There is no evidence University of Science and Technology student Chow Tsz-lok was tear-gassed, pepper-sprayed or poisoned before his fatal fall from a Tseung Kwan O carpark, a toxicologist told the coroner's court.

Chow, then 22, died from brain injuries four days after he plunged from the third to the second floor of Sheung Tak Estate car park in the small hours of November 4, 2019, while police clashed with anti-government protesters nearby.

Toxicology specialist Tse Man-li testified before coroner Ko Wai-hung yesterday as the inquest entered the 25th day - the last day of witness testimony. Lawyers are scheduled to make their closing submissions at the hearing today.

Tse said Chow's medical report did not mention signs of exposure to tear gas, such as a runny nose or reddened eyes.

If Chow was indeed tear-gassed, some of the particles would have remained on his body and clothes, affecting the medical workers rescuing him as well, according to Tse.

"The odds [of Chow getting tear-gassed] are slim, as no medical staffer noticed that," he added.

Tse also said there was no trace of pepper spray or cyanide - a chemical found in tear gas - in Chow's blood or body fluid samples taken during his hospitalization and autopsy.

He added that while tear gas can affect human vision and bring about mental pressure, it would hardly affect one's sanity, walking ability or muscle strength.

It is "almost impossible" for someone to die from cyanide poisoning unless at least 10,000 rounds of tear gas are fired simultaneously, Tse said.

After watching CCTV footage, Tse said Chow showed no sign of having been tear-gassed while wandering inside the car park before his fall, and other people caught on film at the venue also looked calm and at ease.

A juror then asked him whether the tear gas fired toward the car park's third floor at 1am on November 4 could actually reach its destination under the influence of wind, and whether the smoke could fill out the floor within one minute.

Tse replied that the footage showed the smoke of the tear gas was rising upward, meaning it was not affected by wind, adding it is unlikely for the smoke to diffuse at the speed the juror suggested.

Chow's fall is suspected to have occurred at 1.01am based on evidence presented to the court previously.

Chow Tsz-lok

No sign of tear gas on chow, court hears