Modelling the Excess Mortality Associated with Heat Waves in Hong Kong: 2014-2023

This study quantifies the excess mortality in Hong Kong from 2014 to 2023, estimating that extreme heat caused between 1,455 and 3,238 preventable deaths—disproportionately affecting males and the elderly—and highlighting a mortality burden comparable to diabetes that necessitates urgent, targeted public health interventions.

The Gist

Imagine Hong Kong as a bustling, crowded city where the buildings are stacked so high they create a giant "concrete canyon." In recent years, the sun has been beating down harder and longer than ever before. This study is like a detective report that asks a simple but scary question: "How many extra people died because the city got too hot?"

Here is the story of what the researchers found, broken down into everyday concepts:

1. The "Invisible Killer" vs. The "Obvious Killer"

Usually, when we think of causes of death, we think of things like heart disease, cancer, or diabetes. These are like the "heavy hitters" on a leaderboard.

This study found that extreme heat is now a heavy hitter too. In fact, if we counted heat waves as a single cause of death (like we do for diabetes), it would rank in the top 10 causes of death in Hong Kong.

• The Analogy: Imagine a marathon. For years, diabetes has been running in the top 10 spots. This study says, "Hey, look over there! Extreme heat is sprinting right past diabetes and taking its place in the top 10." In some recent years, heat was actually killing more people than diabetes.

2. How They Counted the "Extra" Deaths

You might ask, "If someone dies of a heart attack on a hot day, how do we know the heat caused it?"

• The Old Way: Doctors look at the death certificate. If it doesn't say "Heatstroke," they don't count it. This is like only counting people who fell off a ladder, but ignoring the people who slipped because the floor was wet.
• The New Way (This Study): The researchers played a "What If" game.
  • Scenario A: What actually happened? (We count all the deaths).
  • Scenario B: What would have happened if the weather was perfect and cool? (We calculate the "normal" number of deaths).
  • The Result: They subtracted Scenario B from Scenario A. The difference? Those are the "Excess Deaths." These are the people who likely would have lived a few more days or years if the heat wave hadn't happened.

3. The "Heat Wave" Rules

Since there is no single rule for what counts as a "heat wave," the researchers tried four different rulebooks to be sure they didn't miss anything:

1. The "Super Hot Day" Rule: Any day the average temperature is above 30.6°C.
2. The "Scorching Afternoon" Rule: Days where the high temperature hits 33°C.
3. The "Sleepless Night" Rule: Nights where it doesn't cool down below 28°C (this is crucial because your body needs to cool down at night to recover).
4. The "Double Trouble" Rule: When both the day and night are dangerously hot.

Depending on which rule they used, the number of "extra" deaths varied, but the message was the same: It's a lot.

4. The Numbers: A Tragic Toll

Over the last 10 years (2014–2023), the study estimates that between 1,455 and 3,238 people died prematurely because of the heat.

• The Analogy: Imagine a large stadium full of people. The number of excess deaths is roughly equivalent to filling that stadium completely, or even filling it twice, with people who lost their lives to the heat.

5. Who Got Hit the Hardest?

The heat didn't treat everyone equally. It acted like a bully that targets the weakest.

• The Elderly: People over 65 (and especially those over 85) were the most vulnerable. Their bodies are like old cars; the engine (thermoregulation) doesn't cool down as well as it used to.
• Men: Men were hit harder than women, dying at nearly double the rate in some scenarios.
• Why? As we age, our bodies lose the ability to sweat and cool down efficiently. Plus, many older people take medications for other health issues that make it even harder to handle the heat.

6. Why Is This Happening in Hong Kong?

The city has a "perfect storm" of problems:

• The Concrete Trap: Tall buildings and narrow streets trap heat, creating an "Urban Heat Island." It's like living inside a microwave.
• The "Save Money" Trap: Many elderly people are afraid to turn on their air conditioners to save on electricity bills. They suffer in silence to save a few dollars.
• Bad Design: Many older apartments have small windows and poor ventilation, so the heat gets stuck inside.

7. The Solution: A "Heat Action Plan"

The study concludes that we can't just shrug this off. We need a Heat Action Plan (HAP).

• What is an HAP? Think of it like a fire drill, but for heat.
  • Early Warnings: Shouting "Fire!" before the building burns down. (e.g., "Tomorrow will be dangerously hot, stay inside!").
  • Cooling Centers: Open public places with air conditioning for those who can't cool their homes.
  • Checking on Neighbors: Doctors and social workers visiting the elderly to make sure they are drinking water and using their AC.

The Bottom Line

This study is a wake-up call. Extreme heat isn't just an uncomfortable nuisance; it is a silent, deadly epidemic that is killing thousands of people in Hong Kong. Just as we have plans for typhoons and rain, we urgently need a dedicated, city-wide plan to protect our most vulnerable neighbors from the sun. If we don't act, the "top 10 list" of causes of death will only get hotter.

Technical Summary

1. Problem Statement

Climate change has intensified the frequency and severity of heatwaves, posing a significant public health threat, particularly in high-density subtropical urban environments like Hong Kong. While previous studies have established the relative risk (RR) of mortality associated with heat, there is a lack of comprehensive data quantifying the absolute burden of excess deaths attributable to heatwaves in Hong Kong.

