Impacts of Urban Heat Island: Air Quality and Public Health Effects

Introduction

Urbanization, driven by multiple social, economic, and environmental processes, is one of the biggest social transformations of modern times. According to the United Nations, more than 50% of the global population now resides in urban areas, and this figure is projected to rise significantly in the coming decades. This urban sprawl, characterized by the conversion of natural landscapes into built environments, presents multifaceted challenges that extend from the local to the global scale, making a comprehensive review both timely and crucial. Urbanization affects the microclimate and creates a unique urban climate, such as the Urban Heat Island (UHI), which refers to the phenomenon in which urban areas are often several degrees warmer than the surrounding rural areas. 

Land surface temperatures or near-surface air temperatures in urban regions are higher than those in surrounding rural areas. They may contribute to additional warming beyond that already caused by climate change. Urbanization brings changes to urban meteorology that, in turn, influence ambient air pollutants. Topics such as air pollution, the built environment, and human health are subjects of complex debates that span the disparate fields of urban planning and design, environmental engineering, public health, transportation planning and engineering, and tree and plant ecology. UHI exacerbates the impacts linked with air pollution.

Causes of Urban Heat Island

Developing/developed cities have resulted in massive anthropogenic modifications of the environment through the replacement of natural land cover with concrete, bricks, asphalt, and metal. This reduces evapotranspiration, increases the heat storage, and alters air movement. Furthermore, the total built-up area and the increase in impervious surface cause an adverse impact on UHI intensity and high thermal capacity. UHI intensity is also significantly influenced by land-use/land-cover changes (LUCC) such as vegetation distribution, urban–suburban transitions, and urban–wildland interface. 

While human activities consume large amounts of electricity and fossil fuels, buildings release large amounts of carbon emissions and other pollutants into the atmosphere, trapping heat. Decreasing vegetation cover alters the radiative properties of the surface due to reduced shading and evapotranspiration. Built-up areas act as storage materials that warm up and store much more heat than natural cover during the daytime. The albedo of roofing materials significantly affects the rise in noon temperature in urban areas. These materials (mainly concrete and asphalt) not only alter the heat proportions but also reduce albedo values.

Impact of UHI on Air Quality

Concentrations of ground-level ozone depend on the relative concentrations of precursors such as NO_x and volatile organic compounds (VOCs), some of which are emitted by vegetation (biogenic sources; BVOCs), as well as ambient temperature; higher urban temperatures will lead to increased ground-level ozone formation. Anthropogenic heat and locally induced thermal circulations are strongly associated with ozone (O₃) concentrations in urban environments. Recent studies of the UHI effect on ozone concentration have shown that daytime formation of O₃ was affected by thermally induced air circulation and NO_x dilution due to a change in boundary-layer height. It was observed that air temperature and vegetation regulate the production of BVOCs, precursors of ground-level O₃. 

The photolysis reaction rates are accelerated by high air temperatures, enhancing the production of tropospheric O₃, secondary inorganic and organic aerosols. In one such study, it was shown that alterations in urban land surfaces were responsible for increases in lower air surface temperature and PBL (Planetary Boundary Layer) height, which have caused increases in surface O₃ concentrations but decreases in PM_2.5 concentrations. With increasing PM_2.5 in urban areas, daytime UHII (Urban Heat Island Intensity) weakens, while night-time UHII strengthens, leading to a continuous reduction in DTR. PM_2.5  affects UHII via aerosol-radiation interaction under clear sky and via aerosol-cloud interaction under cloudy conditions.

Public Health Implications

With the growing global urban population, exposure to heat-related health risks in metropolitan cities and larger urban centers has increased proportionately. A recent study, based on data from 1,300 cities worldwide, estimates that nearly a quarter of the world's population (about 1.7 billion people) is exposed to extreme heat. It is also noted that the human health implications of heatwaves are relatively severe in mild and cold climates compared with warm ones. Barrow and Clark noted that mortalities attributed to heat stress are under-reported because heat stress is the driver for apparent causes of death due to cardiovascular, respiratory, and cerebrovascular failures. The main mechanisms that explain global warming and increased temperatures as cardiovascular risk factors are related to an imbalance of the autonomic nervous system towards increased sympathetic tone due to thermoregulation, blood pressure lowering, and dehydration from high temperatures. 

Additionally, high temperatures are also attributed to increased mental health emergencies. Dehydration caused by the heat stress can also result in kidney damage or failure. The severity of these episodes is usually increased for vulnerable population groups, i.e., children, the elderly, and people with pre-existing conditions. Epidemiological studies have shown that the physiological and psychological toll of extreme temperatures is higher in specific subgroups than in others. For instance, the effects of high temperatures differ across age groups; temperature-related mortality and morbidity are amplified at the extremes of the age curve. Outdoor workers and occupations involving intense physical activity are more prone to heat stroke due to high metabolic heat production and heat gain inherent to certain occupations. Vulnerability to heat-related health issues is highest for populations with less/insufficient access to energy resources or relevant equipment (e.g., heating, ventilation, and air conditioning systems).

Conclusion

Urban Heat Island is no longer just an environmental concern but a growing public health and air quality challenge in rapidly expanding cities. By intensifying local temperatures, worsening air pollution, and increasing heat-related illnesses, UHI highlights the urgent need for sustainable urban planning. Integrating greener infrastructure, reflective building materials, improved air quality monitoring, and climate-responsive policies can help mitigate its impacts. Addressing UHI is essential not only for creating healthier urban environments but also for building resilient cities capable of withstanding future climate and population pressures.

A Collective Responsibility

In this modern age, air quality is everyone's responsibility. Each action we take can contribute to a healthier planet. Planting trees, choosing public transportation, or supporting local clean air initiatives can make a difference. Awareness is the first step toward change.

Join us in advocating for cleaner air. Together, we can help nature regain its voice.

Let’s work together to ensure our world thrives in harmony with nature.