Rising urbanization, booming industrialization, and associated anthropogenic activities are the prime reasons that lead to air pollutant emissions and poor air quality. It is expected that by 2030, around 50% of the global population will be residing in urban areas (Gurjar, Butler, Lawrence, et al. 2008). More than 80% of population in urban areas is exposed to emissions that exceed the standards set by World Health Organization (WHO 2016). Air pollution is one of the key global health and environmental concerns (Nagpure, Gurjar, Kumar, et al. 2016) and has been ranked among the top five global risk factors of mortality by the Health Effects Institute (HEI 2019). According to HEI’s report, particulate matter (PM) pollution was considered the third important cause of death in 2017 and this rate was found to be highest in India. Air pollution was considered to cause over 1.1 million premature deaths in 2017 in India (HEI 2019), of which 56% was due to exposure to outdoor PM2.5 concentration and 44% was attributed to household air pollution. As per WHO (2016), one death out of nine in 2012 was attributed to air pollution, of which around three million deaths were solely due to outdoor air pollution.
The rising trends in population growth and the consequent effects on air quality are evident in the Indian scenario. For example, the megacities of Delhi, Mumbai, and Kolkata combined holds a population exceeding 46 million (Gurjar, Ravindra, and Nagpure 2016). Over the years, there has been a massive-scale expansion in industries, population density, anthropogenic activities, and the increased use of automobiles has degraded the air quality in India (Gurjar and Lelieveld 2005). In the last few decades, the greenhouse gas (GHG) emissions and other emissions resulting from anthropogenic activities have increased drastically (Gurjar and Nagpure 2016). As per WHO (2016) estimates, 10 out of the 20 most populated cities in the world are in India. Based on the concentrations of PM2.5 emissions, India was ranked the fifth most polluted country by WHO (2019), in which 21 among the top 30 polluted cities were in India. The Indian cities, on average, exceeded the WHO threshold by an alarming 500%.
The consistent population growth has led to an excessive strain on the energy consumption, thereby affecting the environment and the air quality of the megacities (Gurjar and Nagpure 2016). Kumar, Khare, Harrison, et al. (2015) calculated the increase in the total energy demand for both mobile and point sources and inferred that in Delhi, the energy demand had grown by 57.16% from 2001 to 230,222 TJ in 2011. A subsequent rise in energy consumption can be expected in the coming years, with no reliable sources available for monitoring the rate of energy consumption.
The continuous degradation of ambient air quality in the urban centres of India demands effective measures to curb air pollution. Though various policy measures are being introduced by the Government of India (GoI) to reduce the vehicular and industrial emissions, the extent to which these measures are implemented is questionable. The lack of infrastructural facilities, inadequacy of financial resources to implement advanced infrastructural innovations, difficulty in relocation of the industries from the urban centres even after mandatory court decisions, and above all, the behavioural patterns among people in accepting the green solutions are some of the crucial impediments on the road to environmental protection that our country seems to be struggling to overcome today.
There have been various efforts to study the air quality in Indian cities. The potential of the atmospheric carcinogenic emissions to put human health at risk has been studied by Gurjar, Mohan, and Sidhu (1996). Gurjar, Aardenne, Lelieveld, et al. (2004) framed a comprehensive emission inventory model to understand the emission trends in Delhi, India’s capital, for a period from 1990 to 2000. A multipollutant index (MPI) rating scale was used by Gurjar, Butler, Lawrence, et al. (2008) to rank the megacities with respect to their ambient air quality. According to this study, out of 18 megacities considered worldwide, the Indian cities, namely, Delhi, Kolkata, and Mumbai were ranked 7, 9, and 11, respectively. Gurjar, Nagpure, Kumar, et al. (2010) evaluated the vehicular emissions in Kolkata between 2000 and 2010 and inferred that the older vehicles in the city contributed more to the pollution load and should be phased out. A Vehicular Air Pollution Inventory (VAPI) model was developed by Nagpure and Gurjar (2012) that could estimate the vehicular emissions from road traffic in Indian cities. Later, Gurjar, Nagpure, and Kumar (2015) evaluated the potential gaseous emissions from the agricultural wetlands of Delhi and inferred that man-made wetlands were responsible for 48–49% of the total GHG emissions in the capital city. The study intended to develop an emission inventory for agricultural activities to evaluate their contribution to pollution in Delhi.
Several policy measures have been taken by the Ministry of Environment, Forest and Climate Change (MoEFCC), GoI to tackle the adverse effects of air emissions in short and long terms. The government’s decision to adopt compressed natural gas (CNG) as an alternative fuel to petrol and diesel, the odd-even measures introduced in Delhi, and the improvements in fuel and vehicle quality to lower emissions are some of the recent commendable steps towards curtailment of air pollution. Moreover, the increasing number of studies related to this field shows the importance of research on this subject. Several studies have assessed the trends of air pollutant emissions from different sources across several cities in India. However, there is an urgent need for a comprehensive review of the existing issues in the Indian scenario. More focus is needed on studying the impacts of these pollutant emissions on various forms, such as the ecosystem, biodiversity, buildings and materials, and primarily the health risks that people are vulnerable to due to breathing foul air.
