Pollutants

What is a Pollutant

The definition of a pollutant, in simple words, is something that pollutes. A pollutant is a substance which harms, adversely affects or contaminates the environment in such a way that the quality of air, soil, water or food is compromised.

Primary pollutants are those released directly from a source, such as sulphur dioxide emitted by industries, while secondary pollutants form when primary pollutants react in the atmosphere. For example, acid rain, a secondary pollutant, is created when sulphur dioxide or nitrogen oxides combine with water in the air.

In the case of M.C. Mehta v. Union of India (1996), the acidic emissions of sulphur dioxide from industries near the Taj Mahal led to acid rain that corroded the marble structure, illustrating how primary pollutants can indirectly cause environmental damage through secondary pollutants.

Pollutants can also be classified based on their state of matter. Solid pollutants like microplastics and dust harm ecosystems by contaminating water bodies and the air. Liquid pollutants, such as acids, fertilizers, and oil spills, cause soil and water pollution. Gaseous pollutants, including greenhouse gases like CO₂ and CH₄, as well as toxic gases like ammonia and hydrogen sulphide, contribute to air pollution and pose serious risks to both the environment and human health. Together, these classifications help in understanding how pollutants affect ecosystems in various forms.

Official Definition of Pollutant

As defined in Legislation

Pollutant as defined in the The Environment (Protection) Act, 1986

Section 2(b) of the Environment (Protection) Act, 1986 defines “environmental pollutant” as any solid, liquid or gaseous substance present in such concentration as may be, or tend to be, injurious to the environment”.

Pollutant as defined in the context of air pollution in the Air (Prevention and Control of Pollution) Act, 1981

Section 2(a) of the Air (Prevention and Control of Pollution) Act, 1981 defines an air pollutant as any solid, liquid or gaseous substance  [(including noise)] present in the atmosphere in such concentration as may be or tend to be injurious to human beings or other living creatures or plants or property or environment”. ‘Emission’ is defined under Section 2(j) as any solid or liquid or gaseous substance coming out of any chimney, duct or flue or any other outlet.

Pollutant as defined in the context of water pollution in the the Water (Prevention and Control of Pollution) Act, 1974

Section 2(e) of the Water (Prevention and Control of Pollution) Act, 1974 defines pollutants as any sewage or trade effluent or of any other liquid, gaseous or solid substance into water (whether directly or indirectly) as may, or is likely to, create a nuisance or render such water harmful or injurious to public health or safety, or to domestic, commercial, industrial, agricultural or other legitimate uses, or to the life and health of animals or plants or of aquatic organisms’.

Further, sewage effluent and trade effluent, two common pollutants found in water are described in Section 2(g) and 2(k) wherein Section 2(g) defines sewage effluent as effluent from any sewerage system or sewage disposal works and includes sullage from open drains, and Section 2(k) defines trade effluent as any liquid, gaseous or solid substance which is discharged from any premises used for carrying on any  [industry, operation or process, or treatment and disposal system], other than domestic sewage.

As defined in Legal Provisions

Pollutants are classified as ‘waste’ in various legal provisions wherein the disposal or emission of the waste results in a harmful effect to the nearby surroundings and the degradation of the environment.

Plastic Waste Management Rules, 2016

Section 3(q) of the Plastic Waste Management Rules, 2016 defines plastic waste as any plastic discarded after use or after their intended use is over.

E-waste (Management) Rules, 2016

Section 3(r) of the E-waste (Management) Rules, 2016 defines 'e-waste' as electrical and electronic equipment, whole or in part discarded as waste by the consumer or bulk consumer as well as rejects from manufacturing, refurbishment and repair processes.

Bio-Medical Waste (Management and Handling) Rules, 2016

Section 3(f) of the Bio-Medical Waste (Management and Handling) Rules, 2016 defines biomedical waste as any waste, which is generated during the diagnosis, treatment or immunisation of human beings or animals or research activities pertaining thereto or in the production or testing of biological or in health camps.

