NO2 or nitrogen dioxide is one of the main air pollutants: a toxic irritant gas that affects the respiratory system. NO2 can irritate the airways in the human respiratory system, aggravating respiratory diseases (such as asthma) and leading to increased hospital admissions and visits to emergency rooms. In the long term, NO2 exposure can lead to the development of asthma and increase susceptibility to respiratory infections. Its main source is motor vehicle emissions, particularly from diesel engines (the older they are, the more they pollute). NO2 levels are generally higher in cities due to their higher volume of traffic and are usually related to the way cities are designed.