S4.14.5 Chapter 5

Air pollution

On some days a brown haze hangs over Priya’s town and her eyes sting. Where does it come from?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

A It is cloud that has sunk to the ground.
You picked A. Read on. At the end, the chapter comes back to your guess.
B It is smoke and gases from burning fuel, held near the ground.
You picked B. Read on. At the end, the chapter comes back to your guess.
C It is dust that blows in only from deserts.
You picked C. Read on. At the end, the chapter comes back to your guess.
D It is water vapour that has come in from the sea.
You picked D. Read on. At the end, the chapter comes back to your guess.

By the end you can

Explain how burning fuels causes smog and acid rain
Explain why smoke from indoor cooking fires harms health

Air pollution is a harmful substance in the air. Some pollution is natural: smoke from wildfires, ash from volcanoes and dust from deserts. Most of the pollution in towns comes from burning fuels, such as petrol, diesel, coal, oil, wood and dung.

When a fuel burns in plenty of air, it makes carbon dioxide and water. For carbon: C + O₂ → CO₂. But real burning is not so tidy. Fuels have other things in them. Air has more than oxygen. A fire or engine seldom gets just the right amount of air.

Four harmful things come from burning. With too little oxygen, fuel makes carbon monoxide and soot instead of carbon dioxide. Carbon monoxide is a poison that you cannot see or smell. It sticks to the red blood cells and stops them from carrying oxygen.

See it three ways

As a picture

In words

Carbon burns to carbon dioxide when there is plenty of oxygen. With too little oxygen it makes carbon monoxide and soot.

In symbols

C + O₂ → CO₂. 2C + O₂ → 2CO.

Third, coal and oil have a little sulfur in them. It burns too, to make sulfur dioxide, S + O₂ → SO₂. It has a sharp smell and makes people cough. Fourth, the flame of an engine is so hot that the nitrogen and oxygen of the air itself join. They make nitrogen oxides, N₂ + O₂ → 2NO. NO then turns into NO₂ in the air.

Four pollutants that come from burning fuel, and the harm that each does.

Soot is made of tiny solid particles. Particles less than 2.5 µm across, from soot and from other sources, are called PM2.5. They are many times thinner than a hair. They go deep into the lungs and some pass into the blood. The World Health Organization sets a guide for a day. Averaged over a day, the air should hold no more than 15 µg of PM2.5 in each m³. Only 3 or 4 days a year should pass that. Averaged over a year, it should hold less than 5 µg in each m³. We call 15 the guide for a day.

Worked example

On one day a town’s air has 45 µg of PM2.5 in each m³. The guide for a day is 15 µg per m³. The reading is how many times the guide?

Divide the reading by the guide.
45 ÷ 15 = 3.
So the air has 3 times the guide.

Answer: 3 times

The reading of the day is three times the guide.

Now think about how these gases and soot make smog. The word is smoke and fog put together. Smog is a haze of pollution that hangs near the ground. The air in a high is still, and nothing blows the haze away.

Smog is worst under a warm layer of air. Usually the air gets colder with height, so warm polluted air rises and spreads out. On some days a layer of warm air lies over colder air. This is called a temperature inversion. The warm layer works as a lid. The smoke cannot rise through it, so it spreads out and gathers under it.

On a normal day smoke escapes upward. Under a warm lid it is trapped, and smog builds up.

There are two common kinds of smog. In cold weather, coal smoke and sulfur dioxide mix with fog. In 1952 a smog like this stayed over one big city for five days, and thousands of people died of it. In hot, sunny weather, sunlight acts on nitrogen oxides and petrol vapour. This makes ozone and other irritants near the ground. It is the brown haze of a busy city.

The ozone high in the sky, in chapter 1, was a shield. Ozone down here, where we breathe, is a pollutant. It hurts the lungs and damages crops. It is the same gas in a different place.

Now acid rain. Sulfur dioxide and nitrogen oxides rise into the air and dissolve in the water droplets of clouds. There they turn into acids: sulfuric acid, H₂SO₄, and nitric acid, HNO₃. They fall in rain, snow, fog and dry dust, sometimes hundreds of km from where the gases were made.

From the chimney or the exhaust to acid rain, and what it harms.

Clean rain is slightly acid already, because carbon dioxide dissolves in it. Its pH is about 5.6. Acid rain has a pH below that, often between 4 and 5. Each step of 1 on the pH scale is a factor of 10 in acid.

Acid rain has a pH of about 4 to 5. It is far weaker than vinegar, but it adds up over years.

Worked example

Natural rain has a pH of 5.6. Acid rain has a pH of 4.6. How many times more acid is the acid rain? Each step of 1 on the pH scale is a factor of 10.

Find the difference in pH: 5.6 − 4.6 = 1.
One step on the pH scale is a factor of 10.
So the acid rain is 10 times more acid.

Answer: 10 times

Acid rain is too weak to hurt your skin. But over years it harms lakes, where it frees metals that poison fish and their young. It leaches nutrients from soil and damages leaves, so forests suffer. It wears away stone and corrodes metal. Marble and limestone are calcium carbonate, and an acid reacts with it: CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂.

There are ways to cut these gases. Low sulfur fuel makes less sulfur dioxide. Power stations can pass their smoke through scrubbers, which use limestone to catch the sulfur dioxide. A catalytic converter in a car cleans the exhaust. It turns carbon monoxide, nitrogen oxides and unburnt fuel into carbon dioxide, nitrogen and water. One reaction is 2CO + 2NO → 2CO₂ + N₂. In many places the rules have cut sulfur dioxide sharply, and acid rain has eased.

