Urban heat inequity

Urban heat inequity, also termed thermal inequity, is an unequal distribution of heat in urban areas or neighborhoods within them, which causes disproportionate impacts to people living in those communities. Unequal threat of heat stress in urban environments is often correlated with differences in demographics, including racial and ethnic background, income, education level, and age. While the general impacts of urban heat inequity depend on the city studied, negative effects typically act on historically marginalized communities. The idea is closely tied to the urban heat island effect, where a major cause to urban heat inequity is increased urbanization.

Observations
There is a correlation in the U.S. between ethnicity and exposure to UHIs. Within most U.S. cities, people of color are more likely to live in areas of high Surface Urban Heat Island Intensity than white people in the same cities. According to a study by climatologist Angel Hsu and colleagues, "the average person of color lives in a census tract with higher SUHI intensity than non-Hispanic whites in all but 6 of the 175 largest urbanized areas" in the U.S.

Economic status also plays a role in the human effects on UHIs. Not only are people of lower economic classes more likely to live in UHIs, but they are also less likely to be able to afford commodities such as air conditioning. Like the correlation between SUHI intensity and ethnicity, a similar pattern can be seen when comparing households under the poverty line against those with an income more than double the poverty line.

UHIs can have particularly strong effects on African Americans with chronic diseases. African Americans have higher rates of various chronic diseases, such as asthma and diabetes, than the general population. Per Professor Pamela Jackson and colleagues, these diseases can be exacerbated by extreme heat, leading to health problems such as hypertension or stroke.

On a global scale, there are discrepancies in the effects of UHIs in different regions of the world. While overall heat exposure is increasing worldwide, its effects have increased faster in the Global South in recent decades, per a study by Professor Kanging Huang and colleagues.

The disproportionate impact of UHIs on the Global South exacerbates already occurring environmental injustices. Because many countries in the equator are naturally hot and humid, these areas are particularly susceptible to the effects of UHIs. One World Bank study has found a 7.0° disparity between the hottest and coolest neighborhoods in Bandung, Indonesia.

Researchers have also noted that the spread of impervious surfaces, such as concrete, tar, and asphalt, is correlated with neighborhoods of low socioeconomic status across various U.S. cities and states. The presence of these materials serves as a predictor of "intra-urban variation in temperature".

Increased urbanization
Increased urbanization is a major leading cause for increased urban heat risks, as it replaces vegetated areas with impervious surfaces and concentrates people into smaller land areas.

Spatial distribution
Historic urbanization processes, such as redlining in the United States, have lasting impacts on land use and may perpetuate disparities in urban heat through unequal distribution of vegetation. Access to healthcare, public transportation, adequate housing, and reliable energy typically govern disparities to heat, of which many of these communities may lack access to. Such differences in spatial distribution leave certain, localized neighborhood at risk for micro urban heat islands. Socially vulnerable groups are known to live in more densely populated areas with little vegetation that in turn have higher threats to heat exposure and often have adapting to and mitigation their exposure.

Housing conditions
Housing conditions are a factor that contributes to urban heat inequity. Living on the top-floor, having a home with a dark roof, and poor insulation exacerbate heating conditions during hot weather. Lower income individuals may also not have access to air-conditioning or be unable to afford the increased electricity usage. In condensed cities, opening the windows for airflow and cooler air could introduce air pollution and odor into the home. High-density housing is Asia often has constraints on building updates that promote cooling.

Space poverty
Another contributor to inequality is space poverty, which refers to a significantly reduced indoor residential space in a home. Space poverty is a significant issue in Hong Kong, where low-income individuals live in extremely small, sub-divided units, which has led to overcrowding and indoor air pollution. The units promote little movement, and all furniture, kitchen appliances, and bathroom necessities must fit into one sub-divided room. These units are typically developed from derelict buildings that have poor ventilation and less access to proper space cooling. High-density housing is Asia often has constraints on building updates that promote cooling, and residents often lack the space and authority to complete any improvements to their condition. These residents typically seek out adequate third places, such as libraries, parks, and cafés, to access proper cooling and space.

Heat stress vulnerability
Heat stress vulnerability refers to an individual's predisposition, typically due to demographic factors, to be negatively impacted by heat. The measurement takes into account daily routines, location, sensitivity due to demographic factors, including age. Some demographic groups known to have higher heat stress vulnerability include the elderly, women, children, low-income households, and those who are chronically ill. Elderly populations are included because they are often unable to regular their body temperatures and typically have pre-existing medical conditions that make them more susceptible to heat. Children are also sensitive to increased heat, which may cause stress on their developing bodies. Intensified indoor heat causes stress on the mind and body and strains domestic relations.

Urban green infrastructure
Urban green infrastructure (UGI) are integrated networks of green spaces in cities and are developed in both private and public areas of the city. Examples of UGI include urban parks with trees and shrub cover, rows of trees along a street, private gardens, rooftop gardens, and other green space throughout a city. Properly utilized UGI promotes more equitable heat distribution, where improper usage only perpetuates heat inequity. UGI provides shading and evapotranspiration, which reduces both air and land temperatures. Therefore, a lack of UGI in marginalized communities reduces the land's temperature regulating capabilities promotes heat inequity. UGI is widely recognized as an efficient, sustainable, and non-controversial solution to heat inequity.