Heat Risk Assessment at Union Level Using Multiplicative Hazard-Exposure-Vulnerability Framework Integrating Remote Sensing and Socio-Economic Data

Heat is not just a feeling of discomfort; in many parts of the world, it is a growing physical threat that interacts with the places we live and the resources we have. When scientists talk about heat risk, they are not just measuring how hot the air gets. They are looking at a combination of three things: the intensity of the heat itself, how many people and buildings are exposed to it, and how vulnerable those people are to suffering because of it. A person living in a dense city with few trees faces a different kind of danger than someone in a rural village with poor housing, even if the thermometer reads the same. Understanding where these dangers overlap is crucial for saving lives, yet in many developing regions, detailed maps of this risk have been missing, especially outside of the largest cities.

A team of researchers from Khulna University in Bangladesh has filled this gap for the Rajshahi Division, a vast and diverse region in the northwest of the country. By combining satellite images of the earth's surface with detailed census data, they created the first-ever heat risk map for every single administrative union in the area, covering 654 distinct communities. Their work reveals a striking pattern: the most dangerous heat is concentrated in a few dense urban cores, while the people most likely to suffer from the effects of heat—due to poverty, poor housing, or lack of healthcare—are often living in different, quieter areas. This mismatch means that a one-size-fits-all approach to cooling will fail; instead, solutions must be tailored to the specific needs of each neighborhood.

The researchers began by looking at the heat itself. Using satellite data that measures the temperature of the ground, they identified areas where the surface was significantly hotter than the surrounding average. This is a common phenomenon in cities, where concrete and asphalt soak up the sun and release it slowly, creating an "urban heat island" effect. In Rajshahi, this thermal hazard is most intense in the central parts of Rajshahi City and in the town of Pabna. However, knowing where the heat is does not tell the whole story. The team then layered on data about exposure, counting how many people live in these hot zones, how many buildings are packed together, and how far residents are from the nearest hospital. They also looked at the built environment, measuring how much of the ground is covered by buildings versus how much is covered by vegetation or water.

The final piece of the puzzle was vulnerability. The researchers examined census data to understand the social fabric of each community. They looked at factors such as the average size of a household, the percentage of people living in homes with non-permanent floors, access to safe drinking water, and the rate of literacy. These details help determine how well a community can cope when a heatwave hits. A neighborhood with large families living in temporary housing and limited access to clean water is far more vulnerable than a neighborhood with the same heat levels but better resources. By multiplying these three factors—hazard, exposure, and vulnerability—the team calculated a single risk score for every union in the division.

The results painted a clear and somewhat surprising picture. The risk is not spread evenly across the landscape. Instead, it is heavily concentrated. Nearly half of all the unions in the division face very low heat risk, and these areas are home to about 43 percent of the population. At the other extreme, only 10 unions, all located within the Rajshahi City Corporation, fall into the "very high" risk category. These ten small areas, which make up just a tiny fraction of the total land, are where the intense heat of the city meets the highest density of people and buildings. The researchers found that the risk in these urban cores is driven by the sheer intensity of the built environment, where the combination of heat, crowds, and concrete creates a perfect storm.

However, the study also uncovered a critical disconnect. While the city center faces the most intense heat, the people with the least ability to cope are not always there. The district of Nawabganj, for instance, has the highest overall vulnerability in the entire division. Its residents face significant challenges, including higher rates of poverty, larger household sizes, and less access to financial resources. Yet, because the heat itself is not as extreme there as it is in the city center, the overall risk score for Nawabganj is lower than one might expect. This creates a dangerous blind spot: if officials only focus on the hottest places, they might miss the communities that are most fragile. The study shows that high vulnerability can exist in areas with moderate heat, and high heat can exist in areas with strong social resilience.

To understand how these risks are arranged across the land, the researchers used a statistical method to see if hot spots were clustered together or scattered randomly. They found that the high-risk areas are tightly grouped. The most dangerous zones form three distinct clusters: the heart of Rajshahi City, a corridor connecting Pabna to nearby towns, and a group of unions in the Natore and Nawabganj districts. Conversely, the safest areas form a long, cool belt stretching across the northern and western parts of the division, characterized by agriculture and floodplains. This clustering suggests that heat risk is not a random occurrence but a structured feature of the landscape, shaped by the history of urbanization and the distribution of resources.

The implications of these findings are significant for how the region should prepare for a warming future. The researchers argue that adaptation strategies must be different depending on where you are. In the high-risk urban cores, the priority should be cooling the built environment. This means planting more trees, creating green corridors along rivers, and using materials that reflect sunlight rather than absorbing it. In contrast, in the high-vulnerability rural districts like Nawabganj, the focus should shift to strengthening social infrastructure. Improving access to healthcare, ensuring reliable energy, and upgrading housing quality are the most effective ways to protect people there, even if the heat is not as intense as in the city.

This study demonstrates that it is possible to create detailed, life-saving maps using only publicly available data, such as satellite imagery and national census reports. The approach used by the team can be copied for other regions in Bangladesh and beyond, offering a way to see heat risk clearly without needing expensive field surveys. By revealing exactly where the heat is, where the people are, and where the weakness lies, this work provides a roadmap for saving lives. It shows that fighting the heat is not just about lowering temperatures; it is about understanding the complex relationship between the ground we walk on and the communities that call it home.