Nitric oxide plus nitrogen dioxide are known as NOx. The interaction of nitrogen and oxygen create these inorganic gases that stay in the air. The main formation seems nitric oxide, but after an oxidation nitrogen dioxide appears.
The presence of NOx is common after burning fuel thus the main sources are transportation and the industry. There are limits trying to establish safety although several sources cite how they are frequently surpassed as a logical consequence of the excessive amount of pollution sources.
In terms of health, these gases destroy ozone creating an easier path to UV radiation toward the atmosphere. The effect of UV radiation possess risks by different ranges of action. After interactions with organic chemicals it's a viable path to the formation of nitrosamines, a highly toxic molecule also with a tested activity to promote abnormal cell functions. If they are created in the public air, they could end in any place increasing the statistical rate of fatal illnesses with no apparent reason, or categorized as "natural". It occurs similarly with PM 2.5 or PM 10.
Others state that NOx is harmful for the respiratory system, specifically in the bronchial area.