Air pollution and climate change are major threats to rapidly growing cities in present times. The developing nations like India, which are switching from predominantly rural country to increasingly urban, have to face critical challenges in terms of climate action and sustainable development (Van Duijne, 2017; Singh C. et al., 2021). India is projected to have 53% of the national population as urban population by addition of 416 million urban dwellers by the year 2050 (UNDESA, 2018).

The change in land use land cover patterns in urban areas due to ongoing urbanization affects regional climate by altering the surface and boundary layer atmospheric properties (Shepherd, 2005; Ren et al., 2008; Yang et al., 2012). Further, the urbanization by changing land use land cover affects climate via increased anthropogenic emissions, extreme precipitation (that may cause urban flooding), higher temperatures, and frequent heat waves with heat related human health impacts (Chestnut et al., 1998; Ramanathan et al., 2001; Shastri et al., 2015). The regional climate changes are reflected by different meteorological conditions such as changes in temperature and precipitation. The anthropogenic emissions such as greenhouse gas (GHG) emissions trigger these local climate changes.

In addition to the impact of urbanization on climate, the increasing urban population and vehicular traffic increases the pollutant emissions and aerosol load in the atmosphere. The increasing urbanization alongwith growing population and industrialization has been stated as one of the key reasons for high aerosol loading in the Indian sub-continent (Kaskaoutis et al., 2011; Ramachandran et al., 2012; Krishna Moorthy et al., 2013). Thus, the climate change and air pollution remain one of the biggest threats to human health and well-being in cities and are interlinked with each other as discussed later in this study.

According to a report by World Health Organization (WHO) more than seven million people across the world lose their lives due to diseases linked with PM_2.5 pollution (WHO, 2015). India, being a rapidly developing country with increasing population is suffering from severe air pollution; as among the world's 10 most polluted cities, nine of them lie in India [WHO Global Urban Ambient Air Pollution Database (Update 2016), 2016]. The increasing air pollution in most of the Indian megacities over last few decades and its consequential human health impacts (such as asthma and cardio-respiratory illness) have drawn prominent attention in recent years (Sarath and Ramani, 2014; Gautam et al., 2020; Shaw and Gorai, 2020).

The global change in climate has been reported by various workers in the last few decades. The natural process of climate change because of volcanic eruptions, continental drift, and astronomical cycles is now accelerated by human activities (IPCC, 2007). The emission of greenhouse gases (GHGs) is one of the major factors in altering climate by changing atmospheric concentration of certain gases. Further, the role of water vapors in altering the climate is also being well looked into by scientists (Jacob, 2001; Forster and Collins, 2004). Not only this, scientists are also looking into the role of black carbon in climate change due to their ability to strongly absorb incoming solar radiations (Jacobson, 2001; Ramanathan and Carmichae, 2008; Surendran et al., 2013). Menon et al. (2002) used a global climate model to investigate the role of aerosols in altered climate in India and China and reported that precipitation and temperature changes in the model could be correlated to large load of absorbing black carbon in the aerosols. Also, due to heating of air by black carbon aerosols, atmospheric stability is altered leading to changes in hydrologic cycle and large-scale circulations.

Climate change is known to alter the temperature, precipitation pattern and solar insolation over the planet. According to IPCC (2007) report, about 0.65°C increase has been observed in global average surface temperature over last 50 years and is projected to increase by 1.1–6.4°C. The rise in sea level has been observed with ongoing warming trend. Annual sea level rise between 2.5 and 3 mm along the coastline of Mumbai has been reported (Pramanik, 2017). Also, according to a study by NASA, this region has possessed increase in average temperatures by 2.4°C for the period from 1881 to 2015 (NASA, 2015). Further, an increase in frequency of extreme rainfall was analyzed that can cause flooding. This can also be seen in Mumbai, one of the Indian mega-city and home to the largest population threatened by coastal flooding (Intergovernmental Panel on Climate Change IPCC-SREX, 2012). Mumbai has been recognized as one of the world's most vulnerable cities to climate change according to UN-HABITAT, 2010 (Mehta et al., 2019). The changes in rainfall in the past century (1901–2019) were observed over India by Kuttippurath et al. (2021). In the study of 119 years of rainfall measurements, a significant change in the rainfall pattern has been confirmed after 1973 with the declining trend of about 0.42 ± 0.024 mm dec⁻¹. The study shows that in recent decades, the wettest place of the world has shifted from Cherrapunji to Mawsynram.

