Health Secretary Sophia Chan told lawmakers on Wednesday that the manufacturing details and chemical composition of tear gas used by the police can't be revealed as the force wants such information to be kept secret for operational reasons.
Responding to a question about the tear gas from the Democratic Party's Helena Wong, the secretary said the police have been sourcing "safe and suitable equipment globally according to the esablished procedures".
Chan also told Legco that there has been no evidence to suggest that the use of tear gas can produce dioxins or cause dioxin poisoning.
Dioxin are highly toxic and can sometimes cause cancer.
The secretary said the government did look into academic studies on tear gas, but found no evidence to support poisoning claims.
"Given the short time of combustion of a tear gas canister, any cyanide produced will be quickly dispersed in the air," the health secretary said.
She also said after large-scale events, the Food and Environmental Hygiene Department provides workers to clean up areas affected. The Environmental and Protection Department had not detected any abnormalities in the city's air or water quality, she said.