Try it

What an acid does to shell

You need: two small clear cups or jars, vinegar, water, two small pieces of eggshell, seashell or limestone, about the same size.

Be safe: Do not drink or taste anything in the cups. Keep vinegar away from your eyes, and wash your hands afterwards. Throw the shell and vinegar away down a sink with plenty of water.

Pour vinegar into one cup and the same amount of water into the other. Vinegar is a weak acid, but it is about 100 times more acid than acid rain. So it shows in a day what acid rain does over many years.
Put a piece of eggshell into each cup.
Watch the pieces for several minutes. Then look again after a few hours, and after a day.

What you will see: Bubbles of carbon dioxide gather on the shell in the vinegar. None gather in the water. After a day the shell in the vinegar is softer and thinner. Acid reacts with calcium carbonate and slowly eats it away. This is what acid rain does to marble statues, only far more slowly.

If you cannot try it, this is what happens.

Pollution is not only outdoors. Think of a home where people cook on an open fire or a plain stove with no chimney. They burn wood, crop waste, dung, charcoal or kerosene. The smoke fills the room. The World Health Organization says that more than 2 billion people cook with polluting fuels like these.

A room with a fire and no chimney. The smoke fills the room, and the child breathes it.

The smoke has soot particles, carbon monoxide and other gases in it, at levels far higher than outdoors. Breathing it day after day causes lung infections, especially in young children, and long lung diseases, lung cancer, heart disease and strokes. The World Health Organization links smoke from cooking indoors to about 3 million deaths each year. Women and young children are in the smoke the most.

What helps? A chimney or hood that takes the smoke out of the room. A stove that burns cleaner. A cleaner fuel, such as gas, biogas, electricity or sunshine in a solar cooker. Dry fuel, which makes less smoke than damp. And keeping young children away from the fire. A window that lets in fresh air helps too.

The same room with a chimney. The smoke goes out, fresh air comes in, and the room is clear.

Try it

A smoke walk round a kitchen

You need: paper and a pencil, a kitchen or cooking place, with an adult.

Be safe: Do not light a fire for this activity, and stay away from a hot stove. Never stay in a closed room with a charcoal or wood fire, because carbon monoxide can kill without any smell.

Do not light a fire for this. Look at where food is cooked in your home or a home you know.
Draw a plan. Mark the fire or stove, the doors and windows, and any chimney, vent or hood.
Show where the smoke goes when cooking. Mark where children sit or sleep.
List three changes that would make the air cleaner. Talk about them with the people who cook.

What you will see: Often the smoke goes where people sit, because there is nowhere else for it to go. Changes such as a vent, a window or a stove nearer the door can make a big difference.

If you cannot try it, this is what happens.

A wrong idea many people have

Smog is just fog.

Why it feels right: Smog looks like a grey mist, and we see it on foggy days.

The test that breaks it: Fog is only tiny droplets of water. Smog is also made of soot, smoke and gases such as sulfur dioxide, nitrogen oxides and ozone. Fog clears and does no harm to the lungs. Smog stings the eyes and makes people cough.

What is true: Smog is smoke and fog together. The harm comes from the pollutants in it.

A wrong idea many people have

Acid rain is a strong acid that burns your skin.

Why it feels right: The word acid sounds dangerous, like the acid in a battery.

The test that breaks it: Look at the pH scale. Acid rain has a pH of about 4 to 5. Vinegar, which we put on food, has a pH of about 2.5, which is far more acid. You can stand in acid rain without being hurt.

What is true: Acid rain is only a weak acid. It does its harm by falling on the same lakes, forests and stone over years.

A wrong idea many people have

Smoke from a cooking fire is natural, so it cannot be harmful.

Why it feels right: People have cooked on fires for thousands of years, so it seems safe.

The test that breaks it: The World Health Organization links the smoke from cooking indoors to about 3 million deaths a year. Long ago, most cooking was outdoors or in open spaces. Carbon monoxide has no colour and no smell, so people cannot tell when it is in the air.

What is true: Woodsmoke and other fuel smoke harm the lungs and heart, however old the habit is. A chimney, a cleaner stove or a cleaner fuel makes cooking safer.

Key words

air pollution: harmful substances in the air.
carbon monoxide: (CO) a poisonous gas with no colour or smell, made when fuel burns with too little oxygen.
sulfur dioxide: (SO₂) a gas that comes from sulfur in coal and oil. It makes acid rain.
nitrogen oxides: gases (NO and NO₂) made when air is heated in an engine or flame.
smog: smoke and fog together: a haze of pollution near the ground.
temperature inversion: a layer of warm air over colder air, which traps pollution near the ground.

Back to your guess

The answer is B: It is smoke and gases from burning fuel, held near the ground.

A brown haze over a town is mostly made of smoke, soot and gases from burning fuel in vehicles, factories and fires. Sunlight turns some of the gases into more harmful things. On still days nothing blows the haze away, so it builds up.

Keep it

Copy this into your exercise book.

Burning fuel makes CO, soot, SO₂ and nitrogen oxides. Smog is trapped under a warm lid. SO₂ and nitrogen oxides make acid rain. Indoor smoke harms lungs: use a chimney and clean fuel.

Draw: A chain of boxes: fuel burns, gases rise, dissolve in cloud, acid rain falls on lakes, forests and stone.