Besides, the increasing temperature due to climate change can trigger melting of glaciers. A study conducted by Kumar et al. (2021) for monitoring glacier changes in Nanda Devi region of Central Himalaya, India, for three decades, shows the loss of about 26 km² (10%) of the glaciated area between 1980 and 2017. Additionally, the climate change causing extreme weather events causes increase in frequency and intensity of floods, storms, torrential rains, and droughts etc. that take thousands of lives and affect millions of people (Haines et al., 2006; Majra and Gur, 2009). The projected climate change estimates from 2036 to 2060 for 57 Indian cities show that 33 cities are likely to experience rise in extreme rainfall and exacerbated flood risk. The remaining 24 cities will observe precipitation declines, reflecting higher drought risk (Ali et al., 2014; Singh C. et al., 2021).

The land use land cover being an important factor in climate change has been focused in many studies over Indian cities such as Nath et al. (2021) that shows rapid expansion of built-up areas in Guwahati with an overall increase of 103% in area over the last 30 years (1990–2020). The expansion in urban areas causes decline in vegetated areas, cropland, and fallow land thereby contributing to climate change. Paul et al. (2021) also showed expansion in urban areas at annual rates of 38.6% with decline in agricultural area at rate of 2.1% for peripheral Delhi during the 1973–2017. So, climate change is one of the emerging threats to human health in Indian cities. With the increase in climate variability, there is an associated increase in health issues (Bush et al., 2011). Cities, due to UHI occurrence, are supposed to have higher effect of climate extremes such as precipitation extremes and heat waves than rural regions (Shepherd, 2005; Shastri et al., 2017; Chauhan and Singh, 2020).

Air pollution is a matter of serious concern in megacities where the pollution levels often exceed the permissible limits due to its associated health risks for city residents (Chattopadhyay et al., 2010; Debone et al., 2020). The metropolitan cities of India are exposed to unhealthy and unhygienic conditions due to air pollution (Dutta et al., 2021). The continuous and alarming increase of urban air pollution is an emerging environmental issue in the Indian megacities for the last few decades (Faheem et al., 2021).

Major outdoor and indoor air pollutants in urban areas can be primary or secondary air pollutants. Primary air pollutants that are emitted directly include particulate matter {PM_2.5, PM₁₀, suspended particulate matter (SPM), respirable particulate matter (RPM)}, SOx, NOx, CO, ammonia, and dust particles while the secondary air pollutants involve ozone, smog, Peroxyacyl nitrates (PANs) etc.

The developing nations like India with ongoing urbanization are suffering from increased air pollution issues due to the lack of services such as adequate transportation management, suitable roads, and unplanned distribution of industries (Rumana et al., 2014). The congested roads in cities reduce average vehicular speed resulting in higher vehicular emissions adding to air pollution levels (West Bengal Pollution Control Board, 2010). The increasing and unplanned urbanization coupled with industrialization and population growth are posing threat to human health by increasing air pollution levels leading to number of health issues (Dutta et al., 2021). Additionally, the complex and intensive human activities in these urban areas are fueling the problem by increasing emission of pollutants.

In Indian cities, the air pollutants are either emitted from natural sources such as long-range transport of desert dust influx originated from the western arid regions of Africa, Middle-East, and Thar (Rajasthan) regions, predominately during summer and pre-monsoon season (El-Askary et al., 2006) or they can be of anthropogenic origin as given in Figure 1 (Ghose et al., 2005; Habib et al., 2006; Prasad and Singh, 2007; Badarinath et al., 2010; Sharma et al., 2010; Kharol et al., 2011; Dandotiya et al., 2020).

Vehicular emissions (95%) have been identified as prevalent source of high NO₂ concentrations followed by industries and fuel burning, thereby increasing air pollution in urban areas of India (Mondal et al., 2000; Ghose et al., 2004; ARAI, 2010). The combustion of low-quality fuel in Indian cities causes SO₂ emissions (Zou et al., 2007). Air pollutants are also emitted from crude oil wells and flared natural gas (Amakiri et al., 2009). Besides, open burning and landfill fires of municipal solid waste were recognized as chief source of air pollution for Mumbai, India in a study reported by National Environmental Engineering Research Institute (Central Pollution Control Board (CPCB), 2010). Open coal liming, fluoride mining, lime stone mining, thermal power plant, natural and domestic burning of coal, cement industry, and road dust were another primary source of air pollution in India (MPCB, 2010; Maji et al., 2016). Road dust (61%) was identified as major source of high particulate matter concentration followed by vehicular emissions, industrial emissions, vegetation, and solid fuel burning for another Indian city (Pune). Plastic industry, domestic waste burning, and food processing factories were the main sources of air pollution in Nashik city (Maji et al., 2016). Diesel generators, coal based industrial emissions, oil refinery emissions were major source of PM in Agra (Maji et al., 2016). Also, thermal power plants in most of the cities in addition to large- and small-scale industries are contributing to high air pollution levels. For Kolkata, 51.4% of the air pollution is contributed by motor vehicles followed by 24.5% emissions from industries and 21.1% dust particles (West Bengal Pollution Control Board, 2005).

