Calcium oxide is a water-reactive, corrosive, irritating, nonflammable, odorless, white or grayish-white lump or powder. It is used in the manufacture of bricks, cement, glass, mortar, paper, plaster, poultry feed, pulp, stucco, and chemicals such as alkalis, bleaches, drilling fluids, fungicides, insecticides, and lubricants. It is used to make fluxes; in the manufacture of such metals as aluminum, magnesium, and steel; as a refining agent for metals, petroleum, and sugar; and in the treatment of industrial waste, sewage, and water.

An obsolete use for calcium oxide is as a light-emitting substance. When calcium oxide is heated to a temperature near its melting point, it produces a blinding white light. In the days before electric spotlights were available in theaters, the light from heated calcium oxide was reflected by mirrors through lenses to produce a spotlight bright enough to highlight actors on the stage (the term “in the limelight” evolved from this use, since one of calcium oxide’s synonyms is “lime”).

Calcium oxide often is shipped in 50-pound bags. When it is shipped in larger quantities in bulk form, calcium oxide must be protected from moisture in the air.


Calcium oxide is a nonflammable solid; therefore, it has no flash point, ignition temperature, or flammable range. It has a specific gravity of 3.34 and a molecular weight of 56.08. It melts at 4,661°F, boils at 5,162°F, and reacts violently with water. Its chemical formula is CaO.


Calcium oxide’s major hazard is its reactivity with water. Whenever calcium oxide contacts water, the resulting reaction is so violent that enough heat is generated to ignite ordinary combustible materials. This reaction can be very surprising to onlookers who do not expect a large amount of very wet material to suddenly break into flame. What happens, of course, is that when the calcium oxide contacts the moisture in the wet material, such a tremendous amount of heat energy is generated that the moisture boils off, leaving dry, combustible material.

When calcium oxide is added to water, the severity of the reaction depends on the amount of water present (or the amount of calcium oxide added to the water). Regardless of the amount of water, an instantaneous reaction will take place on the water’s surface, with the accompanying evolution of large amounts of heat energy. The disposition of this heat energy depends on the amount of water present. Water, which has the highest specific heat of any common material, will absorb large amounts of heat, while its temperature rises. If a large enough amount of water is present, the heat energy will be absorbed harmlessly (even though the water may get hot). If not enough water is present to absorb the heat safely, the water’s temperature will rise rapidly, and the water eventually will boil away, leaving many combustible materials at or above their ignition temperatures. Almost instantaneous ignition of these materials will take place.

If water is added to calcium oxide (rather than calcium oxide being added to wet material), the reaction will depend on how much water is added and how rapidly. As the water initially contacts the calcium oxide, a violent reaction will begin. As more and more water is added (again, depending on the amount added and at what rate), much boiling and spattering may take place. This may throw some of the calcium oxide around, producing the possibility that fire may be generated wherever the calcium oxide lands. The more water present (or rapidly added), the greater the chance that the liberated heat will be absorbed without harm.

The reaction with water that produces such heat also will produce calcium hydroxide, an alkaline compound with corrosive properties. The corrosiveness of the calcium hydroxide solution will depend on the amount of material that has dissolved in the water. Calcium hydroxide is also known as slaked lime or hydrated lime. Its chemical formula is Ca(OH), which is the addition reaction of CaO (calcium oxide) plus HOH (or H0, water).

Calcium hydroxide belongs to a family of compounds known as bases. Bases are the chemical opposite of inorganic acids and may be used to neutralize those acids. Since they can be used to neutralize acids, the assumption often is made that bases are not hazardous. Quite the contrary is true. Contact with calcium hydroxide, which may occur when the water containing the material evaporates, may result in severe burns to the skin.

The bigger danger is the skin burns that can occur from direct contact with calcium oxide. As the calcium oxide reacts with the skin’s moisture, enough heat will be evolved to produce third-degree burns, assuming more than a trace of calcium oxide is present. In addition to the thermal burn caused by water reactivity, the calcium hydroxide formed in the reaction will cause a chemical burn.

Very small amounts of calcium oxide, usually present as dust, can irritate the skin, eyes, nose, throat, and all other mucous membrane surfaces. Any inhaled material also may travel to the lungs, producing irritation and possible tissue damage by thermal burn. Ingestion is unlikely, due to the severe burning that would occur in the mouth when the calcium oxide contacts the moisture there. If calcium oxide were swallowed, burns also would occur in the esophagus and stomach. This would be followed by stomach cramps, vomiting, diarrhea, and eventually collapse.

