What If? Lab
What if pollution disappeared from cities?
City air can have tiny bits of dust and smoke in it. People are working hard to make it cleaner. What if it were all clean? Let's imagine it with Olly!
Vehicles, factories and other sources can put pollution into city air. Since 1970, EPA says total emissions of six main air pollutants in the United States have dropped by 78 percent, and cities such as Beijing have cut their pollution too. Imagine city air with no pollution at all, and follow what might change.
EPA reports that U.S. emissions of the six principal air pollutants fell 78 percent between 1970 and 2025 while the economy and driving grew, and UNEP documents Beijing's fine-particle pollution falling 35 percent from 2013 to 2017. This thought experiment pushes the trend to its imagined end: city air with no pollution.
The big question
Sometimes city air looks hazy. Tiny bits too small to see float in it.
What if all of that went away? Let's imagine clean, clear city air.
On some days, a city's air looks hazy, and faraway hills or buildings seem blurry. That haze is often made by tiny floating particles, too small to see one by one.
What if pollution disappeared from cities completely? In this thought experiment you can follow three paths: clear skies, healthier lungs, and the electric buses, bikes and other changes that are already making city air cleaner.
Urban air pollution includes ground-level ozone, particle pollution (PM2.5 and PM10), carbon monoxide, sulfur dioxide and nitrogen dioxide, the five pollutants in EPA's Air Quality Index.
This thought experiment imagines all of them gone from city air. Real cities can only reduce pollution, not erase it, so the experiment uses measured improvements, in the United States, Southern California and Beijing, to reason about what complete removal might change. Each outcome of the imagined clean city is an idea, not a forecast.
Explore the idea
Make a prediction
Since 1970, what happened to the amounts of pollution put into the air in the United States?
From 1970 to 2025, what happened to U.S. emissions of six main air pollutants?
According to EPA, how did total U.S. emissions of the six principal pollutants change from 1970 to 2025?
- They went down a lot
- They went up a lot
- They stayed the same
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Established fact
People have made city air much cleaner, and they keep working on it.
U.S. air pollution emissions have fallen sharply since 1970 (EPA), and cities such as Beijing have cut fine particles (UNEP).
Measured reductions in the United States, Southern California and Beijing show that policy and technology lower urban pollution (EPA, UNEP, USC).
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An idea scientists discuss
A city with no pollution is pretend. We imagine what might happen.
No real city has air with no pollution. What might happen in one is an idea built from what cleaner air has already done.
Every consequence of zero urban pollution here is extrapolated from partial reductions and might differ greatly in reality.
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Established fact
The air in the United States is much cleaner than it used to be.
From 1970 to 2025, U.S. emissions of six main air pollutants dropped by 78 percent.
Total emissions of the six principal air pollutants fell 78 percent from 1970 to 2025 while GDP grew 349 percent and vehicle miles traveled 197 percent (EPA).
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Established fact
Tiny bits of pollution can make the air look hazy.
Fine particles are the main cause of hazy air in parts of the United States, including many national parks.
Fine particles (PM2.5) are the main cause of reduced visibility (haze) in parts of the United States, including many national parks and wilderness areas (EPA).
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Established fact
Green on the air number means the air is good.
On EPA's Air Quality Index, 0 to 50 is green and means good air; higher numbers mean more pollution.
The AQI's six color-coded categories run from good (0–50, green) to hazardous (301 and higher); 100 generally matches the short-term national standard (EPA AirNow).
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An idea scientists discuss
With no pollution, the sky might look clear every day.
With no pollution, haze might vanish, faraway views might look sharp, and the AQI might stay green every day.
Eliminating particles and ozone might keep visibility high and every AQI reading in the good range; weather and natural dust might still cause some haze.
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Established fact
When the air got cleaner, children in a study coughed less.
In a USC study of 4,602 children in Southern California, cleaner air went along with fewer cough and chest problems, in children with and without asthma.
PM2.5 fell 47 percent and nitrogen dioxide 49 percent in the study region (1992–2011); with cleaner air, children with asthma were 32 percent less likely to have bronchitic symptoms and those without had 21 percent fewer (USC, 2016).
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Established fact
Some bits of pollution are so tiny they can get deep inside us.
Particles smaller than 10 micrometers can get deep into the lungs; children are among the people most likely to be affected.
Particles under 10 micrometers can penetrate deep into the lungs and some may enter the bloodstream; children, older adults and people with heart or lung disease are among those most likely to be affected (EPA).
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An idea scientists discuss
With no pollution, people might breathe even more easily.
If cleaner air already helped children's lungs, air with no pollution at all might help even more.
