What are the hazards of nitrogen oxides to human body?

In the normal atmospheric composition, nitrogen accounts for about 79% of the total atmospheric volume. As a single free atom, nitrogen has high reactivity. But there are a lot of chemically stable nitrogen molecules in the atmosphere. Harmful to human health mainly refers to the combination of nitrogen and oxygen into various forms of compounds, including nitrous oxide (N2O), nitric oxide (NO), nitrous oxide (N2O3), nitrogen dioxide (NO2), nitrous oxide (N2O4), nitrous oxide (N2O5) and so on.

Nitrogen oxides have little irritation to eyes and upper respiratory mucosa, and mainly invade bronchioles and alveoli in the deep respiratory tract. After nitrogen oxides enter alveoli, the reaction is accelerated due to the increase of surface humidity of alveoli, of which about 80% can stay in alveoli and some become nitrogen tetroxide. Nitrogen tetroxide and nitrogen dioxide can react with the moisture of respiratory mucosa to generate nitrous acid and nitric acid, which have strong stimulation and corrosion effects on lung tissue, thus increasing the permeability of capillaries and alveolar walls and causing pulmonary edema. After nitrite enters the blood, it can also cause vasodilation and blood pressure drop, and can react with hemoglobin to generate methemoglobin, causing tissue hypoxia. High concentration of nitric oxide can also change oxygen and hemoglobin in blood into methemoglobin, causing tissue hypoxia.

Therefore, generally speaking, when the pollution is mainly nitrogen dioxide, the damage to the lungs is obvious, and pulmonary edema can occur in severe cases. But when there is a lot of nitric oxide in the mixed gas, the formation of methemoglobin is dominant. At this time, the poisoning developed rapidly, with symptoms of methemoglobinemia and central nervous system damage. When the content of nitric oxide is above 100× 10-6, people and animals can die within a few minutes, and inhaling nitrogen dioxide with a concentration of 5× 10-6 can cause harm to the respiratory system within a few minutes. Nitrogen oxides are more harmful because they participate in the formation of photochemical smog and acid rain.