A group representing construction companies on Tuesday said they're concerned that a new heatstroke warning system aimed at protecting outdoor workers may be difficult to implement.

The Labour Department announced on Monday that its "Heat Stress at Work" warning will come with amber, red or black alerts, with those working outside advised to rest or even suspend work if the warnings are triggered.

Under a red or black alert, for example, bar benders and porters – classified as very heavy workers – will be advised to stop work completely.

However, the executive director of the Hong Kong Construction Association, Godfrey Leung, told RTHK that the new system will create confusion as different workers will be subject to different arrangements.

"There may be a hundred or so workers at a small construction site, maybe a thousand or two at a big one," he said.

Leung pointed out that officials should simply suspend work for everyone if the mercury reaches a certain level.

"There are so many different types of workers in there of varying workloads. It's difficult for a foreman or worker to know how they are classified under the alert system.”

Mickey Yan from the Hong Kong Property Services Alliance also expressed a similar concern, saying cleaners work in different places and it may be hard to notify them of the warnings.

Unionist lawmaker Lam Chun-sing, meanwhile, said while the new guidelines are generally an improvement from existing risk assessment rules, he's concerned that some workers may be left out.

"For example, for workers at the airport, they don't carry out heavy work but they stay in the apron. Apart from the sunlight, the emission from the aircraft will [raise] the temperature," he told RTHK.

A deputy commissioner for labour, Vincent Fung, said he hopes the new guidelines can serve as references for different industries to take appropriate, preventative measures against heatstroke.