Extrapolating from partial reductions, full removal might bring further health gains; the size of such gains cannot be estimated here.
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Established fact
Electric cars and buses give off no smoke from a tailpipe.
Electric vehicles produce no tailpipe emissions. Charging them may add some pollution at power plants, but their total emissions are usually lower than gasoline cars'.
EVs produce no tailpipe emissions; total emissions associated with driving them are typically lower than for gasoline cars, particularly with renewable electricity (EPA).
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Established fact
Beijing made its air much cleaner.
Beijing began a big clean-air program in 1998. From 2013 to 2017, its fine particle pollution fell by 35 percent.
Beijing's measures included energy infrastructure changes, coal-fired pollution control and vehicle emission controls; PM2.5 fell 35 percent in the city and 25 percent regionally in 2013–2017 (UNEP, 2019).
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Established fact
Safe paths for walking and biking help cities.
UNEP says cleaner transport needs a mix: better-designed cities, safe walking and cycling paths, more public transport and cleaner vehicles, including electric ones.
UNEP's prescription for cleaner transport combines better-designed cities, walking and cycling facilities, more public transport and cleaner, more efficient fleets including EVs.
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An idea scientists discuss
If every bus and car were electric, city streets might have much less smoke.
If every vehicle in a city were electric, or people walked and cycled more, the air on its streets might get much cleaner, though power plants and other sources might still need cleaning up.
Full electrification and active travel might remove tailpipe emissions, but zero pollution might also require clean power, industry and heating; the experiment cannot rank these.
In our world
People check the air every day. A special number tells them how clean it is.
Green means the air is good. Other colors mean it is less clean.
Many places measure their air every day. In the United States, EPA's Air Quality Index, or AQI, turns those measurements into a number and a color. A value from 0 to 50 is green, which means good air. The higher the number, the more pollution there is.
The good news is that U.S. air has been getting cleaner. EPA says emissions of six main air pollutants dropped by 78 percent from 1970 to 2025, even though people drove almost three times as many miles.
The AQI has six color-coded categories; 0–50 is good, and a value of 100 generally matches the short-term national standard for a pollutant. EPA monitors show large improvements: between 1980 and 2025, carbon monoxide concentrations fell 88 percent, annual nitrogen dioxide 68 percent, one-hour sulfur dioxide 96 percent and eight-hour ozone 29 percent.
Emissions of the six principal pollutants fell 78 percent from 1970 to 2025 while GDP rose 349 percent and vehicle miles traveled 197 percent. Even so, EPA counts about 115 million people living in counties above a primary air quality standard in 2025.
Imagine perfectly clean air
Let's pretend city air has no pollution at all.
The sky might look clearer. Faraway hills might look sharp. These are only ideas.
Now imagine a city where the air has no pollution at all. Haze might vanish, so faraway hills and buildings might look sharp and clear. The AQI might show green every day.
People might breathe more easily, especially children and people with asthma: a USC study in Southern California found that as the air got cleaner, children had fewer cough and chest problems. Statues and buildings might stay clean for longer. These are ideas: no real city has air with no pollution at all.
With all five AQI pollutants removed, the haze that EPA attributes mainly to fine particles might disappear, and every day might fall in the AQI's good category.
Health outcomes might improve beyond what real reductions have shown, since EPA lists children, older adults and people with heart or lung disease among those most affected by particle pollution. Stone and monuments that particles can stain might weather more slowly. These are reasoned ideas extrapolated from partial reductions, not predictions.
Why clean air matters
Some bits in dirty air are too tiny to see. Wind can carry them a long way.
Cleaner air helps people breathe easily.
EPA explains that small particles in polluted air, less than 10 micrometers across, can get deep into our lungs, and EPA says these small particles pose the greatest problems. Children, older people and people with heart or lung disease are more likely to be affected.
The tiny particles also make haze. EPA says fine particles are the main cause of hazy air in parts of the United States, including many national parks.
EPA links particle size directly to health risk: particles under 10 micrometers can penetrate deep into the lungs, and some may enter the bloodstream, affecting both lungs and heart. Fine particles (PM2.5) are the main cause of reduced visibility in parts of the United States, and, depending on their chemistry, particles carried by wind can acidify lakes, deplete soil nutrients and stain stone.
UNEP calls air pollution the greatest environmental health risk of our time and notes that air pollution and climate change share common sources, so cleaning the air can benefit both health and climate.
Cities cleaning their air
Some cities have made their air much cleaner.
Electric buses and cars give off no smoke from a tailpipe. Bikes and walking help too.
