Tear gas residue has been found in six areas of Hong Kong, in some places two to three weeks after it was used by police, posing health risks to residents.

A team of 30 chemical engineers voluntarily conducted surveys in six tear gas-affected districts and found CS gas residue on various surfaces. Hong Kong police started using tear gas to disperse protesters in June.

The team collected samples from 200 objects at 75 sites in Admiralty, Sheung Wan and Sai Ying Pun on Hong Kong, Kowloon Bay and Sham Shui Po in Kowloon as well as in Tsuen Wan in the New Territories from August 18.

After laboratory tests, they found residual amounts of two chemicals used to make tear gas on streets, the walls of buildings, footbridges and around residential buildings.

Five places in Admiralty and Sai Ying Pun were found to have chemical residue two to three weeks after tear gas was used, and some residue was found on an apartment window and on furniture placed near the window.

A sample collected at the intersection of Des Voeux West Road and Sutherland Street showed residue 21 days after police fired tear gas there. Reports said 408 tear gas canisters were fired in Sheung Wan during clashes between police and protesters on July 28.

If police fire tear gas in a district with narrow streets, due to low ventilation, the chemicals stay longer. The team found residue on nearby streets and at the entrance to a home for the elderly in Sham Shui Po where police fired 35 rounds of tear gas on Yen Chow Street on August 14.

Over the past three months, Hong Kong police have used a total of 3,060 tear gas canisters to disperse protesters during clearance operations, the Ming Pao Daily reported.

Lee Ho-wun, who led the team doing the survey, said the results were inconsistent with police claims that residue only lasts for a short time.

Lee also said the team found chemical residue under a bicycle footbridge in Tsuen Wan Park close to a children’s playground. The park was 250 meters from the site of a major clash on Yeung Uk Road.

Tear gas irritates the eyes, nose, mouth and lungs, causing symptoms such as crying, sneezing, coughing, breathing difficulties, eye pain and temporary blindness. It can also cause chemical burns to exposed skin and set off allergies.

A clinic in Sai Wan said one week after clashes on July 28, about 10 adults and elderly people sought medical treatment due to allergies and respiratory infections, the Hong Kong Economic Times reported.

The team urged police to use less tear gas and to properly handle the casings of the tear gas canisters. They also urged authorities and the relevant departments to test for residue in possibly contaminated areas and to clean the areas thoroughly.

Meanwhile, US lawmakers and members of the House of Representatives Jim McGovern and Chris Smith introduced a bill to halt the sales of crowd-control equipment from US companies to Hong Kong police.

“I am deeply concerned that American-made police equipment is being used to violently crack down on peaceful protesters in Hong Kong,” said McGovern in a press release.

“Hong Kong police are targeting their own citizens who are guilty of nothing more than peacefully protesting government threats to their freedoms and liberties,” Representative Smith said.

They cited journalists and citizens who had provided credible evidence showing that the Hong Kong police force had used tear gas, pepper spray, rubber bullets, batons and other crowd control equipment against peaceful protesters in violation of manufacturer guidelines and international standards.

Some of the equipment used by Hong Kong police had been supplied by US companies.

But a spokesperson for a junior police officers’ union said the bill, if passed, would only have a minimal effect, the South China Morning Post reported.

Other police sources quoted by the newspaper said the force had no shortage of equipment and could buy from other countries if the US bill passed.

Local media earlier reported that the police force had purchased 500 sets of protective gear from mainland China.

Read: Will HK police have to buy tear gas from China?

Read: Excessive use of tear gas poses public health risk