Calcium oxide’s TLV-TWA (threshold limit value-time weighted average) is two mg/m3 (milligrams per cubic meter of air); its PEL (permissible exposure limit) is five mg/m3. It is unusual for a substance’s TLV-TWA and PEL to be different. The two organizations that set the standards, the American Conference of Governmental Industrial Hygienists (ACGIH) and the Occupational Safety and Health Administration (OSHA), try to agree on the maximum amounts to which workers may be exposed during the normal workweek without harm. Whenever the two organizations’ numbers disagree, the organization with the higher number usually reexamines its value and adjusts downward.

Calcium oxide is considered a stable material, but it will react violently with some chemicals. Specific chemicals to avoid contacting or mixing with calcium oxide include boron trifluoride, chlorine trifluoride, fluorine, hydrochloric acid, all other strong acids, and, or course, water.

The danger to workers involved in manufacturing processes that include the use of calcium oxide comes from handling the bags of product, cutting the bags open, and dumping the calcium oxide out into another container. Not only is the dust created by this action a problem, but handling the “empty” bags presents a hidden danger. When a product is in powder form, as it is when bagged, a few ounces of the product will remain in the bag after the bag is emptied. It is this small amount of calcium oxide that often is ignored. The powder finds its way onto workers’ skin and into their eyes or lungs, causing severe health problems.


A release of calcium oxide may not mandate the implementation of the community’s emergency response plan, since the spill may not produce a dangerous situation very far from the point of release. Calcium oxide is a solid and may be present as lumps of material or as a fine powder. Obviously, the powdered material poses a greater hazard, since the material may be blown about by the wind. In addition, if the powder comes in contact with water, it will present a larger surface area to the water than lumps would, and the resulting reaction will release its heat in a shorter amount of time. Should calcium oxide spread from an original release site, environmental experts would have to determine the degree of harm posed to the environment.

Material released as lumps or powder may be salvaged by shoveling it into secure containers. Front-end loaders, backhoes, or other power equipment can be used to remove large quantities of the material. The material also may be vacuumed and transferred to secure containers. Any equipment or tools used in salvage operations must be compatible with calcium oxide and resist corrosion. Any personnel near the salvage operation must wear proper eye, skin, and respiratory protection.

Firefighters should not be involved in the salvage of any hazardous materials. Product releases are the responsibility of the sellers, buyers, and/or shippers; therefore, they are the ones who should provide workers who are properly educated, trained, and equipped to carry out salvage operations.

If the wind threatens to disperse powdered calcium oxide, or if rain is imminent, cover the released material with sheets of compatible, waterproof material, such as sheets of natural rubber, nitrile rubber, or neoprene. Contact the manufacturer of the calcium oxide involved in the accident should any question concerning compatibility arise.

In the event that a very strong wind disperses the powdered calcium oxide, anyone caught in the cloud will be in serious danger. Quite likely, concentrations of calcium oxide will be present in amounts considerably greater than the TLV-TWA or PEL.

It is very important to keep water from contacting spilled calcium oxide. Temperatures of 600°F to 800°F may be evolved from the reaction, and even higher temperatures are possible. This heat will not only ignite ordinary combustibles but also force emergency personnel away. If a large amount of water contacts a large amount of calcium oxide, incredible amounts of heat energy along with large amounts of calcium hydroxide will be generated; and the material may begin to flow away from the source, spreading the heat and contamination.

Prevent calcium oxide from entering sewer systems. Sewers usually contain considerable amounts of organic material (fuel) and water. In this scenario, the calcium oxide acts as the ignition source, generating enough heat energy when it contacts the water to ignite the organic material. Sewer fires can produce explosions if methane gas, generated by the decomposition of sewage in the absence of oxygen, is present. Warn all sewage treatment plants serving the system of the material’s entry and the subsequent conversion of the calcium oxide to calcium hydroxide from the water that will be used to flush the system out and extinguish any fires.

Prevent calcium oxide from entering waterways. Since the material is a solid, it would not enter a waterway, pond, or lake by flowing from the source of the release, as a liquid would. In most cases of accidental release into water, a vehicle carrying the calcium oxide dumps the product directly into the water, either as a loose solid (lump or powder) or in a container, or an entire vehicle enters the water. In any case, the severity of the reaction, the degree of contamination, and the damage done will depend on the amount of material released into the water, the amount that actually reacts with the water (this amount may be the same), the volume of water into which the material fell, and whether the water is moving (and if so, how fast).

Obviously, if a large amount of calcium oxide enters a relatively small pond, the reaction will release a great amount of heat, and the resulting solution will be highly alkaline. This will have a catastrophic effect on any aquatic life or waterfowl present in the pond.

If a large amount of calcium oxide were released into a large body of water, the heat would be carried away from the point of entry by the water, and the resulting solution would be less caustic. The overall effect on the body of water would be less dramatic than it would be in a smaller pond, but the effect on aquatic life and waterfowl could be just as catastrophic.