Cities really have cleaned their air. UNEP reports that Beijing began a big clean-air program in 1998, controlling pollution from coal and from vehicles. From 2013 to 2017, fine particle pollution in Beijing fell by 35 percent.
Transport is changing too. EPA says electric vehicles produce no tailpipe emissions. UNEP helps countries and cities bring in electric buses, and it supports safe paths for walking and cycling, along with more public transport.
Beijing's intensified program since 1998 combined energy infrastructure changes, coal-fired pollution control and vehicle emission controls; PM2.5 fell 35 percent in the city and 25 percent in surrounding regions from 2013 to 2017 (UNEP, 2019).
EPA notes that EVs produce no tailpipe emissions; charging may add pollution at power plants, but total emissions are typically lower than for gasoline cars. UNEP's transport work combines better-designed cities, walking and cycling facilities, more public transport and cleaner fleets, including electric buses in 16 countries and cities.
Amazing facts
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People in the United States now drive much more, yet the air is cleaner.
From 1970 to 2025, the miles driven in the United States went up 197 percent, while emissions of six main air pollutants went down 78 percent.
Between 1970 and 2025, U.S. GDP grew 349 percent and vehicle miles traveled 197 percent, yet aggregate emissions of the six principal pollutants fell 78 percent (EPA).
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Lead in the air dropped a lot after cars stopped using leaded gasoline.
After leaded gasoline was phased out, along with other rules, the amount of lead in U.S. air fell by 98 percent between 1980 and 2005.
The permanent phase-out of leaded gasoline, air toxics controls and other regulations cut airborne lead concentrations in the United States by 98 percent between 1980 and 2005 (EPA).
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Tiny bits of pollution can even make statues dirty.
Particle pollution can stain and damage stone, including statues and monuments.
Particulate matter can stain and damage stone and other materials, including culturally important statues and monuments; some effects relate to acid rain (EPA).
Words to know
- air pollution
- harmful gases and tiny particles in the air
- particle pollution
- tiny bits of solid or liquid floating in the air, also called particulate matter
- haze
- air that looks blurry or milky, so faraway things are hard to see
- Air Quality Index
- a number and color that tells how clean or polluted the air is each day
- emissions
- gases and particles that are let out into the air
- tailpipe
- the pipe at the back of a car or bus where exhaust comes out
Discovery challenge
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On the air number, which color means the air is good?
On EPA's Air Quality Index, which color means good air?
Which AQI color marks the good category, 0–50?
- Green
- Purple
- Maroon
Show the answer
Green
Green means good air.
Green, for values from 0 to 50.
Green; purple is very unhealthy and maroon is hazardous (EPA AirNow).
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What makes city air look hazy?
What does EPA say is the main cause of haze in parts of the United States?
According to EPA, the main cause of reduced visibility in parts of the United States is…
- Tiny particles in the air
- Birds flying overhead
- Tall buildings
Show the answer
Tiny particles in the air
Tiny bits floating in the air.
Fine particles, called PM2.5.
…fine particles (PM2.5) (EPA).
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How much smoke comes out of an electric car's tailpipe?
How much tailpipe pollution does an electric vehicle produce?
According to EPA, the tailpipe emissions of an all-electric vehicle are…
- None
- The same as a gasoline car
- Twice as much as a gasoline car
Show the answer
None
Nothing! Electric cars have no tailpipe smoke.
None. Charging may add pollution at power plants, but total emissions are usually lower than a gasoline car's.
…zero; total emissions are typically lower than for gasoline cars (EPA).
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What did Beijing work on to clean its air?
Which measures does UNEP say Beijing used to cut its air pollution?
UNEP's 2019 review lists which of these among Beijing's air pollution measures?
- Controlling pollution from coal and vehicles
- Counting the birds in the city
- Painting buildings white
Show the answer
Controlling pollution from coal and vehicles
It cleaned up smoke from coal and from vehicles.
Controlling coal pollution and vehicle emissions; fine particles fell 35 percent from 2013 to 2017.
Energy infrastructure optimization, coal-fired pollution control and vehicle emission controls; PM2.5 fell 35 percent in 2013–2017 (UNEP).
Experiments to try
- Watch What If? Lab What if a city had no trees? Cities are full of roads, roofs and parking lots that soak up the Sun's heat. Street trees shade them, cool the air, catch rain and give birds a home. Imagine a city with every tree gone: hotter streets, more rain rushing down the drains and fewer birds. Then see how people plant trees to help.
- Watch What If? Lab What if snow were black? Clean snow is white because it reflects most of the sunlight that hits it, which helps it stay cold and frozen. Dark things soak up sunlight and warm up. In some mountains, dust blowing onto snow already makes it melt weeks earlier. Imagine snow that was black, and find out why color matters so much.
