Pollution and Solutions

Pollution is any substance or energy released into the environment in amounts that can harm people, other living things or the places they depend on. Some pollutants are human-made chemicals; others, like soil, nitrogen or heat, are natural things in the wrong place or in the wrong amount.

Point and nonpoint sources

A point source comes from one identifiable place. The Clean Water Act describes point sources as "discrete conveyances such as pipes or man-made ditches," and anyone discharging pollutants from one into U.S. waters needs a permit. A nonpoint source comes from many places at once. EPA explains that nonpoint pollution happens when rain or snowmelt moves over and through the ground, picking up pollutants such as fertilizer, oil, sediment and bacteria, and carrying them into lakes, rivers, wetlands, coastal waters and groundwater.

Primary and secondary pollutants

A primary pollutant is released directly, like carbon monoxide from a tailpipe or sulfur dioxide from a smokestack. A secondary pollutant forms later, in the air, from reactions among other pollutants. EPA explains that ground-level ozone "is not emitted directly into the air"; it forms when nitrogen oxides and volatile organic compounds react in sunlight. Most fine particles also form in the atmosphere from reactions of chemicals such as sulfur dioxide and nitrogen oxides, and acid rain forms when those same gases react with water, oxygen and other chemicals to make sulfuric and nitric acids.

Dose and exposure

Whether a pollutant harms you depends on more than whether it is present. Exposure is contact with it, and the dose is how much actually gets into the body. For mercury, for example, EPA lists the factors that decide the health effect: the form of mercury, the amount, the age of the person (unborn babies are the most vulnerable), how long the exposure lasts and how the person is exposed, whether by breathing, eating or skin contact.

EPA scientists turn these ideas into a risk assessment with four steps after planning:

The Clean Air Act requires EPA to set National Ambient Air Quality Standards (NAAQS) for six common outdoor pollutants, called the criteria air pollutants.

Good ozone and bad ozone

Ozone is the same molecule wherever it is, but EPA describes it as "good" or "bad" depending on where it is found. High in the stratosphere, ozone forms a protective layer, the ozone layer, that shields us from the sun's harmful ultraviolet (UV) rays. Near the ground, ozone is a pollutant that people breathe. It is most likely to reach unhealthy levels on hot, sunny days in cities, though it can still reach high levels in colder months.

Acid rain

EPA notes that normal rain has a pH of about 5.6. It is slightly acidic because carbon dioxide dissolves into it and forms weak carbonic acid. Acid rain (or acid deposition) is more acidic because of sulfuric and nitric acid, and it can fall in wet forms (rain, snow, fog) or dry forms (particles and gases). EPA's acid rain page says burning fossil fuels to make electricity is a major source of both gases: electric power generators account for two-thirds of the SO₂ and one-fourth of the NO_x in the atmosphere.

Its effects are clearest in streams and lakes. As acidic water flows through soil, it can leach aluminum from clay particles into the water, and EPA notes that at pH 5 most fish eggs cannot hatch. Acid rain also removes minerals and nutrients from the soil that trees need, and acidic fog at high elevations can strip nutrients from leaves and needles. EPA also describes damage to building materials and haze that reduces visibility.

A global problem

The World Health Organization (WHO) estimates that in 2019, 99% of the world's people lived in places where WHO's air quality guideline levels were not met, and that outdoor air pollution caused about 4.2 million premature deaths that year, mostly from fine particulate matter. WHO links the combined effects of outdoor and household air pollution to about 6.7 million premature deaths annually.

Some of the most serious air pollutants are found indoors, where people breathe them for hours at a time.

Around the world, WHO estimates that about 2.1 billion people, roughly a quarter of the global population, cook with open fires or inefficient stoves burning kerosene, wood, animal dung, crop waste or coal. WHO attributes about 2.9 million deaths in 2021 to household air pollution.

The Air Quality Index (AQI) is the number you see in weather apps and on AirNow. It runs from 0 to 500. For each pollutant, an AQI of 100 generally matches the level of the short-term national air quality standard, so values at or below 100 are generally thought of as satisfactory. Above 100, air quality is unhealthy, first for sensitive groups and then for everyone as values rise. Each category has an official color, but the name and number carry the same information.

