Dr. Sven-Eric Jordt has experienced tear gas firsthand.
As a student in Germany in the 1980s, he participated in protests against nuclear waste transport, and ran afoul of police with water cannons and tear gas.
"Exposed people, including myself, experience severe burning and pain of the eyes, profuse tear secretion, blepharospasm -- spasm of the eye muscles resulting in eye closure -- nasal pain and strong mucus secretion, and shortness of breath," he said. "My eyes and nose remained irritated for days, and I had to rinse with saline frequently, although I was fine afterwards."
If that sounds a bit academic, there's a good reason.
Dr. Jordt is now a scientist at Yale University. Six years ago, he discovered the human pain receptors that interact with tear gas, which, it turns out, are the same receptors that respond to mustard and wasabi.
His findings could take on a very personal cast for Pittsburghers in about three weeks, when protesters confront police during the G-20 world financial summit Sept. 24-25.
Although public safety officials declined to comment, Elizabeth Pittinger, executive director of the Citizen Police Review Board, said she knows the city has purchased hundreds of canisters of CS gas, the most common form of tear gas, and has been training officers in its use.
That doesn't guarantee it will be used, and Ms. Pittinger hopes it doesn't come to that. But she said there is no way to know, because public safety officials have refused to reveal what guidelines or orders will govern the use of riot control measures such as tear gas, pepper spray or police dogs.
CS gas has been used by police in several protests against financial and political leaders over the past decade, including the World Trade Organization protests in Seattle, free trade summits in Quebec City, Canada and Miami, and both the Republican and Democratic conventions last year.
CS gas has become the dominant type of tear gas because it is considered less toxic than two of its predecessors, CN gas (also known as Mace) and DM gas. Its active ingredient is the compound chlorobenzylidene malononitrile.
The symptoms are potent, but often can clear up quickly when people get into clear air. For those who have never experienced it, said Dr. Jordt, "if you have ever eaten wasabi or mustard and the vapors got in your nose or eyes, you actually experience something like tear gas to some extent, but tear gas is much more potent."
There are few reports of deaths or long-lasting injuries from tear gas, but it can be dangerous for people with asthma or other lung conditions.
In fact, "in police training, they won't spray CS gas on officers who are asthmatic," noted Sam Rosenfeld, a former British infantry officer who heads a company that advises governments on riot control tactics.
"If we judge it's not safe to use on a theoretically fit healthy man or woman, how do we use it against a whole crowd where we have no idea what the health implications are?" Mr. Rosenfeld said.
Dave DuBay, chief technology officer for ALS Technologies, Inc., of Bull Shoals, Ark., one of the major manufacturers of tear gas in America, agreed that CS gas can be risky for people with lung conditions.
"If I were one of those individuals, would I go somewhere where there was a likelihood I'd be exposed to any agent that would exacerbate my condition? I wouldn't do it, but that would include exposing myself to soot or air pollution or pollen."
Mr. DuBay said most studies of CS gas show no long-lasting health effects. As someone who works with a company that manufactures the substance and attends training events where it is used, he has been exposed to the chemical many times, he said, and has suffered no long-lasting effects.
CS is actually a crystalline powder, Mr. DuBay said, and can be launched in three different ways -- mixed with a solvent and sprayed as an aerosol; mixed with fine powder and lobbed in canisters that send up a cloud of dust; or, in the method used in many protests, mixed with a smoke chemical and fired in canisters that explode with high heat, creating viscous clouds.
The smoky form of tear gas can be swept by gusts into air intakes of buildings, affecting innocent bystanders, said the review board's Ms. Pittinger.
At the Republican National Convention last year in St. Paul, Minn., one hospital in the protest area had to shut down its air conditioning system to keep tear gas from spreading through the hallways.
"In our city we have wind whipping around throughout the downtown and this stuff is going to fall -- where are the buildings' intake mechanisms for their heating and air conditioning systems?" she said. "Those intakes are unintended targets."
That is one of many reasons why she and Mr. Rosenfeld do not favor using tear gas in densely populated areas or against demonstrations that include largely peaceful protesters mixed with a few violent ones.
"Our problem is that an indiscriminate technology -- a so-called less-lethal device like tear gas -- will affect not only the up to 5 percent of the crowd who are the violent offenders, but will also affect the bystanders, the vast majority of people who had nothing to do with the conflict in the first place," said Mr. Rosenfeld, who chairs the Densus Group consulting firm of Plano, Texas.
Many cities in the United Kingdom, Northern Ireland and continental Europe no longer use tear gas in protests, he said. Their police now employ an alternative philosophy that says "you make people responsible for their actions. So the trick is to identify those with violent intent, go and arrest and prosecute and convict those individuals, and facilitate protests by everyone else."
But that approach can ignore some of the realities of mass protests in America, Mr. DuBay said. For one thing, police often are outnumbered by protesters.
"If you have thousands of protesters, how do you deal with a ratio that is very disadvantaged toward police officers? If I can move people away from a high target area [with tear gas], the tactical objective has been met," he said.
"It's true there are people who want peaceful protest, and there are people with another agenda. But it's very difficult to understand a mob, because it's a fluid, moving body, and sometimes the best way to handle that is to move it out of an area."
Alex Bradley, a Bloomfield resident and veteran of anti-globalization protests in Seattle and Quebec City, disputed Mr. DuBay's theory on one point.
While he has never been able to stay in an area with strong concentrations of tear gas, he has been able to return to those areas once he got into clear air and recovered, Mr. Bradley said. "I think the most important part about tear gas is it's not going to be effective at dispersing people who don't want to be dispersed," he said.
Mr. Bradley, who is helping organize the protests here, said he didn't want to pick or choose among better or worse crowd control tactics by police. "I'd be against all use of police force. I think the G-20 is violent by definition" because of how its policies affect poor and unemployed people around the world, he said. "Pittsburghers have a right to oppose that violence."
For all the concern about tear gas, Mr. DuBay noted it is not a growth area in his company's business, and that 90 percent of the product is sold for military and police training exercises, not for use against protesters.
Still, Mr. Rosenfeld said using tear gas sends the wrong message. "I think indiscriminate use of CS is counterproductive. What happens if CS gas is used and someone dies? What happens if CS gas is used in Pittsburgh and people start asking, why are we gassing our own population when President Obama condemned similar activity in Tehran?"
Ms. Pittinger agreed: "Wouldn't it be wonderful to show the world that this can happen in a peaceable way, that wrongdoers will be held accountable, and innocent people aren't going to be hurt?"