Hong Kong recorded its hottest day since 1884 on Aug 9 with temperatures reaching as high as 36.9 C. For outdoor workers, construction crews, street cleaners, delivery riders, security guards and maintenance teams alike, extreme heat is a grueling endurance test. Yet on that day, the Labour Department’s Heat Stress at Work Warning system, based on the Hong Kong Heat Index (HKHI), remained at amber rather than moving to red or black. That gap, between what a central alert system records and what a worker may actually experience on an exposed worksite, is what Hong Kong’s heat policy must now address.

Many of us have family members, friends or neighbors who work outdoors. But outdoor heat risk is not only a personal concern; it is also a citywide resilience issue. Hong Kong needs practical action; not necessarily new laws, but better measurements, smarter site decisions, fairer contracts, and sector-by-sector collaboration.

We grew up judging hot weather by air temperature. Yet a simple thermometer cannot show how humidity, weak wind, direct sunlight, exertion, clothing, shade and rest affect the body’s ability to cool itself. That is why the Wet Bulb Globe Temperature (WBGT) is widely used internationally to manage occupational heat stress. By accounting for humidity, wind and radiant heat, the WBGT is more relevant than air temperature for outdoor work.

The locally developed HKHI has clear value as a public heat-health indicator, reflecting broader population risks, including hospital-admission patterns. But an index designed for general public-health risk is not necessarily fit-for-purpose as the sole trigger for protecting outdoor workers exposed to direct sunlight, heavy exertion and site-specific microclimates. This distinction matters.

A rare joint public intervention by three former directors of the Hong Kong Observatory — Lam Chiu-ying, Shun Chi-ming and Lee Boon-ying — questioned the current use of the HKHI for occupational heat warnings and urged a shift toward the WBGT as the relevant standard for assessing outdoor heat stress.

Singapore offers a useful reference point not only because it uses WBGT-based action bands, but because it requires localized measurement. Under Singapore’s Ministry of Manpower guidance on heat-stress measures for outdoor work, employers must ensure that the WBGT is monitored hourly during periods when outdoor work is being carried out. Larger construction sites, shipyards and process-industry workplaces must also keep a WBGT meter on site for localized measurement. Hong Kong need not copy Singapore outright. But the principle is sound: Heat risk should be assessed using occupationally relevant metrics and, where outdoor work is prolonged or exposed, measured close enough to the worksite to reflect actual conditions. Clear WBGT-based action bands, supported by site-level data, would reduce guesswork and help supervisors, workers and clients manage heat-risk predictably.

A weather-station reading is essential public information, but it cannot fully represent conditions at a particular worksite. The heat felt on an exposed podium, a road-maintenance site or a pavement corridor can differ dramatically because of concrete surfaces, traffic heat, restricted airflow, solar exposure, and the urban heat-island effect. Aug 9 made this problem visible. The fact that the Labour Department’s alert system never moved past amber raised questions about whether the current mechanism fully captures on-the-ground risk.

A single citywide signal cannot by itself protect a workforce scattered across vastly different microclimates. The sensible solution should be to encourage the use of portable, calibrated WBGT meters at the workplace for major projects and high-risk outdoor operations.

By treating extreme heat as a measurable and contractually recognized variable rather than an unforeseeable emergency, Hong Kong can protect workers’ health, safeguard daily wages and give contractors a fairer basis for planning. There is no time to waste

These meters could be integrated into smart site safety systems so site managers receive real-time alerts when local conditions reach higher-risk bands. The Construction Industry Council could support this by recommending suitable devices, common measurement protocols, and regular calibration. The result would be better day-to-day management: rescheduling strenuous tasks, rotating crews, increasing hydration, opening shaded recovery areas, and deferring nonurgent work during peak exposure.

Heat is also a commercial and industrial-relations issue. When a contractor pauses or slows work because conditions are unsafe, who bears the delay, labor cost, and program risk? Without a shared framework, supervisors may feel pressure to keep crews working, workers may fear lost earnings, and contractors may fear liquidated damages.

As the city’s largest infrastructure and development project client, the Hong Kong Special Administrative Region government has significant leverage to lead by example. Through public procurement and contract guidance, bureaus and departments can normalize heat-resilient practices without waiting for new legislation, which typically takes time.

The industry is not starting from scratch. The new form of contract published by the Hong Kong Institute of Surveyors already recognizes heat stress as an event that may qualify for an extension of time when it affects the progress of work. This is significant because it acknowledges extreme heat as a project-management and risk-allocation issue, not merely a welfare concern.

The remaining bottleneck is the trigger. Contractual recognition is only meaningful if the mechanism that activates relief reflects actual site conditions. If entitlement depends on a Labour Department warning derived from a centralized HKHI system, relief may be unavailable, even when workers on exposed sites face genuinely hazardous conditions. The fastest practical step is therefore for the Labour Department to review how the Heat Stress at Work Warning is calculated and triggered, with greater reliance on the WBGT and appropriately standardized site-level measurement.

Construction is highly visible, but it is not the only sector under heat pressure. Street cleaners, delivery couriers, gardeners, security guards, utility crews and property-management employees face prolonged exposure, often with limited control over pace, route or access to shade. Because each sector operates differently, a blanket policy would be poorly suited to Hong Kong’s diverse economy. A workable framework must therefore be sector-specific.

The Labour Department should lead structured pilot programs with municipal departments, property-management alliances, logistics platforms, employers, labor unions and occupational-health specialists. These pilot programs could test practical measures such as accessible water points, shaded rest areas, route adjustments, flexible delivery windows, buddy systems, and protection for workers who report heat symptoms.

Extreme weather is likely to affect Hong Kong with greater frequency in the years ahead. The Labour Department is right to conduct an emergency review, but it should be a starting point, not a final destination. The goal is operational continuity with human protection at its center. By treating extreme heat as a measurable and contractually recognized variable rather than an unforeseeable emergency, Hong Kong can protect workers’ health, safeguard daily wages and give contractors a fairer basis for planning. There is no time to waste.

The author is a senior lecturer at the Hang Seng University of Hong Kong and co-chair of the Advocacy and Policy Research Committee, the Hong Kong Institute of Human Resources Management.

The views do not necessarily reflect those of China Daily.