Disproportionate exposure to urban heat island intensity across major US cities

The urban heat island effect, where built environments are hotter than surrounding rural areas, contributes to significant public health problems, including heat-related mortality and morbidity. Heat exposure is linked to various adverse health outcomes, including heat stroke, dehydration, reduced productivity, and impaired learning. The built environment's characteristics (green space, urban form, city size, reflectance) influence both the temperature difference between urban and rural areas and the intracity temperature variation. This variation may lead to unequal distribution of heat-related health burdens across sociodemographic groups. Previous small-scale studies have suggested that low-income and marginalized communities experience disproportionately higher heat intensity. However, large-scale evidence assessing systematic disparities in heat island exposure across diverse US cities has been lacking. This study aims to fill this gap by combining high-resolution satellite temperature data with comprehensive US Census data to examine the relationship between race, income, age, and summer daytime SUHI intensity in the 175 largest US urbanized areas. These areas represent approximately 65% of the US population and are where most heat-related deaths have occurred.

Existing research on the relationship between intracity heat exposure and sociodemographic factors is limited in scope and generalizability. Several studies have examined individual cities or small sets of cities, using varying methodologies for quantifying SUHI intensity and sociodemographic data from disparate sources. One study examining 18 out of 25 global cities found that higher-income areas tended to have lower SUHI exposure, but this sample was not representative and lacked detailed demographic data. Another study found higher summer surface temperatures in historically redlined neighborhoods, but the relevance to current racial and income disparities was unclear due to demographic shifts and urban growth over time. These studies highlight the need for a large-scale, nationally representative study that uses consistent methodologies and data to evaluate the relationship between heat exposure and sociodemographic factors.

This study uses a three-component environmental risk analysis framework: hazard (spatial distribution of SUHI intensity), exposure (intersection of population distribution with the hazard), and vulnerability (propensity to suffer harm). The hazard is measured using a newly developed, high-resolution census tract-level database of SUHI intensity for the USA. Exposure is assessed using census tract-level demographic data from the 2017 American Community Survey (ACS). While a comprehensive vulnerability assessment is beyond the scope of the study, age is considered as a key vulnerability factor, as both young children and the elderly have compromised thermoregulation. The study focuses on summer months (June-August) when SUHI intensity is most pronounced. The analysis considers both mean SUHI intensity across racial and income groups and the Kolm-Pollak (KP) inequality index to evaluate intra-group variation and potential hotspots of high heat exposure. The 175 largest US cities are included, representing ~65% of the total US population. Data on race/ethnicity, income, and age are analyzed in relation to mean summer daytime SUHI intensity. The KP inequality index helps to assess the unequal distribution of SUHI within each demographic group. The analysis accounts for background climate differences using Köppen-Geiger climate zones.

The study's key findings demonstrate a consistent pattern of disproportionate heat exposure for people of color and low-income individuals across nearly all major US cities. People of color, on average, live in census tracts with significantly higher summer daytime SUHI intensity than non-Hispanic whites in all but six of the 175 cities. A similar pattern holds for those below the poverty line versus those above twice the poverty line. Remarkably, in nearly half of the urbanized areas, the average person of color experiences higher SUHI intensity than the average person living below the poverty line, despite only 10% of people of color living below the poverty line. This suggests that racial and ethnic disparities in heat exposure are not solely explained by income differences. The analysis of the KP inequality index indicates that within-group variation in SUHI intensity is not significantly different across demographic groups, suggesting that disparities in mean exposure are not masking significant variations in hotspots of extreme heat. While those over 65 generally have lower SUHI exposure, the racial/ethnic disparities in SUHI exposure persist across age groups, indicating that older people of color still face disproportionately higher exposure than their white counterparts. Illustrative examples using Baltimore, MD, and Greenville, SC, show the varied spatial distribution of heat exposure relative to race and income across different cities. These findings confirm and expand upon previous smaller-scale studies, offering compelling national-level evidence of environmental injustice.

This study provides robust evidence of environmental inequality related to urban heat exposure in the United States. The consistent pattern of higher SUHI intensity for people of color and low-income individuals across diverse geographic locations and climate zones strongly suggests that systemic factors beyond individual choices contribute to this disparity. The findings highlight the crucial need for policies that address both the overall reduction of SUHI and the equitable distribution of its impacts. The nearly identical SUHI exposure levels for people of color and those below the poverty line, despite the relatively small overlap of these groups, suggests a complex interplay of racial and economic factors that influence residential patterns and heat exposure. The persistence of racial disparities across age groups underscores the vulnerability of disadvantaged populations, regardless of age. These results should inform the development of locally tailored SUHI mitigation strategies that consider the specific spatial distribution of vulnerable populations within each city.

This study offers a comprehensive framework for quantifying intercity and intracity disparities in SUHI exposure by race, income, and age. The findings consistently demonstrate that people of color and low-income individuals experience disproportionately higher SUHI exposure across nearly all major US cities. This highlights the urgent need for equitable SUHI mitigation strategies that consider the complex interplay of race, income, and residential patterns. Future research should focus on longitudinal studies to track changes in SUHI exposure disparities over time and on investigating the direct link between SUHI exposure and heat-related health outcomes.

The study's limitations include the use of census tracts as the unit of analysis, which may mask finer-grained variations in heat exposure within tracts. The use of satellite-based SUHI intensity data may slightly overestimate UHI magnitude compared to ground-based measurements. The study also does not consider all factors contributing to heat stress (humidity, wind, radiation) and assumes that individuals remain within their census tract throughout the day. Finally, the analysis does not address temporal changes in SUHI exposure disparities.