Hong Kong authorities have revised the Heat Stress at Work Warning system in an effort to better protect outdoor workers from extreme heat, but concerns remain over whether it provides sufficient safeguards, industry representatives and lawmakers said.

The Labour Department updated the warning mechanism on Thursday in response to criticism following record-breaking temperatures earlier this month, when the existing system failed to trigger the highest levels of alert. The revised system uses the Hong Kong Heat Index—a measure that factors in temperature, humidity, wind speed, and solar radiation—to issue tiered heat stress warnings: amber, red, and black. These alerts are designed to guide employers and employees on appropriate precautions for working in high-heat conditions.

Under the current framework, when an amber warning is issued, employers are advised to provide additional rest breaks to workers. The red alert calls for suspension of duties for those engaged in “very heavy” physical work, while the black warning requires work suspension for “heavy” workloads. However, workers performing “light” or “moderate” tasks are not required to stop work even under the highest black warning, though they are recommended to receive extended rest periods.

Lam Chun-sing, chairman of the Federation of Hong Kong and Kowloon Labour Unions, said the thresholds for the black warning remain too high and do not adequately protect outdoor workers with moderate workloads, such as street cleaners. He pointed out that under typhoon warnings, non-essential outdoor work is halted regardless of workload intensity, a level of precaution not mirrored in the heat stress alerts.

“Allowing work suspension to cover workers of moderate workload under extreme heat conditions will offer them much better protection,” Lam said, urging the authorities to review whether the revised system guarantees sufficiently frequent rest periods. He also suggested that real-world worker experiences should guide further improvements rather than relying solely on scientific measures.

On the construction front, Lawrence Ng San-wa, permanent honorary chairman of the Hong Kong Construction Sub-Contractors Association, supported the adoption of a site-wide suspension policy during black heat stress warnings, regardless of workload differences. He added that the potential loss of up to 10 working days annually under such suspensions would pose limited disruption to most projects but urged the government to consider legislating these requirements. At present, the heat stress warnings remain advisory, which can complicate decisions for subcontractors facing tight project deadlines and possible financial penalties for delays.

The revised system also introduces changes to its triggering criteria. The black alert now activates if the Heat Index reaches 34 at the King’s Park Meteorological Station or two other monitoring sites. If temperatures rise to 35 degrees Celsius or more across 40% of weather stations, lower thresholds of 30 and 32 will prompt red and black warnings respectively. Previously, those thresholds required index readings at King’s Park or four of the nine other stations.

Despite these changes, the authorities did not adopt the internationally recognised wet-bulb globe temperature (WBGT) index, which accounts more heavily for direct sunlight exposure. Lam urged a reconsideration of the WBGT measure, emphasizing that direct sun dramatically amplifies heat stress experienced by outdoor workers.

The Labour Department estimated that the red warning could be issued four to seven days per year, while the black alert might appear one or two days annually under the updated system. Observations during the record heatwave on August 9 indicated only one of ten monitoring stations recorded a heat index above 32, explaining why previous warnings did not escalate.

As Hong Kong faces increasingly frequent extreme heat events, calls persist from labour representatives and industry bodies for stronger legal protections and clearer guidelines to better safeguard outdoor workers from the health risks of high heat stress.