- Watch What If? Lab What if a mountain wore away? Even the tallest mountains are slowly broken apart by ice, rain, plant roots and wind, and the pieces are carried away by rivers and glaciers. Imagine following a mountain as it wears away: its rock might end up as sand on a beach, mud on a farm or new land at a river's mouth, while moving plates push other mountains up.
Explore next
- Watch What If? Lab What if a city had no trees? Cities are full of roads, roofs and parking lots that soak up the Sun's heat. Street trees shade them, cool the air, catch rain and give birds a home. Imagine a city with every tree gone: hotter streets, more rain rushing down the drains and fewer birds. Then see how people plant trees to help.
- Watch What If? Lab What if snow were black? Clean snow is white because it reflects most of the sunlight that hits it, which helps it stay cold and frozen. Dark things soak up sunlight and warm up. In some mountains, dust blowing onto snow already makes it melt weeks earlier. Imagine snow that was black, and find out why color matters so much.
- Watch What If? Lab What if a mountain wore away? Even the tallest mountains are slowly broken apart by ice, rain, plant roots and wind, and the pieces are carried away by rivers and glaciers. Imagine following a mountain as it wears away: its rock might end up as sand on a beach, mud on a farm or new land at a river's mouth, while moving plates push other mountains up.
- Watch Try it What If? Lab What if Earth had no atmosphere? Earth's air works like a jacket: it keeps heat in and carries it around. Take the air away in our model of a sandy patch of ground on the equator, and the coldest part of the night drops from about 13 °C to about 2 °C.
- Watch Try it What If? Lab What if there were no greenhouse gases? Greenhouse gases such as carbon dioxide and water vapor hold some of Earth's heat in, like a blanket. Take them all away in our energy-balance model and Earth's average temperature falls from about 15 °C to about −19 °C, well below freezing.
- Watch Try it What If? Lab What if there were more greenhouse gases? Adding greenhouse gases makes it harder for Earth's heat to escape, so the planet warms until it balances again. In our model a small change in how freely heat escapes warms Earth by about 2.6 °C, and the oceans make the warming take years.
- Watch Try it What If? Lab What if Earth were covered in ice? White ice bounces sunlight back to space, so an icy planet soaks up less of the Sun's warmth. In our energy-balance model, making Earth reflect 60 percent of its sunlight instead of 30 percent drops its average temperature from about 15 °C to about −23 °C.
- Watch Try it What If? Lab What if glaciers melted? Glaciers are huge, slow-moving masses of ice on land. When land ice melts, its water runs into the ocean and raises the sea. In our model, melting every mountain glacier on Earth would lift the sea by about 40 centimeters; scientists' best estimate is a little less, about 32.
For parents & teachers
What children will learn
- Air pollution includes tiny particles and gases such as ozone and nitrogen dioxide.
- The Air Quality Index turns measurements into a daily number and color.
- Real places have cut pollution a great deal, and cleaner air has gone with better children's breathing.
- Electric vehicles, walking, cycling and public transport are part of how cities clean their air.
Recommended ages: 7 to 17 · About 15 minutes
Talk about it
- What do you notice about the sky or faraway views on different days?
- Which ways of getting around in your town make the least pollution?
- Why do you think U.S. air got cleaner even though people drove more?
Try at home or in class
- With a grown-up, look up your area's Air Quality Index on AirNow each day for a week and color a chart with the AQI color (green, yellow, orange and so on).
- Choose a faraway view from a window, such as a hill or a tall building, and note each day whether it looks sharp or hazy; compare your notes with the AQI, since EPA says fine particles are the main cause of haze in parts of the United States.
Key vocabulary: air pollution, particle pollution, haze, Air Quality Index, emissions, tailpipe
Where these facts come from
- Air Quality: National Summary , U.S. Environmental Protection Agency
- Air Quality Index (AQI) Basics , AirNow (EPA, NOAA, NPS and partners)
- Health and Environmental Effects of Particulate Matter (PM) , U.S. Environmental Protection Agency
- Electric & Plug-In Hybrid Electric Vehicles , U.S. Environmental Protection Agency, Green Vehicle Guide
- Cutting smog has improved lung health for Southern California children, USC study shows , Keck School of Medicine of USC
- A Review of 20 Years' Air Pollution Control in Beijing , United Nations Environment Programme
- Electric mobility , United Nations Environment Programme
- Electric buses , United Nations Environment Programme
- Active mobility , United Nations Environment Programme
- Air quality , United Nations Environment Programme
Checked, not yet human-reviewed. Last updated October 24, 2026. Spotted a mistake? Tell us.