The physical characteristics of the environment mediate the impacts of extreme heat in populations. The role of the physical environment is especially crucial in an urban setting where the morphology of the built environment has been physically altered and where large populations are typically concentrated. Previous studies suggest a correlation between urban design and heat health vulnerability. Of particular significance is the urban heat island effect (UHI), a phenomenon where urban air temperatures are higher in comparison to surrounding rural areas. Furthermore, elevated nighttime temperatures in urban areas result in urban populations being at greater risk due to the retained elevated nocturnal temperatures.

The present study investigates specific characteristics in the built-up environment that may affect morbidity to extreme heat in the City of Chicago. Specifically, the research involves an examination of the physical land cover characteristics of the City, including the measurement of vegetation cover, percentage of impervious surface and surface reflectance (also know as albedo), along with an examination of daytime and nighttime surface temperatures. As an estimate for vegetation cover, we calculated for the Normalized Difference Vegetation Index (NDVI) using satellite images from NASA's Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER). We also used ASTER images to calculate for albedo, and surface temperatures utilizing the ENVI software. In addition, we characterized land cover in the City using eCognition, an object-based classification software in conjunction with Quickbird, a high resolution satellite image. Using data from the processed images, we created GIS (Geographic Information System) layers of these physical attributes of the City and overlaid them with the locations of heat-related emergency dispatch (HRD) data within the City of Chicago. We examined the spatial patterns of these variables over a five year period (i.e., 2003 to 2007).

Our preliminary research findings indicate that high HRD numbers are associated with lower albedo, minimal vegetation cover, and high percentage of impervious surfaces. A significant negative correlation (-0.86) was found between albedo and the number of heat calls when we examined the top five areas having high incidents of HRD. That is, in the top five HRD neighborhoods, the higher the HRD, the lower the albedo. Also, a significant negative correlation (-0.81) was observed between NDVI and the numbers of HRD in top five HRD areas; that is, the lower the vegetation cover, the higher the HRD. The present results suggest that the physical characteristics within a City do impact, to a certain extent, the vulnerability of people to heat events.