Hazardous And Other Wastes (Management and Transboundary Movement) Rules, 2016

Section 3(17) of the Hazardous And Other Wastes (Management and Transboundary Movement) Rules, 2016 defines hazardous waste as any waste which by reason of characteristics such as physical, chemical, biological, reactive, toxic, flammable, explosive or corrosive, causes danger or is likely to cause danger to health or environment, whether alone or in contact with other wastes or substances.

As defined in International Instruments

Convention On Long-Range Transboundary Air Pollution 1979

The 1979 Convention On Long-Range Transboundary Air Pollution (1979) defines ‘air pollutants’ as “substances or energy which when released into the air result in deleterious effects of such a nature as to endanger human health, harm living resources and ecosystems and material property and impair or interfere with amenities and other legitimate uses of the environment”.

United Nations Convention on the Law of the Sea

The United Nations Convention on the Law of the Sea with regards to marine pollution defines pollutants as “substances or energy into the marine environment, including estuaries, which results or is likely to result in such deleterious effects as harm to living resources and marine life, hazards to human health, hindrance to marine activities, including fishing and other legitimate uses of the sea, impairment of quality for use of sea water and reduction of amenities”.

Stockholm Declaration of 1972

Principle 7 of the Declaration of the United Nations Conference on the Human Environment states that, “States shall take all possible steps to prevent pollution of the seas by substances that are liable to create hazards to human health, to harm living resources and marine life, to damage amenities or to interfere with other legitimate uses of the sea.”

The Stockholm Convention on Persistent Organic Pollutants (2004) is a global treaty to protect human health and the environment from chemicals that remain intact in the environment for long periods, become widely distributed geographically, accumulate in the fatty tissue of humans and wildlife, and have harmful impacts on human health or on the environment.

Minamata Convention on Mercury

The Minamata Convention on Mercury is a global treaty adopted in 2013 to protect human health and the environment by controlling the entire lifecycle of mercury as a pollutant. It requires Parties to phase out mercury-added products and processes, regulate its trade, manage waste, and reduce emissions from sources like coal-fired power plants, cement production, and small-scale gold mining. It entered into force in 2017 and has been ratified by over 150 countries.

As defined in case laws

Subhash Kumar v. State of Bihar (1991)

In Subhash Kumar v. State of Bihar, the Supreme Court expanded the definition of Article 21 to include the right to have a clean environment, specifically the right to have clean drinking water to the residents living around the Bokaro river, which was being polluted by discharge of pollutants such as slurry/sludge from the washeries of the Tata Steel and Co. plant.

Vellore Citizens Welfare Forum v. Union of India (1996)

The Supreme Court’s judgment in the case of Vellore Citizens Forum v. Union of India, was a landmark decision addressing the pollution caused by tanneries and other industries in Tamil Nadu. The court recognised the severity of the pollution and the urgent need for action to protect the environment and the health of the residents.

Indian Council for Enviro-Legal Action v. Union of India (1996)

The Court in Indian Council for Enviro-Legal Action v. Union of India (1996) ordered the closure of several hazardous industries and factories in Bichhri Village that caused the environmental pollution and public health crises. These chemical factories operated without the necessary permits, producing substances such as oleum and H-acid, which are known for their toxic properties. The factories disposed of untreated hazardous waste in the village’s vicinity, leading to widespread contamination of soil and groundwater. It emphasized that industries operating without necessary permits must cease operations immediately.

M.C Mehta v. Union of India (Oleum Gas Leak Case) (1986)

In M.C Mehta v. Union of India (Oleum Gas Leak Case) (1986) the court emphasized that the oleum gas leak from a factory, which led to the death of one person and the hospitalisation of several others, was also a pollutant in the nearby surroundings that led to environmental degradation and had a profound effect on people’s public health.

Appearance in Official Database

NAQI Database

National Air Quality Index showing the AQI Data in a particular state, city and station

Pollutants examined in the NAQI - PM2.5, PM10, NO2, NH3, SO2, CO, Ozone

Central Pollution Control Board Dashboard - Real Time Water Quality Monitoring System

The RTWQMS Dashboard is an online portal maintained by CPCB to display real-time water quality data from various monitoring stations across India.

