The introduction of compounds that are harmful to the environment is termed as pollution. Polluted air is a harmful combination of primary and secondary pollutants. The U.S Environmental Protection Agency (USEPA) listed the six most common pollutants as criteria air pollutants under the Clean Air Act (1970) viz. particulate matter (PM), tropospheric ozone (O3), carbon monoxide (CO), nitrogen dioxide (NO2), sulfur dioxide (SO2), and lead (Pb).

According to the origin and target criterion, such pollutants are categorized as primary and secondary pollutants. Primary pollutants are emitted directly from their sources, while secondary pollutants are either formed by the atmospheric transformation of primary pollutants or other chemical compounds.

primary vs secondary pollutants (types of pollutants)

There is a whole variety of pollutants that can be either natural or synthetic. Primary pollutants directly affect human beings while secondary pollutants affect humans indirectly, through their effect on the ecosystem.

The identifications and the differences of primary and secondary criteria pollutants are very important in the control and prevention of atmospheric pollution.

Difference between primary and secondary pollutants

Primary pollutants Secondary pollutants
Primary pollutants are directly emitted from the sources to the atmosphere Secondary pollutants are the result of chemical and photochemical reactions of primary pollutants
Primary pollutants can affect living things directly and indirectly (by forming secondary pollutants) Secondary pollutants can affect living things directly. Athough, they can also affect indirectly via some intermediate
They are usually unstable pollutants They are usually stable or inert
These pollutants damage living organisms These pollutants damage the ecological system
These pollutants can be control by reducing anthropogenic emissions These pollutants are complicated to stop because of the interlinked chemical reactions
Examples of primary pollutants are; particulate matter (PM), hydrocarbons (CH), carbon monoxide (CO), sulfur dioxide (SO2), nitrogen oxides (NOx), volatile organic compounds (VOCs), and heavy metals, etc
Examples of secondary pollutants are; tropospheric ozone (O3), peroxy nitrates (PAN), acid rain, nutrient enrichment compounds, photochemical oxidants, secondary particulate matter (SPM), etc

Primary Pollutants

Primary pollutants are raw materials for further pollutants formation. The most common primary pollutants are nitrogen oxides (NOx), sulfur oxides (SOx), carbon monoxide (CO), particulate matter (PM), volatile organic compounds (VOCs), ammonia (NH3), carbon dioxide (CO2), di-hydrogen sulfide (H2S), mercury (Hg), and lead (Pb). In addition to these five common ones, there are dozens of other types of pollutant gases.

Primary pollutants are directly emitted into the air from human or animal activities. According to the United States Environmental Protection Agency (USEPA), primary pollutants are responsible for about 75% of all air pollution and approximately 90% of overall water pollution in the United States.

Anthropological aspects of primary Pollutants

1. Chemical industries

A major source of such chemical substances comes from various chemical industries where combustion reactions occur on a large scale. For example, power plants driven by fossil fuels, oil refineries, etc.

2. Vehicular emissions

Cars and bikes release a large number of hazardous gases into the environment.

3. Bio pollutants

Among bioforms of primary pollutants, microorganisms living in soil and aquatic ecosystems produce exudates through their life cycle which contain organic matter. Bacteria secrete organic compounds known as exudates while fungi produce mycelium made up of chitin.

Bacterial secretions include alkaloids, amines, amino acids, etc while fungi secrete enzymes.

Related topics

Secondary Pollutants

Secondary pollutants are formed when primary pollutants react with other gases or particles. Such transformations always occur in an open atmosphere. They can be just as harmful to people and the environment as primary pollutants.

Secondary pollutants, unlike primary pollutants, do not become immediately visible in the materials they damage. Eventually, they build up to cause further changes. This accumulation often happens invisibly and may take years before its effect is noticed. It is difficult to link back to the source of this pollution.

Anthropological aspects of secondary Pollutants

Excess sulfur dioxide and nitrogen oxides from coal-fired power plants cause acid rain. Acid rain is made of dilute sulfuric and nitric acids that damage crops, trees, aquatic life, buildings, etc.

When mercury (Hg) released by industries falls into rivers, it forms methylmercury radical (CH3Hg+), which then becomes concentrated in fish tissues. It is highly poisonous and extremely toxic to humans.

Effects of secondary pollutants

Secondary pollutants can either be chemical or biological substances. They harm our environment and cost us dearly. For instance;

1. Acid rain

Chemical substances combine to make other chemicals like nitric acid or sulfuric acid, which then dissolve with rainwater to become acid rain. This harms trees and causes human health problems. Acidic rains worsen the level of air pollution which may result in the increase in heart disease.

2. Tropological ozone

It forms when sunlight chemically reacts with cars’ carbon monoxide and nitric oxide gases. When our skin absorbs UV rays from these gases, we develop a sunburn. The long-term effects of being exposed to UV rays include skin cancer.

3. Nutrient enrichment compounds

Compounds rich in nitrogen and phosphorous are the main culprits of eutrophication (disruption of the chemical balance of water bodies).

