Tear gas, CS smoke canisters, and #Ferguson

Over the past week, we’ve seen tear gas and CS smoke canisters used on people of Ferguson, Missouri.  On social media – and in the general media – there seems to be confusion as to the difference between tear gas and CS smoke canisters.  Is there a difference?

The Center for Disease Control’s (CDC) ‘Facts About Riot Control Agents Interim document’ explains what tear gas is:

  • Riot control agents (sometimes referred to as “tear gas”) are chemical compounds that temporarily make people unable to function by causing irritation to the eyes, mouth, throat, lungs, and skin.
  • Several different compounds are considered to be riot control agents. The most common compounds are known as chloroacetophenone (CN) and chlorobenzylidenemalononitrile (CS). Other examples include chloropicrin (PS), which is also used as a fumigant (that is, a substance that uses fumes to disinfect an area); bromobenzylcyanide (CA); dibenzoxazepine (CR); and combinations of various agents.

This image from @WPowell19 is purportedly of a CS smoke canister employed by law enforcement in Ferguson.

In the CDC fact sheet referenced above, CS is the chemical chlorobenzylidenemalononitrile.  This chemical, whose structure is shown at the top, targets the eyes, skin, and respiratory system. From the CDC – National Institute of Occupational Safety and Health (NIOSH) ‘Pocket Guide to Chemical Hazards’ , the symptoms of CS are:

Pain, burn eyes, lacrimation (discharge of tears), conjunctivitis; erythema (skin redness) eyelids, blepharospasm; irritation throat, cough, chest tightness; headache; erythema (skin redness), vesiculation skin

Tear gas is a board classification and a CS smoke canister would fall into this group.  To state it explicitly, CS is a type of tear gas. Firing a CS smoke canister at or near individuals in Ferguson means tear gas is being used.


featured image is the structure of CS from chemspider

This post was inspired by @DNLee’s tweet.


DrRubidium is an analytical chemist that spends her days finding needles in needlestacks. Also a science communicator, she focuses on the the science behind everyday stuff and pop culture.

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  1. When a CS munition is marked “SMOKE” that’s the delivery mechanism, not the function. It burns CS-laced pellets to release a cloud of crystals. Technically CS isn’t really a tear gas either; it’s much worse. It was developed for the British Army as an incapacitating agent and I can testify from personal experience that it’s an extremely effective one. Some people are naturally immune for some reason, but for most it’s an absolute nightmare. It’s actually pretty safe in terms of long-term effects but the immediate effects are horrific.

    1. Varying definitions or categories is likely behind some of the media confusion. If one goes with the fairly board definition offered by the CDC, CS is a type of “riot control agent” (tear gas). For each of the agents listed by the CDC, their symptoms vary severity – quite a lot in some cases.

  2. Yeah, I was gonna say… I’ve had enough lungfuls of the stuff to know that it isn’t smoke. It is the stuff they gassed us with for “training” in the Marines. Sometimes grenades, but I seem to remember them also heating up a disc of the stuff one time in a “gas chamber” tent maybe at Ft. Bragg.

    1. Definitely not smoke as most people know it! As Fergus Mason says above, “smoke” refers to delivery method. ‘CS smoke’ is sometimes labeled ‘CS gas’.

    2. I gassed hundreds of British soldiers with it. We had grenades, portfire flares and spray cans, but for the testing chamber we lit small pellets. One pellet for other ranks, five for officers :-)

    1. Wind, temperature, rain, and related factors can all play a role in dispersion, stability, dilution, etc. when it comes to chemical agents. Rain may have no effect, may dilute, may promote hydrolysis – it depends on the agent and the influence of other conditions.

  3. I didn’t know the best way to point out a typo on here, but I think it is supposed to say that tear gas is a “broad” classification not a “board” classification.

  4. I was reading that story on the weekend. I could not believe how many hits I got for “pepper spray, black man, Seattle” when I tried to find it again.

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