To turn a pollutant concentration into an AQI number, EPA's technical guide uses breakpoints: the concentration range that matches each category. You find the range your reading falls in and scale it in a straight line between the ends:

Nitrogen and phosphorus are nutrients that plants and algae need. Too much of them in water is nutrient pollution, and it comes from fertilizer, manure, sewage and runoff. The National Oceanic and Atmospheric Administration (NOAA) calls the result eutrophication: water so enriched with nutrients that algae and plants overgrow. NOAA reports that 65% of the U.S. estuaries and coastal waters researchers have studied in the lower 48 states are moderately to severely degraded by excess nutrients.

NOAA uses dead zone as the common name for hypoxia, water with 2 milligrams per liter of oxygen or less. Most marine life there either dies or, if it can swim, leaves.

The Gulf dead zone

A large dead zone forms in summer in the northern Gulf (called the Gulf of America in current NOAA reports, formerly the Gulf of Mexico), fed by nutrient runoff carried by the Mississippi River. NOAA-supported scientists measure it on a summer research cruise:

Pathogens

Sewage and animal waste carry germs. WHO estimates that in 2022 at least 1.7 billion people used a drinking water source contaminated with feces, and that contaminated drinking water causes about 505,000 deaths from diarrhea each year. In 2022, 73% of the world's people used a safely managed drinking water service: one at home, available when needed and free from contamination.

Toxic chemicals that build up

Some pollutants break down quickly. Others are persistent: they last for years and collect in living things. Bioaccumulation is a chemical building up in one organism over its life. Biomagnification, as OpenStax Biology defines it, is the increasing concentration of persistent, toxic substances in organisms at each trophic level, from the producers to the top consumers. Each predator eats many contaminated prey, so the chemical becomes more concentrated at every step.

Groundwater

The U.S. Geological Survey (USGS) explains that soil and rock filter particles out of water as it soaks into the ground, but dissolved chemicals and gases can remain. Groundwater can be contaminated by industrial, household and agricultural chemicals from the surface; by road salt, a major concern in the northern U.S.; by leaking fuel tanks and spills; and by nitrate. USGS says the most common water quality problem in rural areas is bacteria from septic tanks.

Making water safe to drink

CDC describes the steps many water utilities use. Different plants use different steps depending on how clean the source is; water from lakes and rivers usually needs more treatment than groundwater.

Cleaning wastewater

Used water from homes and businesses goes to a wastewater treatment plant before it returns to a river, lake or ocean. USGS explains that primary treatment removes about 60% of suspended solids, and secondary treatment removes more than 90%. Under the Clean Water Act, EPA's secondary treatment standards set the minimum performance every U.S. municipal treatment plant must meet, measured by oxygen-demanding waste (BOD₅), suspended solids and pH.

In 2018 the U.S. generated 35.7 million tons of plastic, 12.2% of all municipal solid waste, and recycled 8.7% of it, according to EPA. Some containers do better: PET bottles and jars were recycled at 29.1%. Plastic that escapes into rivers and the ocean joins what NOAA calls one of the most widespread pollution problems facing the world's ocean and waterways: marine debris, including consumer plastics, metals, rubber, textiles and lost fishing gear.

NOAA defines microplastics as plastic pieces less than 5 millimeters long, about the size of a pencil eraser. They come from larger plastic that breaks apart, from resin pellets used to make plastic products and from microbeads once added to some cleansers and toothpastes. The Microbead-Free Waters Act, signed December 28, 2015, banned plastic microbeads in cosmetics and personal care products in the U.S.

The "garbage patch" is not an island

NOAA says the name "Great Pacific Garbage Patch" does not paint an accurate picture. Debris concentrates in several regions of the North Pacific, and much of it is small pieces of floating plastic not easily seen. It is possible to sail through a "garbage patch" and see very little on the surface, and its size is hard to estimate because its borders and content keep changing.

Pollution problems are not all solved, and people still debate the costs and design of environmental rules. But several big efforts have long records of measurements, so we can look at what changed after they started.

