“You can actually save lives if you do better heat health action planning. These are preventable deaths,” he said.

The researchers calculated the number of excess deaths using a methodology model from a past study that looked at possible links between fatalities and different temperatures, with the team finding about 93 extra deaths had occurred during each heatwave.

The latest study defines a heatwave as a period of time where the daily mean temperature reaches 30.6 degrees Celsius (87.1 Fahrenheit) or higher. The team would then check for any excess deaths over the subsequent 20-day period.

The team also specified that the first day of a heatwave and the last day of a studied period should be at least 20 days apart.

Researchers used publicly available data from the Observatory to calculate that the city recorded one to two heatwaves annually between 2014 and 2022, followed by three in 2023, bringing the total to 18.

They also found that a higher number of excess deaths were recorded among people aged 60 to 84 compared with other age groups.

Bishai said that while the team had not investigated whether the deaths covered under the projection were directly related to high temperatures, he noted that people’s bodies generally experienced more stress during such weather.

“The heat makes you need to sweat, and in order to sweat, you have to generate a lot of blood flow through your body,” he said. “So for elderly people’s bodies, their heart has to pump harder.”

The physical stress was also worse for people using medications that could lower their blood pressure, since the body needed a higher blood pressure to pump blood to the sweat glands.

“It’s extremely stressful for somebody with a pre-existing medical condition to be in the heat,” Bishai said.

The academic, who is also a medical doctor, said some psychiatric medications, such as antidepressants, could also increase a person’s heat stress since the drugs could affect their ability to sweat.

The World Health Organization has cited heat stress as the “leading cause of weather-related deaths”.

The research team also noted that Hong Kong had experienced more extremely hot days annually in recent years, rising from six in 2014 to 29 in 2022.

The study defined such days as having a mean temperature of 30.6 degrees or higher.

Observatory data showed the number of “very hot weather” warnings issued each year had increased from 31 to 42 between 2014 and last year.

Over the same period, two to five people died annually due to the condition, the data showed. Provisional figures put the number of related fatalities at four for last year.

Bishai called on government departments to work together to devise an action plan to reduce heat-based health hazards.

He said the Labour Department had already introduced a hot weather warning system for workers, while authorities had yet to formulate a plan for elderly residents.

He said that such an action plan should look into the overall health impact of extreme heat and also cover elderly residents who had no close contacts to turn to for help.

“They are epidemiologically known to be at higher risk during heatwaves. Isolation is a risk factor,” the academic said.

He said the team had reviewed similar plans used overseas and found the best ones included measures to convince elderly people to use air conditioners, good weather warning systems and robust medical infrastructure that could bring in extra staff amid heatwaves.

Local authorities could use the care team initiative unveiled by the government in 2022 to check up on older residents during heat spells, a measure adopted in places such as Paris, he said.

Hong Kong’s Centre for Health Protection currently issues public reminders on hot days, which warn the public to drink more water, and wear loose and light-coloured clothing, among other tips, during periods of high heat.

Ivan Lin Wai-kiu, a social worker with the Society for Community Organisation, said underprivileged elderly people were also more vulnerable to hot weather.

“Low-income elderly are sensitive to electricity bills. They might just turn their air conditioner on for a short while before switching it off or only using it when the weather is very hot,” he said.

Lin said those living in very old buildings or subdivided flats generally dealt with poorer ventilation.

“There might not be windows at their subdivided flats, or they cannot open their windows as rats will climb in, … broken air conditioners might also not be fixed by flat owners,” he said.

The social worker said the government should look into the impact of climate change on underprivileged residents.

Health-related issues are currently not included in the government-led Climate Action Plan 2050, which focuses on areas such as energy saving, as well as green buildings and transport.

A government spokesman said the Centre for Health Protection had been working with the Observatory to strengthen public education about the adverse health effects of extreme heat and how to prevent heatstroke.

“The Department of Health will continue to monitor the health impact of extreme heat in Hong Kong and implement measures to prevent and mitigate adverse health effects for the public, particularly vulnerable groups,” he said.