Tear gas is a chemical weapon that the Geneva Convention bans from use in international warfare. Multiple journalists have reported its use the night of November 24, 2014, during the protests that erupted after a grand jury did not indict white police officer Darren Wilson in the shooting and killing of black teenager Michael Brown.

Jake Tapper says there’s definitely some tear gas, it’s not just smoke. Don Lemmon confirms it as well. As does Cuomo. — John Aravosis (@aravosis) November 25, 2014

St. Louis Police denied the use of tear gas in Ferguson in November, saying they only used smoke to disperse crowds.

Police are not deploying tear gas. They are using #smoke to break up unruly crowds. #Ferguson — St. Louis County PD (@stlcountypd) November 25, 2014

But tear gas was confirmed to be used during protests in Ferguson last summer immediately following the Brown shooting, a move that alarmed experts.

"I'm very concerned about the increased use, and the much laxer attitude we've developed towards the potential health effects," says Sven-Eric Jordt, a scientist at Duke University who researches tear gas.

Jordt says we know a decent amount about how tear gas effects the body in the short-term: it activates pain receptors, especially in the eyes, forcing the eyelids to squeeze shut and tear uncontrollably. Jordt, who himself was tear-gassed during a protest in Germany in the 1980s, describes the sensation as "like cutting an onion but about 100 times more severe."

There is little known, however, about whether the main chemical in modern tear gas — a compound called 2-chlorobenzalmalononitrile — can have longer lasting effects on the body. That's something Jordt and his colleagues are currently researching.

Jordt and I spoke Thursday morning about the history of tear gas, how the compound effects the body and what we still don't know about how tear gas works.

Sarah Kliff: Can you start by walking me through the history of tear gas. How was it developed, and why does it exist in the first place?

Sven-Eric Jordt: The first tear gas agents were developed during the First World War as warfare agents. These were highly aggressive, organic compounds used in trench warfare and other situations alongside other, more lethal war gases like mustard gas.

The tear gas that's used by law enforcement now is typically CS gas. It's a compound that was developed because it's less toxic. It's used for clearing wider areas. It was used in the Middle East and Turkey recently to a scale that was unprecedented. Use has increased tremendously over the last few years; it's also been used in the US more and more. I think it's what's being used in Ferguson.

SK: How do these gases work?

SEJ: The way these gases work, and this is what we do research on, is that they activate pain receptors — the pain sensing nerves in our body. The cornea is densely covered with these receptors. When tear gas activates these pain receptors, that leads to body reflexes like profuse tear secretion and a muscle cramp in the eyelid that causes them to close. These are all protective responses that the body has to pain, and with the gas they become extremely exaggerated.

There are situations where this can be very dangerous or lethal. If somebody has asthma, for example, or a hypersensitivity or an airwave disease that can be very dangerous. It's not very frequent, but it has been a problem in the Middle East and other places.

"There are situations where [tear gas] can be very dangerous or lethal"

Tear gas can also lead to profuse mucus production, and that can lead to the feeling of suffocation. That's especially true if it's used in closed environments, like what you saw in Cairo. That's not the case here in Ferguson.

SK: Can you explain what it feels like to be tear-gassed? I understand that you've experienced it yourself once.

SEJ: That was in the 1980s when I was a student, and I protested against nuclear waste transport in Germany. It's extremely painful. Your face starts burning very quickly and your eyes start tearing. The eyelids shut and you can't do much. It's like cutting an onion but maybe 100 times more severe. It actually is the same pain receptors being activated as what happens with an onion, except you're dealing with something that is about 100 or 1000 times more potent.

What happens next is you get severe pain in your nose and throat and you also get a lot of mucus and snot production, and that obstructs your breathing. It's like a burn injury or a chemical burn that happens.

SK: What do we know about the long-term effects of tear gas exposure?

SEJ: There are very few studies on that. We don't have very much information from humans; we've mostly looked at it in animal models.

What we know is that tear gas causes inflammatory responses that are very rapid, and causes skin and eye inflammation that takes several days to clear. If someone has a chronic, underlying disease — if they have a skin disease, for example — this could aggregate that condition. There are very few long term studies on this though.

"It's extremely painful. Your face starts burning quickly."

SK: I know your work focuses more on the research of how tear gas works, but I was curious if you had any thinking on why we've seen an increase in tear gas use over the last few years?

SEJ: I think tear gas is seen as safe and somewhat effective, especially when the alternatives are weapons and rubber bullets that can cause much more severe injury. There's a perception that these are relatively harmless compared to other riot controlling measures.

SK: As somebody who studies tear gas, do you think that perception is accurate?

I frankly think that we don't know much about the long-term effects, especially in civilian exposure with kids or elderly or people in the street who might have some kind of lung disease already. There's very few follow-up studies. These are very active chemicals that can cause quite significant injury, so I'm concerned about the increased use of these agents.

The question is: what else could the police use when attacked? It's hard to say, and most people do recover okay after being exposed. But then you see, in the Middle East, tear gas being used to essentially fumigate a city with tens of millions of people, and that's a really big concern.

I'm very concerned that, as use has increased, tear gas has been normalized. The attitude now is like, this is safe and we can use it as much as we want.