Biological Consequences of Air Pollution

When we burn fossil fuels — coal, oil, petrol and diesel — in power stations, factories and vehicle engines, useful energy is released. But the burning also releases polluting gases into the air. For Double Award Biology spec 4.12 you need to know the biological effects of just two of these gases:

Pollutant	Where it comes from	Main biological effect
Sulfur dioxide (SO₂)	Burning fossil fuels that contain sulfur	Forms acid rain
Carbon monoxide (CO)	Incomplete combustion of fuels (not enough oxygen)	Binds to haemoglobin, reducing oxygen transport

These two gases harm living things in completely different ways:

• Sulfur dioxide acts indirectly — it changes the rain into acid, which then damages plants, lakes and buildings in the environment.
• Carbon monoxide acts directly inside the body — it poisons the blood by reducing how much oxygen it can carry.

Exam tip. Keep the two pollutants separate in your mind. A common one-mark slip is to say carbon monoxide causes acid rain — it does not. Acid rain comes from sulfur dioxide.

Sulfur dioxide (SO₂) is released when fossil fuels that contain sulfur are burned — for example in power stations that burn coal or oil.

Once in the air, the sulfur dioxide dissolves in water droplets in clouds. This makes the rainwater acidic — we call it acid rain.

sulfur dioxide  +  water (in clouds)  →  acid rain

Acid rain then falls and causes three biological and environmental problems you must learn:

1. It damages plants and trees. The acid damages leaves and can kill the tree. It also washes important nutrients (minerals) out of the soil, so plants grow poorly. Whole forests have been damaged in this way.
2. It acidifies lakes and rivers — killing fish. When acid rain runs into lakes it lowers the pH of the water. Many fish and other aquatic organisms cannot survive in such acidic water, so they die.
3. It erodes buildings and stonework. Acid rain reacts with stone (such as limestone) and metal, slowly eroding buildings, statues and structures.

Here is the chain of events as a simple diagram:

Exam tip. Learn the three effects of acid rain as a list you can recall instantly: damages plants/trees, acidifies lakes (kills fish), erodes buildings. Marks are easy here if you state all three clearly.

Carbon monoxide (CO) is a colourless, odourless and poisonous gas. It is produced when fuels burn in too little oxygen — this is called incomplete combustion. Faulty heaters, car exhausts and cigarette smoke all release carbon monoxide.

To understand why carbon monoxide is so dangerous, remember how the blood normally carries oxygen:

• Red blood cells contain a red pigment called haemoglobin.
• In the lungs, haemoglobin binds to oxygen to form oxyhaemoglobin, which is carried around the body and releases the oxygen to the cells.

The problem is that carbon monoxide binds to haemoglobin far more strongly than oxygen does. When you breathe in carbon monoxide:

1. The carbon monoxide enters the blood in the lungs.
2. It binds to the haemoglobin in the red blood cells, forming a substance called carboxyhaemoglobin.
3. This haemoglobin can no longer carry oxygen — the binding is essentially permanent.
4. So the blood carries less oxygen to the body's cells (reduced oxygen transport).

carbon monoxide  +  haemoglobin  →  carboxyhaemoglobin  (carries no oxygen)

Because the body's cells (including the brain and heart) are starved of oxygen, carbon monoxide poisoning can cause drowsiness, headaches and, in high enough amounts, death. It is fatal precisely because the haemoglobin is "blocked" and cannot deliver the oxygen the body needs.

Exam tip. The full marking chain is: carbon monoxide binds to haemoglobin → forms carboxyhaemoglobin → less oxygen carried/transported → can be fatal. Always name haemoglobin (not just "red blood cells") and say oxygen transport is reduced.

It is easy to mix these two pollutants up, so it helps to compare them side by side. Both come from burning fossil fuels, but they cause harm in very different ways.

Feature	Sulfur dioxide (SO₂)	Carbon monoxide (CO)
Source	Burning fuels containing sulfur	Incomplete combustion of fuels
How it harms	Indirect — forms acid rain	Direct — binds to haemoglobin
What is affected	Plants/trees, lakes (fish), buildings	The blood / oxygen transport in the body
Key word	acid rain	carboxyhaemoglobin
Worst outcome	Forests/lakes damaged, fish killed	Reduced oxygen → can be fatal

A simple way to keep them apart:

Sulfur dioxide = the Sky and Soil (acid rain on the environment). Carbon monoxide = the Cells (it stops the blood carrying oxygen).

Exam tip. If a question describes "fish dying in a lake" the cause is acid rain from sulfur dioxide. If a question describes "less oxygen reaching the cells" or "binding to haemoglobin", the cause is carbon monoxide. Match the clue to the right gas.