Researchers from the Chu Hai College of Higher Education on Wednesday said commuters are likely to fall asleep on buses, especially on the upper deck, due to the high carbon dioxide concentration inside the vehicle.

They carried out testing in March and April, and found that the carbon dioxide concentration inside buses was 60 percent higher than that of MTR trains.

Researchers said the concentration levels inside buses almost reached 3,000 parts per million (ppm) – three times higher than the ideal level.

Professor Louis Lam said this situation may make commuters drowsy, have a headache or even a fast heart rate.

"All the doors on the MTR trains are opened when they reach a station every two minutes. Air from the platform enters the train, lowering the carbon dioxide concentration in there," he said.

"But there isn't a door on the upper deck of a bus. Carbon dioxide accumulates there, leading to a higher concentration."

He said bus operators should look into ways to improve ventilation inside the vehicles.