The dashboard is integrated under CPCB’s WQM (Water Quality Monitoring) related portals. It measures the water quality in terms of parameters such as Dissolved Oxygen, Biochemical Oxygen Demand, chloride, nitrate etc.

Pollutants examined in the water quality : Biomedical Oxygen Demand, Dissolved Oxygen, Nitrate, Chloride, Chemical Oxygen Demand, Total Organic Carbon. Accessible at: https://rtwqmsdb1.cpcb.gov.in/#/

Continuous Tracking of Ambient PM2.5 and PM10 concentration in Air - Central Pollution Control Board Dashboard

The Central Pollution Control Board (CPCB) continuously tracks ambient PM₂.₅ and PM₁₀ concentrations through its network of Continuous Ambient Air Quality Monitoring Stations (CAAQMS) established across Indian cities under the National Ambient Air Quality Monitoring Programme (NAMP). These stations record pollutant levels in real time and transmit hourly averages of PM₂.₅, PM₁₀, and other key parameters like NO₂, SO₂, O₃, and CO to CPCB’s central servers.

Central Control Room for Air Quality Management - Delhi NCR which tracks PM2.5 and PM10 across a 24 hour window.

Central Pollution Control Board Dashboard - Industrial Pollution - Online Emission and Effluent Monitoring System

Online Continuous Emission and Effluent Monitoring System (OCEMS) is an initiative by the Central Pollution Control Board (CPCB) to continuously track and regulate industrial pollution in India. These instruments continuously measure key parameters like Particulate Matter (PM), SO₂, NOx, CO, pH, Total Suspended Solids (TSS), and heavy metals, depending on the process type.

Delhi Pollution Control Committee: Online Emission and Effluent Monitoring System

Official Programmes

Online Continuous Emission and Effluent Monitoring System (OCEMS)[1]

OCEMS refers to systems that continuously monitor and record the emission of pollutants and effluents from industrial units. It collects real-time data on air and water pollution parameters such as particulate matter, sulphur dioxide (SO₂), nitrogen oxides (NOx), carbon monoxide (CO), and other pollutants, depending on the type of industry.

National Air Quality Monitoring Programme (NAMP) 

To check air quality of the country, Central Pollution Control Board initiated National Air Quality Monitoring Programme (NAMP) under which three air pollutants viz., Sulphur Dioxide (SO2), Nitrogen Dioxide (NO2) and Particulate Matter size equal to or less than 10 micron (PM10), have been monitored regularly at all the locations.

Other parameters like PM2.5 (Particulate Matter having an aerodynamic diameter less than or equal to 2.5 µm), Carbon monoxide (CO), Ammonia (NH3), Lead (Pb), Ozone (O3), Benzene (C6H6), Benzo(a)pyrene (BaP), Arsenic (As) and Nickel (Ni) are being monitored at selected locations and are slowly being added to the monitoring network under NAMP.

Continuous Ambient Air Quality Monitoring Stations (CAAQM)

Continuous Ambient Air Quality Monitoring System (CAAQMS) is a specialized system housed in a temperature controlled container and is equipped with all necessary analysers required for ambient air quality monitoring. This system generates real time data and can be remotely managed. The network of Continuous Ambient Air Quality Monitoring Stations (CAAQMS) is spanning over Delhi at present, 40 CAAQM stations covering NCT of Delhi are existing. Under CAAQMS the Particulate Matter (PM10& PM2.5), Sulphur Dioxide (SO2), Nitrogen Dioxide (NO2), Ammonia (NH3), Carbon Monoxide (CO), Ozone (O3) and Benzene (C6H6) are being monitored at all locations. The CAAQMS are also equipped with sensors to measure meteorological parameters such as Wind Speed, Wind Direction, Ambient Temperature, Relative Humidity, Solar Radiation and Rainfall.

