At least three people who live or work in Tuen Mun told a Commercial Radio programme on Tuesday morning they were affected by the unknown smell.

A mother of two surnamed Leung said she first noticed it when she was on her way to pick up her kids at school on a Light Rail train at about 3.55pm. Passengers started coughing before she got off the train at the San Wai stop and realised the smell came from outdoors.

“My eyes and skin were irritated, and I kept on coughing,” she said.

She said she came across the similar “irritating” smell – which she said was similar to the smell of tear gas – on her way to the school, making her worry about her children.

“We still do not know what we have inhaled,” she said, demanding that authorities reveal more details and investigate the incident.

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Another resident, surnamed Cheung, said her eyes became watery at about 4pm, near Shek Pai light rail station.

“When tears came out from my eyes, I originally thought it might be because of the heavy wind. But as I kept walking my throat and nose became uncomfortable as well. Then I also felt irritation on my face.”

A Tuen Mun resident who declined to be named told Commercial Radio smoke had been seen at the PLA’s Tsing Shan Firing Range on Monday afternoon.

Shooting practice did take place at the range on Monday from 8am to 9pm, according to a public notice from the government, dated September 27. The Post contacted the PLA for a response.

At a residents’ press conference on Tuesday, a mother surnamed Chan said her three-year-old daughter suffered from rashes on her back, arms and ears after they walked home from her school at about 4pm on Monday. She said the child rarely had any skin diseases, but she found her neck, back and arms had flared up by the time they got home.

Chan said she did not dare take her daughter to the doctor on Monday fearing further influence from the unknown gas. She planned to take her for a check-up, hoping to find out what exactly happened.

News of the odd smell spread online on Monday afternoon, with protesters suspecting police were testing weapons or other chemicals at their operation base in Tuen Mun. Hundreds gathered in the town at night in protest, resulting in clashes with the force.

At about 10pm, protesters smashed the glass shopfront of a Bank of China outlet and a petrol bomb was hurled into a cafe which protesters said was linked to a pro-Beijing Fujianese clan.

Police then emerged from the base, firing multiple rounds of tear gas outside. A tear-gas canister entered the corridor on the seventh floor of Yat Sang House, a public housing block.

On Tuesday, in a passageway on that floor, a black mark could be seen on the ground, believed to have been caused by the canister. At 11.40am, three men in plain clothes who said they were police went to the floor to check the spot, accompanied by a security guard from the building.

Mrs Cheng, who lives just a few metres from where the black mark was, said her relatives, including her child, a secondary school student, were at home when the tear gas entered the building.

“We were watching the police on the streets through the windows when we smelled the tear gas,” she said, adding that the gas got into her home even though the door was closed.

“We had a running nose and wore masks for the following two hours.”

In a statement on Monday night, police denied that anyone at the Tuen Mun base had used tear gas or “any unidentified gas” before the unrest broke out.

The Fire Services Department said it received 18 emergency calls on Monday afternoon regarding the odour and confirmed no unusual smell was found in the police base. It continued its investigation on Tuesday and did not find any unusual smell in the area.

A Hospital Authority spokeswoman said at least 11 people were treated at Tuen Mun Hospital on Monday night because of gas inhalation. Ten were discharged by Tuesday, but one person was still in hospital, in a stable condition, on Tuesday afternoon.

Some 50 cats and dogs from nearby animal shelter Hand in Paw were also affected by the smell, according to a volunteer surnamed Ng.

Ng said she and other volunteers felt irritation in the eyes and nose and started coughing, with some of the cats and dogs displaying similar symptoms.

“Some even had to use their mouths instead of their noses to breathe,” Ng said, adding that about 10 cats vomited repeatedly as volunteers moved them away from the centre on Monday night and took them to see a vet on Tuesday.

When asked about the source of the smell on Monday, a police community relations officer replied to district councillor Tam Chun-yin that the cause was unknown and there was no use of tear gas at its or the fire service’s base, but he also mentioned rising concentrations of air pollutants in the area.

According to data from the Environmental Protection Department, concentrations of pollutants including nitrogen dioxide, ozone, sulphur dioxide, carbon monoxide, PM10 and PM2.5 saw sudden spikes between 1pm and 2pm on Monday.

The department said that “a northwest wind brought an air mass with higher concentrations of pollutants to several districts” including Tuen Mun on Monday.

The air quality health index in Tuen Mun increased to 7, on a scale of 1 to 10, which is “high” on the index, at 2pm. It reached 8, or “very high”, from 3pm to 4pm.

The level of ozone, a pollutant known to have a sharp smell similar to chlorine and which can cause lung damage and shortness of breath, also rose on Monday.

Steve Yim Hung-lam, an assistant professor at Chinese University’s department of geography and resource management, said the health effects from ozone would not just have affected people in Tuen Mun, as other districts such as Yuen Long and Tung Chung also saw increases in concentrations.

Noting ozone levels in Tuen Mun on Monday reached 153.2 micrograms per cubic metre, Yim said short-term concentrations in 2018 had surpassed the government’s air quality objectives of 160mcg/m³ in eight hours to reach up to 200.

“Even when it was that high, there were no reports of people feeling discomfort,” he said. “While ozone cannot be ruled out as a cause, I don’t think that is the full explanation.”