• The Gap: Existing literature often relies on relative risks or attributes deaths solely to heatstroke, which may underestimate the true burden as heat exacerbates pre-existing chronic conditions.
• The Need: Policymakers require absolute numbers (excess deaths) and age-standardized rates to effectively communicate the severity of the threat and compare it against other leading causes of death (e.g., diabetes) to justify urgent public health interventions.

2. Methodology

The study employed a Monte Carlo simulation approach to estimate excess heat-related deaths (EHD) over a 10-year period (2014–2023).

Data Sources:

• Mortality & Population: Daily age- and sex-specific mortality rates and population data from the Hong Kong Life Tables and the Census and Statistics Department.
• Meteorological Data: Daily mean, maximum, and minimum temperatures from the Hong Kong Observatory (HKO).
• Relative Risks (RR): Localized RR estimates derived from previous studies (Liu et al., 2020; Wang et al., 2019).

Heat Wave Definitions:
Due to the absence of a universal standard, the authors applied four distinct definitions to capture different heat stress scenarios:

1. HWD_Tavg: 20-day period starting when daily mean temperature exceeds the 99th percentile (30.60°C).
2. HWD_Tmax: Consecutive days where daily maximum temperature > 33°C.
3. HWD_Tmin: Consecutive days where daily minimum temperature > 28°C.
4. HWD_Tcombined: Simultaneous occurrence of hot days and hot nights (based on Wang et al., 2019).

Statistical Modeling:

• Counterfactual Approach: Calculated EHD as the difference between observed deaths and expected deaths under "normal" (non-heatwave) conditions.
• RR Adjustment: Adjusted population-level RRs to fit specific age-sex strata using a log-transformation formula:
  $$\ln(RR_{age,sex}) = \ln(RR_{age}) + \ln(RR_{sex}) - \ln(RR_{all})$$
• Monte Carlo Simulation: Generated 1,000 random estimates of RRs assuming a log-normal distribution to calculate 95% Confidence Intervals (CIs).
• Equation for Excess Deaths:
  $$EHD_{ijt} = (RR_{ijt} - 1) \times DD_{ij} \times L_t$$
  Where $DD_{ij}$ is the expected daily deaths for age/sex group $ij$, and $L_t$ is the duration of the heatwave phase.
• Standardization: Results were reported as age-standardized excess death rates per 100,000 population (using WHO 2001 standard population) without discounting or age weighting.

3. Key Contributions

• First Comprehensive Estimation: This is the first study to quantify the absolute number of excess deaths attributable to heatwaves in Hong Kong, covering both direct and indirect impacts.
• Comparative Burden Analysis: The study uniquely benchmarks heat-related mortality against chronic diseases (specifically diabetes), providing a tangible metric for policymakers.
• Multi-Definition Sensitivity: By utilizing four different heatwave definitions, the study provides a robust range of estimates, acknowledging the complexity of defining heat stress in a humid, subtropical climate.
• Demographic Granularity: Detailed breakdown of vulnerability by age and sex, highlighting specific high-risk cohorts.

4. Key Results

Total Excess Mortality (2014–2023):

• Estimated 1,455 to 3,238 excess deaths occurred due to heat exposure, depending on the definition used.
• The HWD_Tcombined definition yielded the highest estimate (>3,200 deaths), while HWD_Tavg yielded the lowest (~1,455 deaths).
• The definition based on combined metrics (hot days + hot nights) resulted in more than double the estimated deaths compared to mean temperature alone.

Annual Rates (2023 Data):

• Annualized excess death rates ranged from 2.95 to 5.09 per 100,000 population.
• Comparison to Diabetes: Under the HWD_Tmax and HWD_Tcombined definitions, heatwave mortality rates exceeded those of diabetes in nearly half the study period (2020–2023). In 2023, the rate under the combined definition was nearly twice that of diabetes.
• Ranking: Heatwaves are now projected to be among the top 10 causes of death in Hong Kong.

Demographic Vulnerability:

• Sex: Males experienced nearly twice the number of excess deaths compared to females (across most metrics).
• Age: Individuals aged 65 and older were disproportionately affected. Those over 85 showed the highest vulnerability, with death counts more than double those of other age groups.

5. Significance and Policy Implications

• Public Health Priority: The findings demonstrate that extreme heat is a leading cause of mortality, comparable to major chronic diseases, necessitating its inclusion in national health statistics and priority setting.
• Urgency for Heat Action Plans (HAPs): The study highlights the critical lack of a tailored HAP in Hong Kong. It cites evidence from Ahmedabad, France, and Montreal where HAPs significantly reduced mortality.
• Targeted Interventions:
  • Elderly Care: Interventions must prioritize the elderly (65+), addressing barriers such as reluctance to use air conditioning due to cost and poor building ventilation.
  • Clinical Role: Clinicians (pharmacists, nurses) should proactively counsel high-risk patients regarding medication interactions and heat risks.
  • Social Support: Social agencies need to engage in culturally appropriate home visits to identify and mitigate risks.
• Future Outlook: With climate change projected to increase hot days and Hong Kong's population aging, the mortality burden is expected to rise without immediate, data-driven adaptation strategies.

Conclusion:
The study provides irrefutable evidence that heatwaves are a major, preventable cause of death in Hong Kong. It calls for the immediate development and implementation of territory-wide Heat Action Plans that integrate health, social, and urban planning sectors to protect the most vulnerable populations.