The Hazard of Mixing Chemicals

We have all heard there are chemicals that we cannot mix together. Even at home, mixing chemicals like bleach and vinegar can create chlorine gas. Have you ever thought about this on a larger scale though? What happens when incompatible mixtures combine at manufacturing or storage facilities?


In a small Kentucky town last month, three people were severely injured and one killed in an explosion. It all started when a waste company began pumping sludge out of a local plant. The sludge mixture combined with some used cooking oil that was already in the tank and exploded. The sludge, mixed with oil, caused a chemical reaction that resulted in an explosion that jettisoned the tank through an exterior wall and two interior walls.


In my personal experience, I have seen reactions happen in containers that were thought to be considered empty. I have seen something as simple as drums of cleaning agents mixed together that created an incompatible mixture that reacted and started to melt a 55-gallon (208-Liter) drum.


On a much larger scale, in May 2019, a chemical explosion occurred at a facility in Waukegan, Illinois, causing multiple deaths, injuries, and damage to multiple buildings. This was caused by the mixing of chemicals from a misidentified drum, where the reaction released flammable hydrogen gas into the building.


How to stay safer


As we know, this is not a new issue, but there are many things that we can do to ensure that we are not mixing incompatible materials, and NFPA® codes and standards give us a lot of guidance.



Information on incompatible materials can be found in safety data sheets or manufacturers’ product bulletins. This is always the first place that you want to look when working with chemicals.



If you look in the 2022 edition of NFPA 400, Hazardous Materials Code, you will see that in Annex A, information on incompatible materials can be found in safety data sheets (SDS) or manufacturers’ product bulletins. This is always the first place that you want to look when working with chemicals.


There are also many materials that are not compatible with water, especially combustible dusts. This causes an issue when there is a possible fire. In the 2020 edition of NFPA 654, Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids, under Annex A.3.3.55, water-incompatible materials are classified as those that dissolve in water or form mixtures with water that are no longer processable—for example, sugar. Although water is an effective extinguishing agent for sugar fires, the sugar dissolves in the water, resulting in a syrup that can no longer be processed through manufacturing equipment.


A similar situation exists with flour. When mixed with water, it becomes dough. These materials are candidates for extinguishing systems that use media other than water until the damage potential of the fire approaches the replacement cost of the process equipment. (Learn more about clean agent fire suppression systems in this blog.)


Water-reactive materials, which are typified in Annex A.3.3.56 of NFPA 654, also represent a very special fire protection problem. The application of water from fixed water-based extinguishing systems or by the fire service without awareness of the presence of these materials could seriously exacerbate the threat to human life or property. For example, many chemicals form strong acids or bases when mixed with water, thus introducing a chemical burn hazard. Additionally, most metals in the powdered state can burn with sufficient heat to chemically reduce water-yielding hydrogen, which can then support a deflagration. These types of materials should be handled very carefully. Even small quantities of water usually make matters worse.


It’s also important to remember that if incompatible chemicals are mixed, there is a potential for off-gassing. In an incident in March 2022, a worker was injured after being exposed to a toxic gas that was produced after two chemicals were mixed. This incident involved incompatible mixing with an oxidizer. Oxidizers are incompatible with many chemicals or other materials. It is essential to follow all storage and handling procedures to prevent conditions that might cause emergencies, such as a fire or explosion. Annex G.6.2 in NFPA 400 reviews the compatibility of dealing with oxidizers.


Remember that you should always know what you have on site. Ensure that you have a site plan outlining what is at your location and your emergency plans, and ensure those who could be affected by chemicals are properly trained. When planning, make sure to check out NFPA 1660, Standard for Emergency, Continuity, and Crisis Management: Preparedness, Response, and Recovery. For more information on hazardous materials, reference NFPA 400.

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Holly Burgess
Technical Lead

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