Speaking after inspecting the Sheraton Hong Kong Tung Chung Hotel on Friday, government advisor Yuen Kwok-yung said the 56-year-old man, who arrived from the UK and stayed in a 15th floor room, had used a cycling machine during his quarantine.
The microbiologist said the man, who later tested positive for Covid-19, must have exhaled a large amount of virus-laden air, which might have flown into the room next door when his neighbour opened the door.
The 62-year-old man next door, who arrived from Thailand, also tested positive later.
The Centre for Health Protection said some 30 people who are still quarantining on the 15th floor of the hotel, and another 20 people who stayed on the same floor but had already completed their quarantine, have to be sent to quarantine centres.
Yuen said he has asked the hotel to retrain staff who are responsible for disinfecting the corridors, and to provide air purifiers for rooms with cycling or jogging machines.
People who are quarantining at hotels should also be reminded to wear their masks properly when they open their room doors, he added.