Categories of pollutants

  1. Natural Pollutants (CO, SO2, Pb, Hg, trace elements, etc)
  2. Synthetic pollutants (pesticides, detergents, pharmaceuticals, organic acids, etc)
  3. Non-degradable pollutants (aluminum cans, mercurial salts, DDT, nuclear wastes, etc)
  4. Bio-degradable pollutants (phosphates, carbonates, nitrates, etc)
  5. Particulate pollutant materials (smoke, dust, mist, fog, aerosol, etc)

Sources of primary and secondary pollutants

1. Stationary sources of pollutants

Chimneys of power plants, manufacturing facilities, waste, etc are stationary sources of pollutants.

2. Natural sources of pollutants

Forest fires, emissions from trees, volcanic eruptions, erosion activities, radioactive decay, methane, dust, etc are natural sources.

3. Anthropogenic activities

Transportation, biomass burning, crop residue burning, industrial activities, vehicular emissions, mining, etc are some anthropological activities.

Effects of primary and secondary pollutants

Air pollutants

There are different effects of air pollutants on the environment.

  • Photochemical smog
  • Acid deposition (Acid rain)
  • Greenhouse effect
  • Ozone depletion, etc

Water pollutants

There are the following effects of water pollutants.

  • Eutrophication of aquatic ecosystem
  • Oxygen depletion
  • Metal toxicity
  • Heat, etc

Soil pollutants

Herbicides, insecticides, halogens, fungicides, polymers are different types of soil pollutants. They affect soil as:

  • Soil erosion
  • Loss of fertility
  • Accumulation of pollutants
  • Leaching of harmful contaminants, etc

In short, global climate change is a major effect of pollutants.

Key Differences (primary vs secondary pollutants)

Primary and Secondary pollutants - Difference table - image

Concepts Berg

What are the most dangerous primary and secondary pollutants?

Both primary and secondary pollutants are dangerous. Although, primary pollutants exceed in hazards than secondary ones.

What is a primary pollutant?

Primary pollutants are emitted from a source. These pollutants may be chemicals, biological materials, or even radionuclides. Primary pollutants enter into the air, water, and soil at their point of emission and travel throughout the ecosystem. For example, carbon monoxide, sulfur dioxide, etc.

What is a secondary pollutant?

A secondary pollutant is a contaminant. These contaminants are generally the products of chemical reactions between atmospheric gases and chemicals decomposition. Secondary pollutants are usually not harmful to humans but can cause ecological damage.

Secondary pollutants include sulfates (from sulfur dioxide), nitrates (from nitrogen oxides), organic compounds, ammonia, and carbonaceous matter. The carbonaceous matter is generally burned soot from fossil fuels like oil or coal and causes acid rain when it falls on earth in rainwater.

What are the 5 primary pollutants?

  • Air pollutants
  • Noise pollutants
  • Water pollutants
  • Thermal pollutants
  • Soil pollutants

Is CO2 a secondary pollutant?

Carbon dioxide is considered as a primary pollutant, as decided by the US Supreme Court.

What are examples of primary and secondary pollutants?

There are different examples of primary and secondary pollutants.

Examples of primary pollutants

  • Carbon monoxide CO
  • Sulfur dioxide SO2
  • Trace elements

Examples of secondary pollutants

  • Acid rain
  • Ozone
  • Photochemical materials

Is nitrogen dioxide a primary or secondary pollutant?

Nitrogen dioxide is a secondary pollutant, as it is produced by interactions between other pollutants. Nitrogen dioxide is a side effect of fuel combustion and cannot be emitted directly into the air. The majority of nitrogen oxides are formed through interactions with volatile organic compounds, such as benzene, toluene, and xylene.

Nitrogen oxides have been described as toxic in high concentrations and some studies have suggested that long-term exposure can lead to asthma or cardiovascular disease. These reactions also form ozone, another pollutant (topological pollutant) that has been shown to negatively affect human health.

What are pollutants?

Pollutants are the building blocks of pollution. There are different types of pollutants. Gases, aerosols, heat, smoke, ash, soot, ozone, and odor molecules can all be considered air pollutants.

Water pollutants are chemical substances that are present in water in concentrations higher than usual. Usually, they cause harm or reduce the usefulness of water by lowering its quality and may include dissolved mineral salts and man-made chemicals including pesticides or herbicides.

What are the harmful components of smog?

Harmful elements contained within smog include nitrogen dioxide, hydrocarbons, carbon monoxide, sulfur dioxide, other organic chemicals, etc.

What is photochemical pollution?

Photochemical pollution is caused by sunlight reacting with pollutants and natural compounds in the earth’s atmosphere.

When gasoline-powered engines burn fossil fuels like gasoline or diesel fuel to make energy, they emit chemicals like nitrogen oxides into Earth’s atmosphere. While nitrogen oxides do not stay in our atmosphere for very long, they can react with other chemicals and form tiny particles called aerosols. They remain in Earth’s lower atmosphere for months to years. Aerosols reflect some of the sun’s energy back into space, which keeps our planet cool.

What are the different types of air pollution?

There are different types of air pollution that affect the environment:


  • Criteria Air Pollutants and their Impact on Environmental Health by Pallavi Saxena (DES, Hindu College, University of Delhi, New Delhi, India), Saurabh Sonwani (SES, Jawaharlal Nehru University, New Delhi, India)
  • Primary Vs. Secondary Pollutants (University of California, Riverside)