A comprehensive review is done to understand the current scenario in the Indian context. The following section comprises a detailed review focusing on air pollution studies in India, the various sources, and the effects of the pollutants on the ecosystem, biodiversity, materials and buildings, and on human health, which are discussed in the later sections of this article. The various air quality standards followed by countries worldwide are included as well. The Discussion section of the article consists of the mitigation strategies adopted for emission control in India, the challenges posed by various sectors in the Indian scenario, and the research gaps that have been identified from the available literature. The key conclusions and a few recommendations form part of the last section.
The present review is divided into three sub-sections: The first sub-section discusses the literature that focuses on air pollution in India on a national scale; the next segment highlights the various sources of air pollution and the effects of the pollutants. The major sources are categorized into seven sectors. Thereafter, the various effects of pollutant emissions are pointed out. The air quality standards adopted by various countries for controlling air pollution have been discussed in the later sections of this article.
Studies on air pollution in India
Though various studies have addressed the issue of air pollution and its impacts on urban Indian cities, most of these studies are limited to specific cities and do not necessarily give a complete picture of the situation. Some of the highlights of these studies are discussed in the following paragraphs.
Pandey and Venkataraman (2014) evaluated the effects of emissions from various modes of transport in India. Their study inferred that on-road transportation contributed over 97% of the estimated emissions in India, when compared to other modes of transport, such as railways, waterways, and airways.
Gurjar, Ravindra, and Nagpure (2016) did a comprehensive study on various anthropogenic emission sources in Indian megacities, such as Delhi, Mumbai, and Kolkata. The global impact of urban pollution is also discussed in their study. Upadhyay, Dey, Chowdhury, et al. (2018) evaluated the major anthropogenic sources of PM2.5 and the potential benefits to human health, if sufficient control measures are applied to curb emissions. A recent study by Jat, Gurjar, and Lowe (2021) examined the extent of pollution during the winter months in India. The study used a WRF-Chem model, that is, Weather Research and Forecasting (WRF) coupled with chemistry, to evaluate the concentrations of pollutants, such as PM2.5, oxides of sulphur (SOX), oxides of nitrogen (NOX), black and organic carbons, and non-methane volatile organic carbons (NMVOCs) that were identified for the winter months. The various sources of air pollution can be classified into seven major sources and the consequent effects are discussed in this article.
Sources of air pollution
The various sources of air pollution are classified into seven major sectors, which include transportation, industries, agriculture, power, waste treatment, biomass burning, residential, construction, and demolition waste.
The transportation sector is the main contributor of air pollutants in almost every city, but this phenomenon is worse in urban cities (Gurjar, Aardenne, Lelieveld, et al. 2004). This could be due to the increased number of vehicles when compared to the existing infrastructural facilities, e.g., roads, fuel stations, and the number of passenger terminals provided for public transport. In India, the amount of motorized transport increased from 0.3 million in 1951 to 159.5 million in 2012 (Gurjar, Ravindra, and Nagpure 2016). A significant share of vehicular emissions comes from urban cities, such as Delhi, Mumbai, Bengaluru, and Kolkata. Carbon monoxide (CO), NOX, and NMVOCs are the major pollutants (>80%) from vehicular emissions (Gurjar, Aardenne, Lelieveld, et al. 2004). Other trace emissions include methane (CH4), carbon dioxide (CO2), oxides of sulphur (SOx), and total suspended particles (TSPs).
In an urban environment, road traffic emissions are one of the prime contributors of air pollution. Road dust is a major contributor to PM emissions in Delhi (37%), Mumbai (30%), and Kolkata (61%). Road transport is the largest source of PM2.5 in Bengaluru (41%), Chennai (34%), Surat (42%), and Indore (47%) (Nagpure, Gurjar, Kumar, et al. 2016). In the Indian context, some of the essential factors of high traffic emissions include extreme lack of exhaust measures, the highly heterogeneous nature of vehicles, and poor quality of fuel.
Over the last few decades, India has witnessed large-scale industrialization. This has degraded the air quality in most urban cities. The Central Pollution Control Board (CPCB) has categorized the polluting industries into 17 types, which fall under the small and medium scale (Gurjar, Ravindra, and Nagpure 2016). Out of these categories, seven have been marked as ‘critical’ industries that include iron and steel, sugar, paper, cement, fertilizer, copper, and aluminium. The major pollutants comprise SPM, SOX, NOX, and CO2 emissions.
The small-scale industries, which are not regulated like the major industries, use several energy sources apart from the primary source of state-provided electricity. Some of these fuels include the use of biomass, plastic, and crude oil. These energy sources are neglected in the current emission inventory studies. In Delhi, after the intervention of the judiciary in 2000, many industries were relocated from urban areas to adjacent rural areas (Nagpure, Gurjar, Kumar, et al. 2016). In Delhi, a major fraction of the pollution load comes from the brick manufacturing industries, which are situated at the outskirts of the city. Rajkot (42%) and Pune (30%) are the two cities where industries play a prominent role in contributing to the highest amount of PM2.5 (Nagpure, Gurjar, Kumar, et al. 2016).
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