The deterioration of air quality has been further aggravated by emission of toxic pollutants such as particulate matter, greenhouse gases like SOx, NOx, and O₃ (Rumana et al., 2014). Emission of aerosols from deserts, oceans, forest fires, and volcanoes into the atmosphere also adds to air quality depletion. Increase in population, urbanization, and industrialization has depleted air quality and hence adversely affects human health (Rumana et al., 2014).

Besides, Particulate matter, Black carbon (BC) has been studied by various workers around the globe. Singh A. et al. (2014) reported mass concentrations of BC in Indo-Gangetic Plains (IGP) that varied from 8.5 to 19.6, 2.4 to 18.2, and 2.2 to 9.4 μg m⁻³ during paddy-residue burning emission in the month of October-November, emission from bio- and fossil-fuel combustion during December–March months and wheat-residue burning emissions duringApril-May, respectively. In contrast, the mass concentrations of Elemental Carbon (EC) varied from 3.8 to 17.5, 2.3 to 8.9, and 2.0 to 8.8 μg m−3 during these emissions, respectively. Not only this, polycyclic aromatic hydrocarbons (PAHs) have been studied by Rajput et al. (2011) during paddy and wheat biomass burning emissions of Indo-Gangetic plains and reported 40 ng m⁻³ of total PAHs are reported from paddy residue burning and 7 ngm⁻³ during wheat burning season.

The interactions between urban climate, air pollution, and human health in cities need to be explored. The cities in developing nations like India are facing high pressure due to air pollution and climate change. Limited studies have been performed on the combined effects of weather, climate variability, increased air pollution, and health impacts in India (Agarwal et al., 2006; Karar et al., 2006).

Climate plays a considerable role in spatial and temporal distribution of air pollutants. Greenhouse warming and ozone depletion in stratosphere are vital factors of climate change. Climate change can influence the air pollutant concentration and catalyze the formation of secondary pollutants. Also, the climatic conditions in addition to atmospheric parameters, topography and urban settlements influence the dispersion, accumulation and transformation of pollutants in the atmosphere. The dispersal of these air pollutants may cause respiratory disorders such as emphysema, asthma, allergy problems and chronic bronchitis (D'Amato et al., 2002).

According to the World Health Organisation (WHO) estimation, in past 30 years, the precipitation and warming trends due to anthropogenic climate change had taken 150,000 lives annually. The alterations in climate had caused many prevalent human diseases such as cardiovascular mortality and respiratory illnesses due to heat waves etc.

Besides, the nexus between urbanization, climate change and air pollution lies in a way such that some of the atmospheric pollutants (aerosols) can enhance the climate change because of their direct and in-direct effects (Ramachandran and Cherian, 2008). These air pollutants not only degrade the air quality with certain human health impacts but also have a considerable impact on climate by heating lower and mid troposphere, causing sea-land temperature gradients, monsoon circulation, distribution of rainfall solar dimming and cloud microphysics (Lau et al., 2006; Gautam et al., 2010; Sharma et al., 2014) thereby modifying the heat wave frequency, intensity of storms and precipitation patterns. These small sized particles can weaken the UHI effect by up to 1 K under heavily polluted conditions (Wu et al., 2017). So, the increased concentrations of air pollutants (such as aerosols) have an impact on global climate change as the increased air pollution (aerosol load) in the atmosphere is associated with the climate system and hydrological cycle (Ramanathan et al., 2001; Jirak and Cotton, 2006). In addition to this, the indirect effect of aerosols can also be seen on optical properties of clouds. Aerosols can affect the surface energy balance by either scattering or absorbing the incoming solar radiations that may cause surface cooling and atmospheric heating (Kaufman et al., 2002; Wu et al., 2017). This influences the radiation equilibrium of Earth via radiative forcing and chemical perturbations (Rosenfeld et al., 2007; Wang et al., 2009; Zhu et al., 2010; Zhang S. et al., 2016; Zhang W. et al., 2016).

Besides, the atmospheric structure and climate is influenced by concentration of atmospheric pollutants that are emitted by human activities (Fischer et al., 2003; Jaffe and Ray, 2007; Yan et al., 2008).