National Ambient Air Quality Standards (NAAQS)

The National Ambient Air Quality Standards (NAAQS) prescribe the permissible concentration of key pollutants in ambient air. For example, for sulphur dioxide (SO₂) the limit is 50 µg/m³ annually and 80 µg/m³ over 24 hours; for nitrogen dioxide (NO₂) it is 40 µg/m³ annually and 80 µg/m³ over 24 hours. Particulate matter PM10 is limited to 60 µg/m³ annually and 100 µg/m³ over 24 hours, while PM2.5 is restricted to 40 µg/m³ annually and 60 µg/m³ over 24 hours.

Official Reports

The annual official reports of the Central Pollution Control Board (CPCB) 2022-23 describes the board's functions and monitors various forms of pollution and pollutants. It also describes the National Water Quality Monitoring Programme (NWMP).

• Monitoring Stations: Over 1,500 monitoring stations across India.
• Parameters Monitored: 28 water quality parameters, including pH, DO, BOD, COD, TSS, and heavy metals.

The CPCB also has reports on Water Quality of river Beas for the year 2023, Status of Water Quality of River Ganga at Interstate Boundaries (April- 2024) and the annual report on ash generation and utilization by coal or lignite based thermal power plants during FY 2022-23.

• Total Ash Generation: Approximately 158.08 lakh metric tonnes (LMT) of ash generated by thermal power plants during the financial year.
• Utilization:
  • A significant portion of the generated ash was utilized in various applications, including cement manufacturing, road construction, and brick production.
  • The utilization rate varied across different plants, with some achieving higher utilization percentages.

• Challenges:
  • Some thermal power plants reported ash utilization in activities like raising ash dykes, which may not align with the intended utilization guidelines.
  • Efforts are ongoing to ensure accurate reporting and compliance with utilization norms.

The report on Ambient Air Quality Status of the Country during 2023 (Integrated data in microgram per cubic meter)by the Central Pollution Control Board (CPCB) describes monitoring of air pollutants being carried out with the help of State Pollution Control Boards (SPCB), Pollution Control Committees (PCC) and other reputed institutes.

• Vishakhapatnam, Begusarai, Brynihat, Sri Ganganagar are some amongst many cities that have higher than annual average levels of PM10
• Pune, Karnal, Jamshedpur, Kolar have higher than annual average levels of SO2
• Arrah, Katihar, Motihari, Delhi have higher than annual average levels of NO2
• Delhi, Patna, Surat, Gurgaon have higher than annual average levels of PM2.5

The Annual Report 23-24 of the Commission for Air Quality Management in National Capital Region and Adjoining Areas outlines the Commission’s actions to monitor, regulate, and improve air quality in Delhi–NCR, highlighting enforcement drives, greening initiatives, policy measures, and progress in implementing pollution-control strategies during the year.

The Continuous Ambient Air Quality Monitoring Stations (CAAQM) have an inventory of major point air pollution sources in Delhi Hotspots and other priority areas.

• Transport, domestic burning and road dust were the three highest contributors of PM2.5 reported in three recent emission inventories in Delhi-NCR.
• Delhi has a total of 40 CAAQM Stations.

International Experiences

Canada

The National Pollutant Release Inventory [NPRI] is Canada’s inventory of pollutants released to the air, water and land. It also includes pollutants that were disposed of or recycled. The inventory tracks almost 500 substances. It is managed by Environment and Climate Change Canada.

Anyone can access the inventory. It includes information that is useful for governments, researchers, the media and the public.

United States of America

The Toxics Release Inventory (TRI) tracks the management of more than 800 chemicals and chemical categories that may pose a threat to human health and the environment. U.S. facilities in certain industry sectors that manufacture, process, or otherwise use these chemicals in amounts above established levels must report how each chemical is managed through recycling, energy recovery, treatment, and environmental releases.  The information facilities submit to the Environmental Protection Agency [EPA] is compiled annually as the Toxics Release Inventory or TRI.  

The Clean Air Act requires EPA to set National Ambient Air Quality Standards (NAAQS) for six commonly found air pollutants known as criteria air pollutants. These pollutants are for the ozone, particulate matter, carbon monoxide, lead, sulphur dioxide, and nitrogen dioxide.

