Harmful Gas Affinity for Haemoglobin

This question asks about a specific harmful gas that poses a significant risk to human respiration due to its interaction with haemoglobin. The key aspect is the gas's affinity for haemoglobin compared to oxygen.

Understanding Haemoglobin and Oxygen Transport

Haemoglobin is a protein found in red blood cells responsible for transporting oxygen ($O_2$) from the lungs to the body's tissues. It achieves this by reversibly binding with oxygen molecules. The efficiency of this transport depends on the relative binding strengths (affinities) of haemoglobin for oxygen and any other competing molecules.

Analyzing Gas Affinities

Let's examine the options provided:

• Methane ($CH_4$): Methane is a simple asphyxiant. It doesn't bind strongly to haemoglobin. Instead, it can impair respiration simply by displacing oxygen in the air, reducing the concentration of available oxygen to breathe.
• Carbon dioxide ($CO_2$): Carbon dioxide is a natural waste product of metabolism. While it interacts with blood and haemoglobin, it is transported mainly in three forms: dissolved in plasma, bound to haemoglobin (at a different site than oxygen), and as bicarbonate ions ($HCO_3^-$). It does not exhibit a significantly higher affinity than oxygen for the primary oxygen-binding site on haemoglobin.
• Carbon monoxide (CO): Carbon monoxide is a colorless, odorless gas produced by incomplete combustion. It is particularly dangerous because it binds to haemoglobin at the exact same site as oxygen. Crucially, haemoglobin has a much greater affinity for carbon monoxide (approximately 200-250 times greater) than for oxygen. When CO is inhaled, it binds tightly to haemoglobin, forming carboxyhaemoglobin ($COHb$). This binding is nearly irreversible under normal conditions and effectively prevents haemoglobin from binding and transporting oxygen. This severely impairs oxygen delivery to tissues, leading to symptoms like headache, dizziness, nausea, and potentially death from asphyxiation. The chemical reaction is: $ Haemoglobin + CO \rightleftharpoons Carboxyhaemoglobin $ $ Haemoglobin + O_2 \rightleftharpoons Oxyhaemoglobin $ The equilibrium strongly favors $COHb$ formation due to the higher affinity of haemoglobin for $CO$.
• Nitrogen dioxide ($NO_2$): Nitrogen dioxide is a toxic gas often found in air pollution. It primarily acts as a respiratory irritant, damaging lung tissue and airways, which can lead to breathing difficulties. However, its primary mechanism of harm is not through having a higher affinity for haemoglobin than oxygen.

Conclusion on Impaired Respiration

Based on the analysis, Carbon monoxide (CO) is the gas that severely impairs human respiration because its binding affinity for haemoglobin is significantly higher than that of oxygen, effectively blocking oxygen transport.