The heat inside: Why urban heat is a hidden health crisis — and how cities can fight back

In Singapore, the air never truly cools. Even at night, concrete walls and asphalt radiate stored heat, keeping temperatures high long after sunset.

Assoc. Prof. Jason Lee, Heat Resilience & Performance Centre at the National University of Singapore's Yong Loo Lin School of Medicine, and chairs the Global Heat Health Information Network's Southeast Asia Hub.

For residents, this is more than an inconvenience. It's a health risk hiding in plain sight.

"People think they're used to heat," says Associate Professor Jason Lee, Director of the Heat Resilience & Performance Centre (HRPC) at the National University of Singapore's Yong Loo Lin School of Medicine. "But chronic, year-round heat has cumulative impacts on health, productivity, and well-being."

Lee leads a multidisciplinary team studying how the human body responds to extreme heat, and how science, design, and digital technology can help cities protect people from it. His work places him at the intersection of medicine, climate science, and urban policy, and offers a sobering view of what lies ahead if cities fail to adapt.

The underestimated threat

Urban heat isn't new, but its intensity is growing. In tropical cities such as Singapore, the urban heat island effect can raise temperatures by 4° to 7°C above the surrounding rural areas. Concrete, glass, and vehicles trap heat by day and release it at night, creating a cycle of relentless warmth.

"Urban heat affects every aspect of life, including how we work, sleep, move, think, and experience our city's livability," Lee says. "It leads to excess mortality, increases hospital admissions, and aggravates cardiovascular, kidney, and metabolic diseases." Less visible effects include mental fatigue, reduced physical activity, lower appetite, and poor sleep quality.

Those impacts are not distributed equally. Elderly residents, outdoor workers, children, and people in low-income housing suffer the most because they have limited access to air conditioning or green space. And because tropical cities face chronic heat, rather than occasional heat waves, the danger is cumulative: a daily toll that erodes recovery and resilience.

"Heat is still treated as a seasonal event," Lee notes, "when in reality, it's a continuous public-health stressor for tropical climates." From research to protection

At HRPC, Lee's team works to understand how heat exposure affects the body, and how the group's findings can be translated into policy. Their research has already informed Singapore's new national guidelines for outdoor labor and sports, introduced by the Ministries of Manpower and of Culture, Community and Youth.

"We've seen research move directly into national practice," Lee says. "Simple, low-cost, well-timed measures such as shade provision, hydration, and structured rest cycles can measurably reduce heat risk and boost productivity when consistently applied."

That translation from data to decision is critical. Singapore's "whole-of-government" approach now coordinates public health, sustainability, and environmental agencies around shared goals of protecting people, maintaining productivity, and planning for a hotter future.

Why cities still get it wrong

Despite mounting evidence, Lee says many city authorities continue to underestimate heat.

"Because heat is frequent, there's a belief that people are 'used to it,'" he explains. "It doesn't get the same preparedness as floods or typhoons."

Another misconception is that only heatwaves are dangerous. In fact, the International Labor Organization reports that 80 percent of occupational heat injuries occur outside heatwaves.

Treating heat as an isolated emergency misses the point. "Urban heat is a systemic challenge," Lee says. "It requires impact-based warning systems, climate-sensitive design, and workplace protections. There is no single technological fix."

The lack of recorded heat-related deaths also doesn't mean heat is not an issue. For many regions, heat also means populations are not living up to their true health potential. Heat affects the whole lifespan, from fertility, maternal health, adverse birth outcomes, children's learning, indoor and outdoor workers, persons with disabilities, and the elderly.

The paradox of adaptation

Left unsupported, citizens inevitably adapt on their own. They stay indoors, use more air conditioning, and cut back on physical activity. These responses are rational, but they can become maladaptive over time.

"Over-reliance on air conditioning encourages sedentary lifestyles, poorer sleep, and even vitamin D deficiency," Lee explains. "We also see mental-health effects from prolonged indoor time and isolation."

Inequity compounds the problem. "Not everyone can afford to adapt. Outdoor workers paid by the piece, or families without access to affordable cooling, face a choice between losing income they need, and risking exposure to harmful or even fatal levels of heat."

The solution, Lee argues, is to reduce dependence on maladaptive behavior through better design and policy: shade, ventilation, and early-warning systems, as well as green and blue infrastructure, which refers to the strategic use of parks, trees, and water bodies to provide shade and promote cooling evaporation.

"Done well, these measures protect both health and livelihoods," Lee says.

Smart cooling strategies

Technology can become a key enabler to support how cities understand and manage heat.

Digital twins such as Cooling Singapore's Digital Urban Climate Twin allow planners to test how tree-canopy coverage, reflective materials, or district-cooling systems affect temperatures before construction begins.

Sensor networks track Singapore's wet-bulb globe temperature (WBGT), a measure of environmental heat stress used to assess the risk of engaging in physical activity, especially for athletes and workers.

The sensors feed the data into Singapore's myENV, a mobile app for information about the environment, water and food safety.

Wearable devices can monitor core and skin temperature or heart rate, with the potential to offer personalized alerts for at-risk populations such as outdoor workers or the elderly.

Smart building systems and cool materials are lowering surface and indoor temperatures while reducing energy use.

"Technology helps us link climate, health, and urban systems in practical ways," says Lee. "AI can also help identify heat-stress biomarkers, guide diagnosis of new heat-related cases, and inform heat-governance design."

But he cautions against expecting tech alone to solve the problem. "These tools don't 'win the battle' by themselves," he says. "They have to be embedded in comprehensive, equity-centered policy."

Lessons from the leaders

Around the world, a handful of cities are showing what that kind of governance looks like. Ahmedabad, India pioneered South Asia's first Heat Action Plan in 2014, combining early-warning systems with community outreach. Phoenix (USA) became the first city to establish a publicly funded Office of Heat Response & Mitigation in 2021.

Singapore, meanwhile, has integrated heat resilience into its national adaptation strategy. The Heat Stress Advisory, launched in 2023, guides residents engaged in prolonged outdoor activity, backed by a network of WBGT sensors and public alerts. A coordinated inter-agency taskforce now enables a national heatwave response plan, with targeted measures for vulnerable populations. Urban greening, reflective materials, and shading are being expanded to lower surface temperatures over the long term.

"The pattern among successful cities is clear," Lee says. "Dedicated leadership, early-warning systems linked to action, inclusive policies, and continuous learning through data and research."

As regional chair of the Global Heat Health Information Network's Southeast Asia Hub, Lee is helping share those lessons across ASEAN — from Bangkok to Manila — to strengthen regional capacity for heat governance.

Warning about warming — and about inaction

After years of research, Lee remains both optimistic and cautious.

"Momentum is accelerating," he says. "Record-breaking heat in 2024 and 2025 has raised awareness. The UN Secretary-General's Call to Action on Extreme Heat has put the issue on the global agenda. Private finance and philanthropy are also bringing innovation and resources to the table. The key is pragmatic progress: don't let perfect be the enemy of good."

Yet awareness alone is not enough. "Climate change continues to intensify the warming trend," Lee warns. "Chronic heat remains under-recognized compared to heatwaves, leading to policy and investment gaps. The risk is awareness without action — plans without enforcement, ambition without implementation."

For Lee, the path forward is clear. "If we treat heat as a health issue, not just a weather issue," he says, "we can design cities that help people thrive, not merely endure, in a warmer world."

For further information on Cooling Singapore's Digital Urban Climate Twin, see Link 1

To learn more about the UN Secretary-General's Call to Action on Extreme Heat, see Link 2