Nitrogen Dioxide is considered as one of the  most polluting compounds in the air. NO2 is  produced by automobiles and other kinds of  combustion.  

Long-term exposure to NO2 may cause a wide  spectrum of severe health problems such as  hypertension, diabetes, heart and  

cardiovascular diseases, and even death. 

NO2 can also form indoors when fossil fuels  like wood or natural gas are burned. Current  guidelines by the WHO says that one hour  exposure to 106 ppb of Nitrogen Dioxide26 is  acceptable. 

At about twice this level, asthmatics may find  it harder to breathe due to reduced  pulmonary function. For normal people,  breathing in NO2 at this level can cause slight  difficulties in breathing.