Officers involved in the April 18 Veishea riot used some tear gas canisters that had expired, students and law enforcement officials said.

Michael Brechler, sophomore in electrical engineering, found one such empty tear gas canister with manufacture date of 1994 in a stairwell by People’s Bar and Grill, 2428 Lincoln Way, the morning after the riot.

The canister’s label said it was not to be used more than four years after its manufacture date.

A label on the canister said it had been manufactured by Mace Security International and contained a form of tear gas called phenylchloro-methylketone, or CN.

Ames Police Sgt. Brian Braymen, who has special training in gas munitions, acknowledged the expiration date was long past but said the past-due date could only cause problems for officers, not health risks for gas recipients.

“The only real concern is that the device won’t work,” Braymen said.

The expiration date was for the protection of the user, not because the chemicals could become more dangerous to the people it is being used on, Braymen said.

The chemicals in tear gas could become less potent, but the major concern is the firing mechanisms in the canister wouldn’t fire, he said.

Ames Police Chief Loras Jaeger said the department had talked to a number of manufacturers about how to properly store the chemical canisters. The Ames Police has a special area it used to store the chemicals that will allow safe storage for many years, although the chemicals may lose potency, he said.

Jon Goodrich, CEO and founder of Mace Security International, said the reason tear gas canisters have expiration dates is because the propellant inside the canister can decrease in strength, potentially putting the user in harm’s way.

“You want it to be in full power when you need it to be,” Goodrich said.

Temperature changes can cause the canisters to decrease in potency or firing pressure, which is why there is a general expiration date, he said.

There have not been any medical reports of anyone experiencing long-term illness from exposure to tear gas, Goodrich said.

Current industry standards say about 10 percent of all gas canisters will fail, whether they are expired or not, Braymen said.

During the riot, many canisters failed to work properly, he said.

Story County Sheriff Paul Fitzgerald said there were three types of chemicals used by the various law enforcement agencies present in the Veishea riot — CN, CS, and OC.

The first two chemicals, CN and CS, are different forms of tear gas, and the third, OC, is a pepper spray, Fitzgerald said. CN is usually used inside, since it dissipates, whereas CS has a tendency to form a cloud, Goodrich said.

“A good way to put it is CN hates itself, CS likes itself,” he said.

CN primarily works as an irritant to the eyes and other mucous glands, while CS causes more irritation to the skin, he said. In that sense, CS makes for a more practical use in riot control.

Goodrich said he was surprised to see people using CN for riot control since it was seldom used, and was difficult to train with.

Braymen agreed CN is not a common choice for riot control.

“[CN] is sort of antique-ish,” Braymen said.

Most of the gas used by Ames police was pepper spray, he said, which works as an irritant to the eyes and skin, but doesn’t cause any major damage, he said.

“It’s a sensory type thing to make you feel unpleasant; it wouldn’t leave a mark,” he said.

— Jason Noble and Tom Barton contributed to this article.