Hong Kong is set to optimize its heat warning system, factoring in exceptional heat in adjusting the threshold for triggering warnings, following a public decry over the record-breaking heat waves in recent weeks.

The move came as the Labour Department issued another amber warning on Tuesday -- the lowest in its three-tier heat stress at work warning system. No red or black warnings have been issued since the system came to effect.

Commissioner for Labour Sam Hui Chark-shum said the department is working with the Hong Kong Observatory to enhance the warning system to make it more responsive to temperatures.  

He explained that the recently revamped warning system isn’t built solely on the Hong Kong Heat Index which is based on the King’s Park Meteorological Station. The overhaul in April had factored in various meteorological stations’ data in issuing temperature warnings in a relaxed manner.

Higher warnings are expected to be issued under the new system, Hui said, adding that the authorities are perplexed when higher warnings haven’t been triggered despite record-breaking temperatures.

Hong Kong recorded its hottest day on Sunday since records began in 1884, with temperatures hitting 36.9 degrees Celsius in certain districts. The mercury even hit 39.8 degrees in Sheung Shui in the northern New Territories – the highest reading registered since automatic weather stations were introduced in the 1980s.

The heat stress at work warning -- formulated by the Labour Department and based on the Hong Kong Heat Index -- serves as an indicator for employers and employees when working outdoors or indoors without air-conditioning systems.

Hui said that regarding heat-stress conditions in the workplace, the authorities do inspect some 20,000 sites annually and issue about 1,000 warnings or statutory improvement notices due to insufficient heat-relief measures. He believes there’s still much room for improvement.

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Addressing the public outcry, Hui explained that the current heat stress model is based on the internationally-recognized Wet-Bulb Globe Temperature index which was then locally adjusted by the Observatory and the Faculty of Medicine of The Chinese University of Hong Kong.

Since Hong Kong has a subtropical Lingnan climate, the human body’s heat stress often stems from high humidity and poor ventilation. Therefore, the current model has raised the proportion of humid heat factors, such as humidity and wind speed, from the internationally-common 70 percent to 80 percent.

Hui admitted that under the current model, the weight of thermal radiation from direct sunlight is relatively lowered. When extremely sweltering weather occurs, the current index may not immediately reflect the actual risks faced by workers exposed to strong outdoor sunlight.

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Therefore, one factor to be reviewed is enhancing the index’s sensitivity to outdoor, sun-exposed and high-temperature work environments. The review is expected to be completed before the end of this year’s summer holidays.

Contact the writer at atlasshao@chinadailyhk.com