The Clean Air Act

The Clean Air Act has been in place since 1970, and Congress added major amendments in 1990. Since 1970, EPA's data show, emissions of the six common pollutants fell while the economy, driving and population grew. EPA reports that new cars, SUVs and pickup trucks are roughly 99% cleaner for common pollutants than 1970 models.

Concentrations in the air fell too. From 1980 to 2025, EPA reports carbon monoxide down 88%, annual nitrogen dioxide down 68%, sulfur dioxide down 96% and 8-hour ozone down 29%. The work is not finished: about 115 million people lived in counties with pollution levels above the primary national air quality standards in 2025, and 24-hour PM₁₀ levels were 23% higher in 2025 than in 2010.

The Acid Rain Program

The 1990 Clean Air Act amendments created the Acid Rain Program, which began in 1995. EPA calls it the first national cap-and-trade program: it capped total SO₂ from power plants and let them buy and sell allowances, so each plant could choose how to cut. Together with later power-sector rules and a changing energy sector, EPA says it has helped deliver annual SO₂ reductions of over 95% and NO_x reductions of over 89%.

Getting the lead out

Starting in 1975, new cars came with catalytic converters that needed unleaded fuel, and on January 1, 1996, the Clean Air Act banned the sale of leaded fuel for on-road vehicles. EPA reports that airborne lead fell 98% from 1980 to 2005. The median blood lead level of U.S. children ages 1 to 5 fell 96%, to 0.6 micrograms per deciliter in 2017–2020, compared with 1976–1980. Around the world, UNEP announced that leaded petrol use ended in 2021 when the last service stations, in Algeria, stopped selling it. Today EPA lists metals processing and piston-engine aircraft using leaded aviation fuel as the major remaining sources of lead in U.S. air.

The Montreal Protocol and the ozone layer

The 1987 Montreal Protocol is a treaty to phase out ozone-depleting substances such as chlorofluorocarbons (CFCs), once used in refrigeration, air conditioning, firefighting foam and even hairspray. UNEP reported in 2023 that nearly 99% of banned ozone-depleting substances had been phased out. The 2016 Kigali Amendment added a phase-down of HFCs, replacement chemicals that don't directly deplete ozone but are powerful greenhouse gases.

The 2026 WMO/UNEP Scientific Assessment of Ozone Depletion reports that chlorine and bromine from controlled chemicals continue to decrease, and a statistically significant upward trend in global ozone strengthens the evidence of recovery. Average global ozone is projected to return to 1980 values around 2040, and Antarctic ozone during the 2060s. In 2025 the Antarctic ozone hole averaged about 7.23 million square miles at its peak, the 5th smallest since 1992, according to NASA and NOAA. It still forms every spring.

The Cuyahoga River and the Clean Water Act

On June 22, 1969, an oil slick on the Cuyahoga River in Cleveland, Ohio, caught fire. EPA notes the river had had at least 13 fires since the 1800s, and the National Park Service points out that stories about the 1969 fire often mix fact and myth. Still, it drew national attention to water pollution. The Clean Water Act, signed October 18, 1972, made it unlawful to discharge pollutants from a point source into U.S. waters without a permit, through the National Pollutant Discharge Elimination System (NPDES).

Recovery has been slow and is still underway. The Cuyahoga became a Great Lakes "Area of Concern" in 1987. EPA lists nine types of damage there, called beneficial use impairments, and five have been removed: degradation of aesthetics (2017), restrictions on eating fish and wildlife (2019), undesirable algae (2021), fish tumors (2023) and beach closings (2024). Two dams that blocked fish from Lake Erie at Brecksville were removed in July 2020. Today, nonpoint pollution, which a pipe permit can't control, is what states report as the leading remaining cause of water quality problems.

Reduce, reuse, then recycle

EPA's waste management hierarchy ranks options from most to least preferred: source reduction and reuse, then recycling and composting, then energy recovery, then treatment and disposal. In 2018 Americans generated 292.4 million tons of municipal solid waste, about 4.9 pounds per person per day. About 32.1% was recycled or composted, 11.8% was burned with energy recovery and 50% went to landfills.

Recycling has real limits. Plastic's 8.7% recycling rate in 2018 was far below the overall rate, and a recycled item only helps if someone collects, sorts and buys the material. That is why the hierarchy puts using less material first.