Bill Birtles via Twitter The Hong Kong police have fired more than 1,800 rounds of tear gas as the mass protests gripping the city have become increasingly fraught. A torrent of tear gas was launched from a tall building onto crowds below. Gijsbert Heikamp A canister was shot into a protester filming the police. Luk Chung Man William/Maddog.HK via Facebook Clouds of toxic smoke filled a subway station. Experts who reviewed videos for The New York Times said the police had repeatedly violated standards for safely dispersing crowds, putting protesters and bystanders in serious danger.

The Hong Kong police have defended their use of tear gas, saying the protests have become more violent in recent weeks, with demonstrators embracing more aggressive tactics. Activists have thrown bricks, blocked streets and set fires.

The police say they have acted with restraint. Often, they hold up large banners warning the crowds about tear gas before they fire.

But a Times review of dozens of episodes involving tear gas show the police force, at times, has used methods that experts describe as indiscriminate and excessive. Many recordings show officers deploying tear gas against crowds that appeared nonviolent and that were not attacking the police.

“This is ridiculous,” said Jim Bueermann, former president of the Police Foundation in Washington, referring to a video that appeared to show tear gas being fired off an office tower.

“I would find this to be completely unacceptable under American standards,” added Mr. Bueermann, a former police chief who advises law enforcement agencies. “You are now taking a less-lethal tool, the tear gas, and making it a potentially lethal object.”

Around the world, tear gas has long been a means to control riots and civilian protests. If used appropriately, it drives people to flee the gas, which irritates their eyes, skin and lungs without causing serious, long-term injuries in most people.

Experts say officers should fire tear gas canisters a short distance toward the edges of a crowd.

Safe use: Tear gas should be dispersed at the edges of a crowd and below knee-level to avoid hitting people. Guilbert Gates

But law enforcement officers at times ignore those guidelines, leading to debilitating injuries, as has happened at protests in South Korea, Bahrain and Turkey.

Most injuries from the use of tear gas in Hong Kong have been minor. Groups have begun documenting health issues like prolonged coughing, skin blisters and diarrhea. Bystanders with no protective gear, including children, have been seen getting caught in tear gas.

Experts reviewing the videos of episodes in various neighborhoods described several ways in which the officers’ methods could have been dangerous.

Using Tear Gas Indoors

Tear gas is a nonlethal chemical, when used appropriately. It spreads quickly over a wide area, limiting exposure to individuals. Experts and tear gas manufacturers warn against using it indoors, where higher concentrations of the chemical can become trapped, and people may not be able to escape.

A week ago, after an hourslong standoff with the police in the Kwai Fong area, antigovernment protesters retreated to a subway station.

Luk Chung Man William/Maddog.HK via Facebook 1 In a video taken by a journalist, protesters can be seen gathering inside the gates of the station. 2 An officer runs in, and protesters set off a fire extinguisher to impede visibility. The officer then fires what appears to be a tear gas canister toward the crowd. 3 Moments later, more officers rush into the station, and they appear to fire two more rounds in the protesters’ direction.

Using tear gas inside is dangerous, whatever the reason, experts said.

“Discharging indoors leads to panic, can lead to stampede, and at its worst it can lead to dire health consequences, including death, if people cannot escape the suffocating effects of the gas,” said Michael Power, a civil rights lawyer based in South Africa who specializes in protests and policing.

Mr. Bueermann, the former police chief, said it was dangerous to fire tear gas into an area with limited visibility because people could be hit.

“In this case, it’s very clear they can’t see very well in that area because of the concentration of tear gas,” he said. “You don’t fire any kind of a weapon into an area that you cannot see what your target is.”

The police initially denied using tear gas in the train station, according to I-Cable News, a local TV station. But later, the police acknowledged that they had fired tear gas.

Firing From High Above

Bill Birtles via Twitter

In several cases, the police have shot tear gas from their perches above the crowds.

Early this month, thousands of protesters gathered outside the government headquarters as part of a general strike across Hong Kong. The demonstration was considered an unlawful assembly because it expanded beyond what was authorized by the city.

1 The police warned protesters they needed to leave. But the crowd on Harcourt Road remained, blocking traffic. 2 Multiple rounds of tear gas are fired from above, landing in the middle of the crowd. Officers’ exact position is unclear, but the rounds’ trajectories suggest that the police are stationed on a building high above.

Shooting from such a position is reckless, Mr. Bueermann said, because the tear gas canisters fall to the ground at a high speed. Most canisters collected by protesters have been six-inch-round metal shells.

“I have never seen that before,” he said. “I think that is hugely problematic. If it hits someone in the head, you could kill them.”

From a distance, the police also don’t have a complete understanding of the protesters’ actions, said Dr. Rohini J. Haar, an adviser at Physicians for Human Rights, an advocacy group. “At such a height, they may not be able to discern whether protesters have become threatening.”

Firing at People

A freelance filmmaker, Ryan Lai, was filming protests outside a police station in Sham Shui Po. After hours of standoff, the protests had dwindled to what looked like dozens of people. Some shouted insults at the police.

When the police fired tear gas, Mr. Lai was struck in the head.

Inmediahk via Facebook

“The thing hit me directly in the forehead, at the corner of my left eye,” Mr. Lai said.

Inmediahk

Gijsbert Heikamp was filming with his cellphone at a protest outside a police station in Tsim Sha Tsui.

He was outside the station, standing behind a barrier, when officers began firing tear gas from behind a fence. Two of the canisters went through gaps in the barrier, hitting him in the stomach and on the right arm.

Five days after the episode, he still had a clear burn mark, bruising and swelling.

Experts say that most injuries from tear gas are blunt force trauma caused by a canister.

“They are being fired at very high velocity; the launchers that are used are basically guns” said Anna Feigenbaum, an expert on tear gas at Bournemouth University in England.

Indiscriminate Use

The police have fired tear gas at crowds from far off or partly hidden positions. The tactic is considered inappropriate, since the crowds don’t have sufficient warning and the police don’t have a full view of the environment.

As a large group of protesters gathered outside a police station in Sham Shui Po one night, the police fired tear gas from inside the gates. The use of tear gas caught many by surprise.

Luk Chung Man William/Maddog.HK via Facebook 1 Protesters have surrounded the police station; most people are dressed casually, not wearing gas masks or other protective gear. 2 A group of officers gathers far back in the station, raising a barely visible warning flag. “If they are raising a flag as a notice, that is ineffective from that far,” Mr. Bueermann said. 3 Tear gas is fired from inside the station. 4 People disperse as the street is covered in a thick cloud of smoke. 5 Less than a minute later, a second round of tear gas is fired. 6 Moments later, a third round of tear gas is fired.

When the tear gas is lobbed over the outer wall of the police station, the police don’t appear to have a specific target or purpose, and it haphazardly hits the crowd.

“Why is it that the police, who seem to be comfortably protected in the police station, are seeking to call for dispersal?” Mr. Power said.

The use of a second and third round of tear gas, to the experts, also seemed to go too far. “If people are leaving, and that was your intent, then stop using it,” Mr. Bueermann said.

Also, the more tear gas is used, the harder it can be for people to leave.

“If you don’t give them enough warning, if you don’t give them a place to go, if you use too much gas, if you charge them too soon, all of those things can be counterproductive of the primary goal of dispersing the crowd,” said Mr. Bueermann.