Commercial and high traffic regions have higher concentration of gaseous pollutants than vegetated areas. Also, the concentration of pollutants varies with the seasons and other atmospheric parameters (Dandotiya et al., 2020). The estimation of pollutant concentration is influenced by atmospheric conditions of that urban area such as temperature, relative humidity and wind speed etc.

The greenhouse gases (GHGs) emissions are estimated mainly by consumption patterns in cities of the developed world that causes climate change. According to IPCC report, ~20% of global emissions were attributed by buildings. Further, transportation was estimated to contribute to 13% of GHG emissions (Diarmid Campbell-Lendrum and Corvala). It can be seen that both buildings and transportation are eminent factors of cities. Also, the cities face higher pollution issues than rural areas with higher vegetation due to higher emissions from transportation and fossil fuel burning in highly populated regions with high vehicular traffic (Dandotiya et al., 2019).

Further, it is notable that the urbanization phenomenon plays important role in both climate change and air pollution either directly or indirectly. The increase in air pollutant emissions and their concentration in the atmosphere increases with the urbanization. The urban characteristics, materials used, vegetation, vehicular traffic etc alters the climatic conditions of an urban area thus leading to formation of strong spatial gradients of heat and air pollution. These conditions exacerbate the risks for human health.

The expanding urban areas with inadequate or improper management accompanied by land use land cover changes, deforestation and decrease in vegetation cover and alterations in climate variables can influence or modify urban climate by transformation of natural land surface to impervious surfaces (Balica et al., 2012; Jha et al., 2012). The urban heat island effect by increased urban temperature due to climate change increases the demand of energy requirement for cooling in cities. The air conditioners used for reducing the high temperature in cities in turn emits harmful GHGs that cause urban air pollution. Also, the concentration of certain pollutants, such as ozone, is influenced by atmospheric conditions and tends to be higher on warmer days. Moreover, the higher demand of electricity consumption leads to higher burning of fossil fuels that also increases air pollution. Certain respiratory issues can be caused by UHI effect due to depleted air quality by certain cooling agents (Liu and Zhang, 2011). The city residents also suffer from thermal discomfort due to elevated urban temperature by UHI effect resulting in exacerbation of heat-waves (Ohashi et al., 2007). The UHI effect influences air quality as the differential heating generates mesoscale winds that facilitate pollutant movement and circulation causing urban air pollution issues (Agarwal and Tandon, 2010). So, air pollution and climate change are interlinked with adverse impacts on human health in cities.

The present review highlights high air pollution levels over most of the Indian megacities with air pollutant levels lying above the permissible limits. The continuous emissions from both anthropogenic as well as natural sources causing high PM concentration with adverse human health impacts highlight the necessity of continuous monitoring of air pollutants over the Indian subcontinent using measurements and remote sensing satellite data.

The essential information regarding air pollutant levels in different megacities of India, provided in this review can help in design of effective mitigation strategies for each city by analyzing vulnerable regions. The data can facilitate a baseline data for air quality modeling studies to predict air pollution levels for effective preparedness, adaption and mitigations plans in tackling air pollution. The high disease burden and mortality linked with air pollution in Indian cities should be emphasized to effectively control air pollutant concentration throughout the nation. Besides, the results depicting reduction in air pollution during COVID-19 lockdown period suggest adoption of such short-time restrictions for pollution mitigation across different cities of India to improve the air quality and thus benefit human health.

Further, as stated in the review, India being a developing country is experiencing adverse human health impacts due to climate change. Indian cities are exposed to extreme weather events such as high precipitation, floods, droughts, heat waves with increased temperatures induced by climate change. The increase in health surveillance for heat waves, floods and for vector-borne diseases linked with climate change can help in combating severe human health impacts in near future in Indian cities. Also, the high population density with ongoing urbanization and industrialization are some of the primary factors to be considered to avoid negative health impacts associated with climate change in India. So, essential mitigation and adaptation strategies are required for current and projected climate change impacts mentioned in the review to avoid myriad human health effects in Indian cities because of climate change.

To conclude, the use of advanced technologies such as satellite data with geospatial techniques can be of great help in monitoring and mapping of spatial-temporal distribution patterns of the air pollution and climate change and associated health impacts. So, while focusing on building smart cities in developing nations like India, proper urban planning and sustainable measures should be taken for sustainable urban environment to avoid adverse health impacts.

RK was involved in review of the chapter and preparation of the manuscript. PP was involved in overall supervision of the manuscript and manuscript review and editing. Both authors contributed to the article and approved the submitted version.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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