Pollution: Air, Water, Soil, and Plastic Debris

Pollution is the introduction of substances or energy into the environment at levels that harm living things or ecosystems, and the main types are air pollution, water pollution, soil pollution and plastic debris. In brief: pollutants come from point sources (a factory pipe) and non-point sources (runoff from farmland). Some, such as DDT, mercury and PCBs, accumulate in organisms and become more concentrated at each trophic level (biomagnification), while nutrients such as nitrogen and phosphorus cause eutrophication in water.

Pollution harms human health and ecosystems, and the WHO attributes millions of premature deaths a year to outdoor and household air pollution. This guide describes the sources and effects of the main pollutants, explains eutrophication, bioaccumulation and plastic pollution, compares the types in a table, works through a biomagnification calculation and corrects common mistakes.

What causes air pollution?

Air pollution comes from burning fossil fuels and biomass, industry, vehicles, agriculture and natural sources such as volcanoes and dust. The main pollutants are:

• Particulate matter (PM_2.5 and PM₁₀): particles smaller than 2.5 or 10 micrometers. The fine particles penetrate deep into the lungs and the blood, and are linked with asthma, lung cancer and heart disease.
• Nitrogen oxides (NO_x): from engines and power stations. They form acid rain and, with sunlight and volatile organic compounds, ground-level ozone and photochemical smog.
• Sulfur dioxide (SO₂): from burning coal and oil. It causes acid rain, which damages forests, lakes and stonework. Emission controls greatly reduced acid rain in Europe and North America.
• Carbon monoxide (CO): from incomplete combustion. It binds to hemoglobin more strongly than oxygen and reduces its transport.
• Ozone-depleting substances: chlorofluorocarbons (CFCs) destroyed stratospheric ozone, which absorbs ultraviolet radiation. The Montreal Protocol (1987) phased them out.
• Lead and other toxic metals, and indoor smoke: from cooking with solid fuels, which affects billions of people.

Note that carbon dioxide and methane are greenhouse gases that cause climate change, which is discussed in a separate article, even though they are not toxic at normal concentrations.

What causes water pollution?

• Sewage: introduces pathogens and organic matter. Bacteria that decompose the organic matter use oxygen, and the biochemical oxygen demand (BOD) measures how much oxygen they consume. Untreated sewage can leave water without enough oxygen for fish.
• Fertilizer and manure runoff: nitrates and phosphates cause eutrophication: algae multiply (an algal bloom), then die, and decomposers use up the oxygen, creating a “dead zone”. The seasonal dead zone in the Gulf of Mexico has covered thousands of square kilometers in many years.
• Industrial waste: heavy metals (mercury, cadmium, lead), solvents and chemicals. In Minamata, Japan, methylmercury from a chemical plant contaminated fish and poisoned thousands of people in the 1950s.
• Oil spills: coat birds and mammals, and damage coastal ecosystems. Examples are the Exxon Valdez (1989) and Deepwater Horizon (2010).
• Thermal pollution: warm water from power plants holds less dissolved oxygen.
• Pharmaceuticals, pesticides and microplastics in wastewater.

What causes soil pollution?

Soil is contaminated by pesticides, heavy metals from mining and industry, leaking tanks, landfill leachate, excessive fertilizers, and salts from irrigation (salinization). Pollutants may be taken up by crops, enter groundwater or harm soil organisms. Remediation includes removal, treatment with microbes (bioremediation), and plants that take up metals (phytoremediation).

Why is plastic debris a problem?

Plastics are cheap, durable and widely used, and therefore accumulate: estimates suggest that several million tonnes of plastic waste enter the oceans each year, and a study published in 2015 (Jambeck and colleagues) estimated 4.8 to 12.7 million tonnes for the year 2010. Most plastics are not biodegradable, but break into smaller pieces. Microplastics are pieces smaller than 5 mm, which include fibers from clothing and beads from products. Effects include:

• entanglement of seabirds, turtles, seals and whales in nets and rope;
• ingestion by animals, which can block the gut and give a false feeling of fullness;
• microplastics have been found in water, soil, air, marine animals and human blood and tissues, but the effects on human health are still being investigated;
• plastics carry toxic additives and can adsorb pollutants;
• large accumulations in ocean gyres, such as the Great Pacific Garbage Patch.

What are bioaccumulation and biomagnification?

Bioaccumulation is the build-up of a substance in an organism over time, when intake is faster than excretion. It occurs with fat-soluble and persistent substances that are not easily broken down. Biomagnification is the increase in concentration at higher trophic levels in a food chain, since each consumer eats many contaminated organisms. DDT, an insecticide, caused thinning of eggshells in birds of prey such as peregrine falcons and bald eagles, and Rachel Carson’s book Silent Spring (1962) raised public awareness. DDT was banned for agricultural use in the United States in 1972, and bald eagle populations later recovered. Other examples are mercury in large fish such as tuna and swordfish, and PCBs in orcas.

Types of pollution compared

How can pollution be controlled?

• Prevention first: cleaner energy, efficient engines, less packaging, lower use of fertilizers.
• Treatment: catalytic converters on cars, flue-gas desulfurization at power stations, wastewater treatment plants, and filters.
• Regulation and agreements: clean air and water laws, the Montreal Protocol, bans on persistent chemicals under the Stockholm Convention (2001).
• Waste hierarchy: reduce, reuse, recycle, then recover energy, and dispose of what remains safely.
• Monitoring: using indicator species such as lichens (sensitive to sulfur dioxide) and mayfly larvae (sensitive to low oxygen), as well as chemical measurements.

Worked example: biomagnification

The concentration of a persistent pesticide is measured in a food chain in a lake. The values are illustrative and use micrograms per kilogram of body mass (µg/kg).

1. Factor at each step: 2 ÷ 0.5 = 4; 10 ÷ 2 = 5; 50 ÷ 10 = 5; 400 ÷ 50 = 8.
2. Overall magnification: 400 ÷ 0.5 = 800. As a check, 4 × 5 × 5 × 8 = 800.
3. Interpretation: the gull has 800 times the concentration in the phytoplankton. The pesticide is persistent and fat-soluble, so it is stored rather than excreted, and each predator eats many contaminated prey, mainly because energy is lost between levels, which means more food must be eaten.

How did pollution control develop?

The Great Smog of London in December 1952 was linked with thousands of deaths and led to the UK Clean Air Act of 1956. Rachel Carson’s Silent Spring was published in 1962, and the US Environmental Protection Agency was founded in 1970, followed by the Clean Air Act (1970) and the Clean Water Act (1972). Mario Molina, Sherwood Rowland and Paul Crutzen explained ozone depletion in the 1970s, which gave them the 1995 Nobel Prize in Chemistry, and the Antarctic ozone hole was reported in 1985. The Montreal Protocol was agreed in 1987, and the ozone layer is expected to recover around the middle of this century. Plastic pollution is the subject of negotiations for a global treaty that began in 2022.

Which mistakes are common?

Mistake 1: Confusing ozone depletion with global warming

Ozone depletion is caused mainly by CFCs and reduces the stratospheric ozone that filters ultraviolet light. Global warming is caused by greenhouse gases. They are different problems, though some chemicals affect both.

Mistake 2: Confusing bioaccumulation with biomagnification

Bioaccumulation is build-up in one organism over time. Biomagnification is the increase along a food chain.

Mistake 3: Believing that nutrients are harmless

Fertilizers help crops, but in water they cause eutrophication and oxygen loss.

Mistake 4: Thinking that plastic disappears

Plastic breaks into smaller pieces and can remain in the environment for decades or longer.