William Yu is giving a tour of a Hong Kong apartment where each of the three bedrooms has been divided into a separate flat - part of a lab he has set up to show how many families live in small, crowded and hot spaces with no air conditioning or fresh air.
"Even if there is a window, there is no ventilation and some flats are very scary," said Yu, of the homes on Hong Kong's Chun Tin street.
Outside the window, black smoke and pounding noises rise from the Hop Lee metal and scrap paper shop on the dead-end street of dilapidated tenement buildings in the Hung Hom district.
The World Green Organization Yu founded has set up the apartments to show how some vulnerable families live in one of the most expensive and densely-packed cities on earth - and how they might cope with global warming.
Average summer temperatures in this city of 7.4 million people have risen swiftly over the past century, according to a study by researchers including Emily Chan, who directs the Centre for Global Health at The Chinese University of Hong Kong.
Temperatures inside these illegally-divided flats are 6 to 7 degrees Celsuis hotter than outside, and research shows heat in some of them rising from a former peak of 36C to closer to 40C in the height of summer, according to Yu.
"In the past we used to say, 'Don't stay outdoors for a long time exposed to intensive sunlight, otherwise you will get heatstroke,'" said Yu.
"Now we have found the opposite in the subdivided flats: Don't stay indoors for a long time as that's bad for your health."
Yu instead tells families to go to a shopping mall with strong air-conditioning and not to keep windows open because of the air pollution outside.
This is but one example of how warming is hitting Hong Kong's vulnerable hardest in this steamy city that last week hosted an international forum on heat risks from climate change.
Global experts on health, weather and climate flocked to the former British colony to try to find ways to manage what Joy Shumake-Guillemot, who leads the joint office of the World Health Organization (WHO) and World Meteorological Organization (WMO) in Switzerland, called "the silent emergency that heat poses to health".
As part of the effort, they launched the Global Heat Health Information Network (GHHIN), made up of experts from more than 30 countries, all keen to work out how to deal with growing heat extremes as the world continues to break heatwave records.
While typhoons and fires grab headlines, heatwaves kill more people than any other weather-related disaster, though the deaths are rarely attributed to heat, the GHHIN said.
With more people moving to cities globally, the risks are rising as well, it said.
"Heat affects everyone," said Shumake-Guillemot. "From healthy athletes and outdoor workers, to the elderly and people with pre-existing medical conditions … many people simply don't realise how serious dehydration and heat stress can be."
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The subtropical city of Hong Kong has a unique set of heat challenges and opportunities that make it a useful case study.
Its dense urban living, combined with a humid climate, means growing heat threats, with average high temperatures in the summer hitting 31 or 32 degrees Celsuis.
The concrete, skyscrapers and lack of greenery in inner city areas lead to what is coined the "urban heat island" effect, which means the city gets - and stays - hotter than surrounding more rural areas.
That particularly affects the city's most vulnerable living in tiny, subdivided apartments, Yu said.
Chan said hospital records showed that an increase in high temperature from 29C to 30C was linked with a 4 percent jump in deaths in urban areas with particular heat vulnerability,
The rate of admissions to hospitals also rose 4.5 percent for every jump of 1C above 29C, a study by her team showed.
There is "not a single health outcome that is not affected" by soaring heat, Chan said.
Faced with growing heat threats, policy makers, urban planners, academics and non-governmental organisations are working together to tackle the problem, Chan said.
The Hong Kong Observatory, for instance, has created a heat index that measures temperature, humidity, wind speed and and other measures and warns the public when key thresholds are passed.
It is also working with a senior citizens' organisation to see how weather data could help growing numbers of elderly people - who are particularly at risk - cope with temperature extremes.
The observatory also works with urban planners to help design effective layouts for the city and plan how buildings fit, taking into account micro-climates in different areas.
It has plans to install sensors at street level to see how hot and steamy life there gets, as well as use crowdsourcing to create real-time weather forecasts on heat impacts and risks to people.
All of this makes Hong Kong a heat pioneer in Asia, Chan said - and heat experts hope to learn from its experiences and those of other cities as the world becomes increasingly urbanised.
"We need to take steps to educate the public on healthy behaviours they should take up, and train medical professionals to be more attentive to heat stress symptoms in patients," said Shumake-Guillemot.
Helping cities plan ahead to accommodate extreme temperatures and become more liveable in the face of them, as well as better coordinating emergency preparedness to manage heatwaves, will be key, she said.
"The likelihood of falling sick from heat has increased for everyone," Shmake-Guillemot said. Heat experts "are taking this new reality seriously, and are working to find solutions to keep people healthy in a warming world."