According to the US Centres for Disease Control and Prevention, the most common compounds used in tear gas are chlorobenzylidenemalononitrile (CS) and chloroacetophenone (CN). Hong Kong police have said they use CS tear gas. So far this year, the police have fired more than 12,000 rounds of tear gas in all districts except Islands district.

How does tear gas work?

Exposure to tear gas causes almost instantaneous irritation. Within seconds, people feel a stinging and burning sensation in their eyes and mouth. They begin to shed tears, start coughing, and may experience shortness of breath. Some break out in rashes, too. The reaction then eases, with symptoms usually gone after a short while.

Professor David Hui Shu-cheong, an expert in respiratory medicine from the Chinese University of Hong Kong (CUHK), said people with asthma or chronic bronchitis are more likely to develop inflammation in their trachea, or windpipe, after being exposed to tear gas.

Hong Kong police buy tear gas canisters, including same model used by PLA

Worldwide, at least two deaths have been linked to tear gas. One was recorded in Bahrain, where a man died of respiratory arrest after tear gas was fired into his home. In another case in Nepal, the tear gas canister was found to have contributed to a death from traumatic brain injury.

Are some people affected more than others?

The use of tear gas in densely populated urban areas with residential blocks, schools and health care facilities has affected young children, the elderly people and patients.

Although the effects are said to be short-lived, some people complained that their rashes persisted for days after they were exposed. An elderly man with asthma said his condition worsened and remained bad for days afterwards.

Does tear gas release poisons?

Eyebrows were raised when the Hong Kong police were found using expired tear gas on some occasions this year, and some pharmacists asked if expired tear gas might release the poison cyanide.

Hui said the release of cyanide is related more to the temperature when tear gas is fired rather than the expiry date. He said that tear gas devices are usually ignited at 150 degrees Celsius. But he cited findings of the US army that such devices could release cyanide if ignited at 275 degrees Celsius or above.

Hui said at the higher temperature, tear gas would also produce more chemical products including chlorobenzene, which releases dioxins when it decomposes. Dioxins are highly toxic compounds which are harmful in a number of ways and can also cause cancer.

Why the worry over dioxins?

A journalist covering the protests at the frontlines where tear gas was frequently released complained earlier this month that he suffered from chloracne, a skin condition linked to exposure to dioxins. People then began asking if the tear gas used in Hong Kong emitted dioxins.

Police said in October that they bought tear gas supplies from mainland China that explode more quickly and produce denser smoke compared to products from Britain. But it remains unclear if mainland-made tear gas burns at a higher temperature.

Parents, children in march against health hazards of tear gas

Professor Chan King-ming from CUHK’s school of life sciences said there is no evidence suggesting that tear gas emits dioxins. “The burning is too short, lasting only one to two minutes,” Chan said, adding that dioxins are emitted when burning occurs for 15 minutes to an hour in an enclosed space. To ease public concerns, Hui suggested that the government conduct tests to check contamination of the environment.

Is the air monitored for poisons?

The Environmental Protection Department monitors levels of dioxins in the air at two stations, in the Central and Western district and in Tsuen Wan. Chan suggested including dioxin checks at all its 16 air quality monitoring stations.

Government officials have pointed out that chemicals including cyanide and dioxins can be released by burning any substance. They said there is no evidence of dioxin poisoning caused by the use of tear gas, and any cyanide produced during the short duration of combustion of tear gas canisters would “quickly disperse in the air”.

Dr Howard Hu, a toxic chemical expert with US-based NGO Physicians for Human Rights who has been following the use of tear gas in Hong Kong, said he is not aware of records of previous incidents in other countries in which tear gas emitted cyanide or dioxins.

But he said there are major concerns about the possibility of tear gas exposure causing long-term chronic conditions, such as triggering asthma in people who have not had the condition before and worsening existing lung or heart conditions. He agreed that Hong Kong’s high-rise urban setting could increase risks of tear gas exposure occurring in enclosed or semi-enclosed spaces. That, in turn, increases the level of exposure.

How have people responded so far?

Concern groups and lawmakers have asked the government to reveal more details of the tear gas used in Hong Kong, including detailed composition and places of origin. The police said this would be inappropriate as it would affect the force’s operational capability.

Some Hong Kong parents have organised rallies, demanding an end to the use of tear gas. Schools in neighbourhoods where tear gas was used brought in professionals to conduct thorough clean-ups before classes resumed. At Queen Elizabeth Hospital in Jordan, close to one of the major areas where tear gas was fired at intense levels about two weeks ago, windows were sealed and portable air purifiers deployed. The hospital also replaced the air filters of its ventilation system.

Just passing by and hit by tear gas? How to treat its effects

The Chinese University, where the police fired more than 1,000 rounds of tear gas in clashes earlier this month, tested air, water and soil samples from its campus for harmful substances. Preliminary results of the soil tests showed no harmful substances.

Some people are moving to get away from the tear gas where they live. Gloria Lee and Sam Tsang, a couple in their 30s living in Yau Ma Tei, are looking for a place in a rural area in Tai Po. Lee, who is planning to get pregnant, said: “We are afraid that our future child will be affected by those chemicals.”

6. What to do if you are exposed to tear gas?

The experts have commonsense advice. First, get away quickly to an area where fresh air is available, and avoid dense, low-lying clouds of tear gas vapour. Remove clothing that might have been contaminated, and avoid pulling such clothing over your head. Cut that t-shirt off instead.

Use plenty of soap and water to wash your body. If you are suffering a burning sensation in your eyes or blurred vision, rinse your eyes with plain water for 10 to 15 minutes.

Hospital Authority toxicologists advised women to avoid breastfeeding for one to two hours after exposure to tear gas. They should also change their clothes, and clean their hair and skin thoroughly before picking up their babies.

Hong Kong’s Centre for Health Protection said those who are indoors when tear gas is fired nearby should close all doors and windows, turn off the air conditioning, and seal the gaps of doors and windows with wet towels. Use soapy water to clean any surfaces with tear gas residue, but avoid using hot water to prevent the residue from evaporating. To avoid stirring up the residue, do not use high pressure water jets, brooms and electric fans.

Illustrations: Adolfo Arranz