If the product were to enter a stream, more damage would be done if the water were moving slowly than if it were moving rapidly. The faster the water is moving, the more quickly the material will react (and the more quickly the heat will dissipate), and the faster the calcium hydroxide solution will be diluted. Aquatic life still may be severely affected, however, and only the proper environmental authorities can judge when the water is safe again for human use.

As in all cases where hazardous materials enter waterways, notify all downstream users of the water at once. This includes agricultural users, industrial users, and any water treatment plants operating to produce water for drinking and other household uses.



(Chemical Abstract Services)



(Registry of Toxic Effects of Chemical Substances)



(United Nations/North America)



(Chemical Hazard Response Information System)



(U.S. Department of Transportation)

Class 8, corrosive

NFPA 704 Rating


Environmental authorities must supervise the removal of any product remaining on land after salvage operations are completed. Any dry, unreacted calcium oxide, including any soil contaminated by the product, must be removed and disposed of in accordance with all appropriate federal, state, and local regulations. Some references state that calcium oxide may he flushed down sewers, using large amounts of water. No hazardous material should ever be flushed down a sewer.

Some references suggest that any calcium hydroxide remaining in a runoff containment pond or pit may be neutralized with dilute solutions of acid. This should not be done until trials are run on small amounts of the solution in a safe place. Litmus test paper can be used to determine if the remaining solution is neutral. The material’s manufacturer may be the best source to recommend neutralization techniques and agents. In most cases, if the material is not rendered completely harmless, what is left after the incident, in addition to any soil or other material contaminated in the incident, should be disposed of in the same manner as the pure material.


Calcium oxide will not bum. The great danger facing emergency responders and anyone else near a calcium oxide release surrounded by fire (or approached by fire) would be contact of the product with the water used by firefighters to control or extinguish the fire. There are no hard and fast rules to determine if water should be used on a fire in the presence of a water-reactive material. The incident commander must weigh the odds after determining what can and might happen if the calcium oxide becomes wet. The IC must factor this additional hazard into all the other information available, determine the probable outcome vs. the desired outcome, act accordingly, and then take responsibility for the results.

If water used to control a fire does come in contact with the calcium oxide, the resulting runoff solution must be contained. Ordinarily, the use of flooding amounts of water to dilute the reactants would be advised, but in this case the resulting volume of solution may be impossible to contain. The resulting contamination would be too widespread and too costly to clean up.

Also remember that calcium hydroxide solutions are corrosive and can cause further damage to exposed metal. The best course of action is to try to prevent the fire from reaching the calcium oxide and, if it does, know the consequences of continuing to fight the fire with water.

Fires in warehouses where calcium oxide is stored also can be a problem. Some references state that sprinkler systems should not be present where water-reactive materials are stored. Since this advice is not always followed, water from an activated sprinkler head might already have worsened the condition on your arrival. Ensure that all buildings in your protection district are preplanned so you know where hazardous materials are stored and what to do if they are involved in a fire.


Choose protective clothing and equipment that will provide the maximum respiratory, eye, and skin protection against calcium oxide. Regular turnout gear that is impervious to powdered materials can provide skin protection. Use methods that will prevent powders from entering through sleeves or pant cuffs. Rubber boots, gloves, aprons, coveralls, and hoods may offer protection. Total encapsulating suits may not be required. If, however, a department’s standard operating procedures call for their use, contact the suits’ manufacturer to find out if the product offers protection against calcium oxide. Some references indicate that natural rubber, neoprene, and nitrile rubber may give some protection against calcium oxide, since they offer protection against similar materials.

Chemical-resistant goggles that prevent the passage of powders are required under a face shield. Positivepressure, self-contained breathing apparatus is required for respiratory protection.


Inhalation. Move the victim to fresh air and keep him/her calm and warm. If the victim’s breathing has stopped or becomes labored, administer artificial respiration, being aware that such action might expose the first-aid giver to the material in the victim’s lungs and/or vomit. Seek medical attention immediately.

Eye contact. Immediately flush the eyes for at least 15 minutes, lifting the eyelids occasionally. Be careful not to wash the product into the clean eye. Seek immediate medical attention.

Skin contact. Wash the affected areas of the body with large amounts of soap and water. Irritation will continue after washing, so seek medical attention.

Ingestion. Ingestion of calcium oxide is rare, but it can happen. Give the victim large quantities of milk or water to drink. Do not induce vomiting. Never try to force an unconscious person to vomit or drink anything. Immediately seek medical attention.


burnt lime


calcium monoxide


fluxing lime


lime, burnt

lime, unslaked


unslaked lime

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