, and triazine herbicides + alcohols.

Be extremely careful when trying to neutralize any spilled product. Seek and follow the advice of a knowledgeable person. Neutralization is not the panacea for corrosive products that many people think.


Reference materials pertaining to the hazards of chemical mixtures are scarce. Among the books that do address this problem are Dangerous Properties of Industrial Materials (Van Nostrand Reinhold, 1984), Handbook of Reactive Chemical Hazards (Butterworths, London, England, 1987), and the NFPA’s Fire Protection Guide on Hazardous Materials. Perhaps the best source, though, is a book that has been out of print for a long time: A Method for Determining the Compatibility of Hazardous Wastes, written by H.K. Hatayama et al. (U.S. Environmental Protection Agency, Cincinnati, Ohio. 1980). This book was written for those involved in management of waste water and industrial wastes, not for emergency responders, yet it contains a wealth of pertinent information. It would be well worth your effort to locate a copy. Still, even this valuable source book evaluates mixtures of two chemicals only. If three or more products mix, no reference materials are available today that will help you at all!

Another very important source of information pertaining to chemical reactivities and incompatibilities is the material safety data sheet (MSDS). An MSDS for each hazardous chemical is required by law to be available to every person working with or exposed to these materials. Be wary when gleaning information from these sheets, however. It is common practice to indicate that the chemical does not react adversely under “normal temperature and pressure conditions.” This can be misleading to firefighters who, of course, do not usually respond to incidents under “normal” conditions.

Hie possibility of having to handle mixtures of chemicals in uncontrolled situations would gain the immediate attention and respect of most chemists. As responders to haz-mat emergencies, this possibility also deserves your utmost attention and respect. Many a hazmat instructor has stood before a class and stated absolutely that there is no such thing as an expert in this field. If ever there was a doubt in your mind about the truth of this statement, ask a few “experts” about potential reactions when chemicals are mixed. You are truly entering the proverbial “uncharted waters” when the incident to which you are responding involves more than one product.

We have discussed examples of explosive and otherwise violent reactions, as well as some in which flammable, toxic, and corrosive products are produced. The dangers that these incompatible chemical mixtures hold for the emergency responder are obvious. What about the mixtures in which the reaction is not so obvious? Lack of explosion or ignition does not mean that there is no additional risk. It is quite possible, for example, that the reaction products can jeopardize the integrity of your protective clothing and equipment without you even realizing it.

The world of chemistry is often one of unknowns. To complicate matters further, the field of chemistry is constantly evolving. New reactions are discovered and new products synthesized every day. Therefore, emergency responders are forced to constantly play “catch up” with the chemical industry.

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