Which of the following gases emitted from automobile exhaust is highly poisonous for human beings?

Introduction:
Air pollution from vehicles is a major urban environmental issue. Automobile exhaust contains several gases and particles, but not all of them have the same direct toxic effect on humans. This question checks if the learner can identify the gas that is especially dangerous because it interferes with oxygen transport in the human body.

Given Data / Assumptions:

• We are considering gases released from the exhaust of petrol or diesel vehicles.
• The options are carbon monoxide, carbon dioxide, hydrocarbons, sulphur dioxide, and nitrogen gas.
• The question focuses on which gas is highly poisonous for humans, not just environmentally harmful.

Concept / Approach:
Carbon monoxide is a colourless, odourless gas produced by incomplete combustion of fuel in engines. It is extremely dangerous because it binds very strongly with haemoglobin in red blood cells, forming carboxyhaemoglobin. This reduces the ability of blood to carry oxygen, leading to tissue oxygen deficiency. Even low concentrations of carbon monoxide can cause headache, dizziness, and in severe cases, death. Carbon dioxide and other gases have different roles and effects.

Step-by-Step Solution:
Step 1: Recall that incomplete combustion in engines generates carbon monoxide.Step 2: Understand that carbon monoxide has a much higher affinity for haemoglobin than oxygen has.Step 3: Recognise that when carbon monoxide is inhaled, it prevents adequate oxygen transport in the body.Step 4: Compare this direct toxic effect with other gases like carbon dioxide that mainly contribute to climate change.Step 5: Conclude that carbon monoxide is the gas that is highly poisonous for humans.

Verification / Alternative check:
Cases of accidental poisoning from running car engines in closed garages or from faulty heaters are widely reported and are linked to carbon monoxide. Safety advisories warn specifically about carbon monoxide poisoning. This supports the textbook emphasis on carbon monoxide as a highly poisonous component of exhaust fumes for human health.

Why Other Options Are Wrong:
Carbon dioxide is a major greenhouse gas and can cause problems at very high concentrations, but at normal levels in open air it is not the primary acute poison from vehicle exhaust. Hydrocarbons in exhaust contribute to smog and may be harmful over long periods but do not have the same direct oxygen blocking effect. Sulphur dioxide is irritating and can worsen respiratory conditions, yet it is not as directly lethal at low concentrations as carbon monoxide in typical vehicle exposure. Nitrogen gas is the main component of air and is largely inert under normal conditions.

Common Pitfalls:
Students sometimes choose carbon dioxide because it is widely discussed in relation to climate change. However, the question is about direct poisonous effects on humans, where carbon monoxide is far more dangerous. Remembering the link between carbon monoxide and carboxyhaemoglobin in blood helps avoid this confusion.

Final Answer:
Correct option: Carbon monoxide