Nitrous oxide is the major component of combustion and can become nitrogen dioxide through photochemical reactions. Nitrogen dioxide is a gas with yellowish color and irritating odor. Under the sunlight, nitrogen dioxide can breakdown into nitrous oxide and oxygen by photochemical reactions. The oxidation reactions of nitrogen dioxide in the air are also a source of acid rain formation. In addition to the natural formation of carbon monoxide such as forest fire, methane oxidation, and biological activities, the major sources of carbon monoxide come from incomplete combustion of fossil fuel.
Carbon monoxide is lighter than air and is a colorless and odorless gas.
Due to its affinity with hemoglobin in the blood system is stronger than the oxygen, the carbon monoxide will compete with oxygen for binding sites in hemoglobin. Thus, the toxic effect of carbon monoxide to human and animals is the result of oxygen depletion in the blood system. Ozone is a secondary pollutant formed through the series reactions of nitrogen oxides and active hydrocarbons under the sunlight. Due to its strong oxidative reactions, ozone can cause irritating effects on respiratory system such as coughing, asthma, headache, tired, and even lung damage. Children, elder people, patients, or person with active outdoor activities are most vulnerable to ozone damage.