European Union

The monitoring of emissions to air and water represents an important element in preventing and reducing pollution from industrial installations and in ensuring a high level of protection of the environment as a whole. Therefore, the Industrial Emissions Directive 2010/75/EU (IED) addresses the monitoring of emissions in a number of instances.

This JRC Reference Report on Monitoring (ROM) summarises information on the monitoring of emissions to air and water from IED installations, thereby providing practical guidance for the application of the Best Available Techniques (BAT) conclusions on monitoring in order to help competent authorities to define monitoring requirements in the permits of IED installations.

Around 60,000 large industrial facilities are operating in the EU. Under the Industrial Emissions Portal Regulation (IEPR)​, industrial operators for these sites are required to report on significant emissions and the use of resources.

The EU aims to improve transparency in data reporting by collecting and disseminating information on the amounts of industrial pollutant releases, off-site transfers of waste and pollutants in wastewater, as well as the consumption of energy, water and key raw materials.

The Industrial Emissions Portal Regulation (IEPR) replaces the European Pollutant Release and Transfer Register Regulation (E-PRTR). It provides public access to key environmental data from industrial facilities in EU Member States, Iceland, Liechtenstein, Norway, Switzerland, Serbia and the UK through the Industrial Emissions Portal (the Portal).

Australia

The National Pollution Inventory [NPI] contains data on 93 substances that have been identified as important due to their possible effect on human health and the environment. The data comes from facilities like mines, power stations and factories, and from other sources such as households and transport.

Facility operators determine their own emissions and transfers, and diffuse emissions from households and other sources like motor vehicles are estimated by government agencies.

Japan

PRTR system (Pollutant Release and Transfer Register) - A system in which a business operator identifies the amounts of a hazardous chemical specified by the law to be released to the environment and amounts of the chemical contained in waste that is to be transferred, notified to the authority, and the information will be released by the government.

China

On January 29, 2021, the China State Council issued Regulations on Management of Pollutant Discharge Permits with an effective date of March 1, 2021 (Regulations). These Regulations complete legislative efforts on the national pollutant discharge permit system and specify compliance obligations for enterprises with operations in China that discharge pollutants in the form of air emissions, water discharges, or wastes.

The China Environmental Protection Law sets out a legal foundation for the establishment of the national pollutant discharge permit system, and both the Air Pollution Prevention and Control Law and the Water Pollution Prevention and Control Law authorize the State Council to draft and issue implementation measures for the pollutant discharge permit system. In November 2016, the State Council issued a policy document, the Implementation Plan of the Permit System for Controlling Pollutant Discharge (Plan), to kick off the evolution of the national pollutant discharge permit system.

What India can take away from these international experiences

India does not have a Pollutant Release and Transfer Register (PRTR) like other countries. Instead, it relies on the Online Continuous Emissions/Effluents Monitoring Systems (OCEMS), managed by the Central Pollution Control Board (CPCB). Industries in the “highly polluting” categories (17 red industries) are required to install Continuous Emission Monitoring Systems (CEMS) and Continuous Effluent Quality Monitoring Systems (CEQMS) to report air and water pollution data in real time to CPCB and State Pollution Control Boards.

However, the system is self-regulated: industries choose their own equipment, calibrate it themselves, and report data without third-party verification. CPCB neither certifies nor assesses the monitoring equipment.

A major concern is the lack of transparency. Out of 28 states and 8 union territories, CEMS links exist for only 18 states and one UT, with only 7 states providing real-time data to the public—and even those datasets are inconsistent and not in open, machine-readable formats. Key industrial regions like Uttar Pradesh, Jharkhand, and Chhattisgarh are excluded from public reporting, indicating significant gaps in accountability and accessibility.

India’s Carbon Pricing System

India is developing a carbon pricing system to meet its climate goals and promote sustainable practices. The government has introduced a rate-based Emissions Trading System (ETS) and a voluntary carbon credit market, with the Carbon Credit Trading Scheme (CCTS) launched in July 2024. The scheme covers nine energy-intensive sectors such as power plants, refineries, and cement industries. Unlike absolute emission caps, it focuses on reducing emissions intensity, rewarding industries that exceed benchmark targets.

