Emissions are gases and particles released into the air, mostly from burning fuel

An emission is something released into the atmosphere. In everyday use, the word usually means gases or tiny particles that come from burning fuel — in cars, factories, power plants, and heating systems. When you burn gasoline in a car engine, coal in a power plant, or natural gas in a furnace, chemical reactions happen that create new substances. Those substances escape into the air as emissions.

Not all emissions are the same. Some are invisible gases like carbon dioxide. Others are visible — the black smoke from a diesel truck or the white plume from a factory smokestack. Some emissions are measured in parts per million because they are so small. The key point is that emissions leave a source and enter the air around us.

Emissions matter because some of them trap heat in the atmosphere, some damage the ozone layer, some make breathing harder, and some fall back to earth as acid rain. That is why governments set limits on how much of certain emissions factories and cars are allowed to release.

Key Takeaways

  • Emissions are gases or particles released into the air, usually from burning fuel in engines, power plants, and heating systems.
  • Common emissions include carbon dioxide, nitrogen oxides, sulfur dioxide, and particulate matter — each with different effects on air and climate.
  • Some emissions are invisible gases and others are visible smoke or haze, but both count as emissions.
  • Governments set limits on emissions because they affect air quality, human health, and the climate.

The main types of emissions and what causes them

The largest source of emissions in most countries is transportation — cars, trucks, planes, and ships burning fossil fuels. A second major source is electricity generation, especially from coal and natural gas power plants. Manufacturing, heating buildings, and agriculture also produce significant emissions.

Within those categories, different fuels create different emissions. Burning coal releases more sulfur dioxide and particulate matter than burning natural gas. Diesel engines release more nitrogen oxides than gasoline engines. Each fuel and each process has its own emission profile — the mix of substances it sends into the air.

Some emissions come from sources you might not think of as "burning." Landfills release methane as garbage breaks down. Livestock farming releases methane from animal digestion. Refrigerators and air conditioners can leak refrigerants. These are still emissions because they are substances entering the atmosphere.

How emissions are measured and reported

Emissions are usually measured in tons or kilograms per year. A power plant might report that it released 50,000 tons of carbon dioxide in a year. A car's emissions are often given as grams per mile or per kilometer driven. The measurement depends on what is being measured — a gas, a particle, or a mix of both.

Governments require certain sources to measure and report their emissions. Large factories have monitors that track what leaves their smokestacks. Cars are tested in laboratories to measure what comes out of their tailpipes. Power plants report their emissions to environmental agencies. This data helps regulators see whether sources are staying within legal limits.

Scientists also estimate emissions from sources that are harder to measure directly. They calculate how much coal a country burned that year, multiply by how much carbon dioxide burning coal produces, and estimate total emissions from that sector. These estimates are less precise than direct measurement, but they give a picture of where emissions are coming from across an entire economy.

The difference between emissions and air pollution

Emissions and air pollution are related but not the same. An emission is the substance released. Air pollution is what happens when those emissions (and other particles and gases) build up in the air around you. You breathe air pollution; you do not breathe an emission directly.

Some emissions become air pollution right away. Nitrogen oxides released from a car tailpipe mix with sunlight and create ground-level ozone, which is a pollutant that damages lungs. Other emissions travel far before they cause problems. Carbon dioxide released in one country spreads around the entire planet and traps heat everywhere.

A city can have high emissions from traffic and factories but relatively clean air if the wind carries those emissions away. Another city might have lower total emissions but worse air quality if geography traps the emissions in a valley. The relationship between what is released and what you breathe depends on weather, geography, and how far the emissions travel.

Why governments set limits on emissions

Emissions limits exist because some emissions harm human health and the environment. Particulate matter — tiny solid particles in smoke — causes respiratory disease and heart disease. Nitrogen oxides and sulfur dioxide cause asthma and acid rain. Carbon dioxide and methane trap heat and warm the planet. Ozone-depleting chemicals damage the layer that protects us from ultraviolet radiation.

A government sets a legal limit on how much of a certain emission a source can release. A coal power plant might be required to remove 95 percent of its sulfur dioxide before it leaves the smokestack. A new car must meet tailpipe standards for nitrogen oxides and particulate matter. A factory cannot release more than a certain amount of a chemical per day.

These limits are based on what scientists know about the health and environmental effects of each emission. The limits are usually stricter for emissions that cause the most damage, and they often become stricter over time as technology improves and makes it cheaper to reduce emissions.

The connection between emissions and climate change

Some emissions trap heat in the atmosphere and warm the planet. Carbon dioxide is the most abundant of these heat-trapping gases. When you burn fossil fuels, you release carbon dioxide that has been locked underground for millions of years. That carbon dioxide stays in the atmosphere for centuries, continuously trapping heat.

Methane is another heat-trapping emission that is released from oil and gas production, landfills, and livestock farming. Methane traps heat much more intensely than carbon dioxide, but it breaks down in the atmosphere after about a decade. Nitrous oxide, released from agriculture and some industrial processes, also traps heat and lasts for over a century.

The more of these emissions released, the more heat gets trapped, and the warmer the planet becomes. This is why reducing emissions from burning fossil fuels is central to efforts to slow climate change. It is also why some emissions are called "greenhouse gases" — they work like the glass in a greenhouse, letting sunlight in but trapping heat inside.

Frequently Asked Questions

Is carbon dioxide the only emission that causes climate change?

No. Carbon dioxide is the largest contributor because so much of it is released and it lasts so long in the atmosphere. But methane, nitrous oxide, and some industrial chemicals also trap heat. Together, these are called greenhouse gases. Carbon dioxide accounts for roughly three-quarters of the warming effect, but the others matter too.

Can emissions be completely eliminated?

Not entirely. Even renewable energy sources like wind and solar produce some emissions during manufacturing and installation. The goal is to reduce emissions as much as possible, especially from burning fossil fuels. Switching to renewable energy, improving efficiency, and capturing emissions before they leave a smokestack all reduce the total amount released.

Why do some emissions smell and others don't?

Whether an emission has a smell depends on its chemical structure. Sulfur dioxide smells like burnt matches. Nitrogen oxides are mostly odorless. Carbon dioxide is invisible and odorless. A smell does not tell you whether an emission is harmful — some dangerous emissions are odorless, and some smelly ones are less harmful than you might think.

Who decides what the legal emission limits should be?

Government environmental agencies set limits based on scientific research about health and environmental effects. In the United States, the Environmental Protection Agency (EPA) sets federal standards. States can set stricter limits. Other countries have their own environmental agencies that set limits. International agreements like the Paris Agreement set targets for reducing certain emissions across countries.

How do emissions from my home compare to emissions from a car?

It depends on how you heat your home and how much you drive. A home heated with natural gas or oil releases emissions continuously through the heating system. A car releases emissions only when running, but concentrated in the exhaust. Over a year, both can be significant. Switching to renewable energy for heating or driving an electric vehicle reduces emissions from both.