A group of around 40 chemical engineers warned on Wednesday that they have found potentially harmful tear gas residue lingering on various surfaces up to three weeks after it was fired by police officers during clashes with protesters.

The group said it had collected more than 200 samples since August 18, from Admiralty, Sheung Wan, Kowloon Bay, Tsuen Wan and Sham Shui Po, where officers had earlier fired rounds and rounds of tear gas.

Michael Lee, one of those behind the study, said tear gas – or CS gas – chemicals were found on various surfaces, including walls and billboards. In one case, in Sheung Wan, the chemcials were still present 21 days after the weapon was deployed there.

Lee said they also found that tear gas can spread much further afield than people might think, especially in well ventilated areas.

For example, he said, they found residue of tear gas 250 metres away from a site in Tsuen Wan where officers had used it.

And he said it was worrying that in Tsuen Wan Park, they found a lot of residue in the children’s area, the bicycle park and in the tennis court.

“We are worried that the gas chemicals may stick on those facilities in public areas. A child or those elderly doing exercises will be contaminated by the CS … harming their health.”

Lee urged the police to stop using tear gas indoors or in poorly ventilated locations, and called on the government to inspect areas where tear gas has been fired.