The Energy Conservation (Amendment) Act, 2022 provides the legal foundation for carbon credits, while other initiatives support the broader climate agenda. The National Green Hydrogen Mission promotes green hydrogen production by using carbon credit methodologies, and the Perform, Achieve, and Trade (PAT) scheme helps industries improve energy efficiency. Additionally, India’s goal of achieving 500 GW of non-fossil fuel capacity by 2030 strengthens its renewable energy transition.

Research that engages with Pollutants

Environmental Justice in India: Incidence of Air Pollution from Coal-Fired Power Plants

This research paper by Jacob Kopas et al., investigates how emissions from coal-fired power plants in India are unequally distributed across socioeconomic and ethnic groups, applying models of pollutant dispersion combined with demographic data. The authors estimate which populations bear the greatest exposure to air pollution from coal plants and find that ethnic minorities and the poor are disproportionately exposed.

Decoding Green Justice: An AI-Assisted Exploration of Indian Environmental Rulings over Three Decades

In this research paper, Behrer et al. (2025) compile and analyze a dataset of 12,615 judicial rulings related to environmental law in India using large language models alongside human coders to classify whether decisions are “pro-environmental.” They find that GPT-4 can approximate human judgments with over 70 % accuracy, though it tends to classify more rulings as “green” than humans do. The work shows both the potential and the limitations of AI in legal analysis, and provides a novel resource for further empirical research on environmental jurisprudence in India.

Public Health

Estimating the effect of annual PM2·5 exposure on mortality in India [2] - The study critiques prior estimates of air-pollution mortality in India that rely on exposure–response functions from places with relatively low pollution, arguing these may misestimate risk in high-pollution settings. The authors use annual mortality counts at district level across India from 2009 to 2019, and high-resolution PM₂.₅ exposure models, to apply a quasi-experimental “difference-in-differences” design to estimate the causal effect of changes in long-term PM₂.₅ exposure on all-cause mortality within India. They find that a 10 µg/m³ increase in annual average PM₂.₅ is associated with an 8.6% increase in mortality. Over the study period, they estimate that ~3.8 million deaths could be attributed to exposures above India’s National Ambient Air Quality Standards, and ~16.6 million deaths relative to the stricter WHO guidelines. The exposure–response curve is near‐linear at moderate levels, with some leveling off at extremely high exposure levels.

Environmental Justice & Inequality

Only seven countries are meeting an international air quality standard, with deadly air pollution worsening in places due to a rebound in economic activity and the toxic impact of wildfire smoke, a report in 2024 had found. Of 134 countries and regions surveyed in the report, only seven – Australia, Estonia, Finland, Grenada, Iceland, Mauritius and New Zealand – are meeting a World Health Organization (WHO) guideline limit for tiny airborne particles expelled by cars, trucks and industrial processes, PM2.5. [3]

This gives rise to a concept called the Global Air Quality Inequality as elaborated by Lutz Sager. The global PM2.5 Gini Index increased from 0.32 in 2000 to 0.36 in 2020, exceeding levels of income inequality in many countries. Air quality inequality is mostly driven by differences between countries and less so by variation within them, as decomposition analysis shows. A large share of people facing the highest levels of PM2.5 exposure are concentrated in only a few countries such as India, Pakistan, Bangladesh, China, and Saudi Arabia.

Pollutant Exposure and Caste

Researchers have empirically demonstrated that regions with higher prevalence of Scheduled Caste (SC) and Other Backward Class (OBC) households experience significantly higher ambient PM2.5 exposure. Each standard deviation increase in the presence of SC or OBC populations correlates with measurable increases in total PM2.5 levels.

Manual scavenging is overwhelmingly performed by Dalits which leads to this community being exposed to higher levels of certain pollutants normally found in sewages than others.

Studies connect hazardous industrial zones like chemical plants, power stations, thermal effluent clusters to spatial caste inequity. For example, Scheduled Tribe communities suffer around 78% more exposure to coal-related emissions compared to non-ST communities. [4]