Nitrogen dioxide (NO2) is a known air pollutant that acts as a strong lung irritant. When inhaled, it reacts with the moisture in the respiratory tract to form acids (HNO2) and HNO3 ) that can damage the delicate lung tissue. Exposure to NO2 is particularly harmful to children, as it can increase their susceptibility to respiratory infections and lead to acute respiratory illnesses. Let's examine the other options. Sulphure dioxide ( SO2 ) is also a potent lung irritant and a major cause of respiratory problems, especially in asthamtics. However, between the given options, NO2 is often specifically cited for its role in causing acute respiratory disease. Carbon dioxide ( CO2 ) is a simple asphyxiant at high concentrations, meaning it displaces oxygen. It is not a long irritant. Carbon monoxide (CO) is a toxic gas that impairs the blood's ability to carry oxygen by binding to hemoglobin. It is not a lung irritant, Therefore, nitrogen dioxide ( NO2 ) is the correct answer.