Over 10,000 tear gas canisters have been been fired in Hong Kong's vicious protests, and it could have long-term consequences for the city's health

• Pro-democracy protests in Hong Kong have sent the city into prolonged chaos since June 9.
• Over the course of over six months, more than 10,000 canisters of tear gas were launched, according to Associated Press estimates.
• An estimated 88% of Hong Kong's 7.4 million residents were affected by tear gas at some point.
• Anna Feigenbaum, author of the book "Tear Gas: From the Battlefields of WWI to the Streets of Today," spoke to Business Insider about the long-term impacts of tear gas exposure on Hong Kong's residents.
• Visit Business Insider's homepage for more stories.

Pro-democracy protests in Hong Kong have consumed the city since June 9, leading to the arrests of nearly 7,000 people, according to Hong Kong news site RTHK, and the injuries of thousands of others.

During that time, a record number of tear gas canisters were fired by police during clashes, exposing an estimated 88% of Hong Kong's 7.4 million residents to the chemical. The Associated Press estimates that more than 10,000 canisters have been fired.

The prolonged use of the riot control method, particularly in confined areas like metro stations and near shopping centers, has prompted concern among experts and locals about the effects of long-term exposure.

"What's going on in Hong Kong is pretty unprecedented," Alistair Hay, a British toxicologist from the University of Leeds, told AP in December.

"I don't think there have been circumstances where there has been this level of repeated exposure for people to tear gas."

Anna Feigenbaum, author of the book "Tear Gas: From the Battlefields of WWI to the Streets of Today" and tear gas researcher, discussed the striking amount of tear gas unleashed on Hong Kong and the possible impact it would have on the health of the city.

According to Feigenbaum, there are several reasons why the firing of tear gas in Hong Kong is of particular concern among researchers.

Hong Kong is a densely-packed city

According to Feigenbaum, the use of tear gas in enclosed or tightly-packed spaces intensifies its toxicity levels.

Firing inside an enclosed space can also cause people to panic, breathe faster, and inhale even more gas. This can lead to serious lung damage or even death by suffocation.

According to the United Nations figures from 2017, Hong Kong has a density of 26,000 people per square kilometer, or about 10,000 per square mile.

Feigenbaum also expressed concern that police in Hong Kong have sometimes used tear gas over and over in short spaces on time.

Hong Kong police have been firing tear gas at close distances

Feigenbaum explained that firing tear gas at a close distance exacerbates the risks.

Although officers firing tear gas at crowds are supposed to account for weather conditions - especially wind - and the layout of the area, video footage has shown police firing tear gas from a close distance and aiming canisters at people instead of the ground.

For these reasons, the relative safety of using tear gas as a crowd dispersal method is difficult to calculate in Hong Kong, and the effects on the population become "more potent."

"The safety of tear gas is determined by the amount of the toxin that is in a canister per metres squared," Feigenbaum told Business Insider.

"Factors such as distance, clear escape routes and wind levels and directions should also be calculated.

"So releasing 50 canisters in an open street with clear ways off the street in normal air conditions is more 'safe' than releasing 50 canisters in a transit station or on a blocked road. Likewise, being next to a canister makes it far more 'unsafe' than being 50 metres from one."

Long-term exposure to tear gas puts the population at risk

According to Feigenbaum, the impacts from tear gas can be seen in both the short and long term, and can be made worse by a variety of factors including stampeding and panic.

Tear gas canisters release a cloud of moisture, designed to seep into everything it reaches, including skin, soil, and buildings.

Tear gas is designed to attack all senses at once. This can lead to physical and psychological trauma, according to Feigenbaum.

Tear gas can cause skin burning, blurred vision, redness, a runny nose, chest tightness, coughing, and nausea.

Long-term effects are harder to study, and little research exists into its broader impacts on whole populations.

"It is very difficult to study because you cannot isolate or make separate what health conditions are caused by tear gas versus other environmental, social and genetic conditions," Feigenbaum said.

However, Feigenbaum said some research suggests that long term exposure can exacerbate respiratory issues like asthma, increase effects of epilepsy, cause long-term tissue and respiratory damage, scarring. She said it has even been linked to miscarriages.

Additionally, long-term impacts of tear gas exposure can lead to psychological damage. She said there are increased diagnoses of PTSD and trauma-related illness associated with locations that have been heavily tear gassed.

Though protests in Hong Kong have quieted in the last few weeks, scores of protesters took to the streets on New Year's Eve, leading to violent clashes between protesters and police, who fired water cannons and pepper spray at crowds.

It is unclear where the movement will go, faced with the stark refusal of the Hong Kong government to concede to the protesters.

In November, pro-democracy candidates won hundreds of seats in Hong Kong's local elections. And last month, US President Donald Trump signed a bill meant to defended human rights in Hong Kong.

But Hong Kong leader Carrie Lam suggested one solution may be slowing the pace of integrating Hong Kong with mainland China, which currently has a deadline of 2047, 50 years since it was returned to China by Britain,

On Thursday, Lam said the status quo could continue beyond 2047 as long as the city's "loyalty" to Beijing was upheld.

"Only if we insist implementing 'one country, two systems' principle and practice it continuously and fully ... then I think there will be enough grounds for 'one country, two systems' to move ahead smoothly and there would be no change after 2047," Lam said.