The principal nitrogen pollutants generated by boilers are nitric oxide (NO) and nitrogen dioxide (NO2), collectively referred to as NOx. A significant portion of the NOx from stationary sources can be attributed to residential, commercial, and industrial sources, including industrial boilers. The contribution from different NOx sources to the total NOx levels varies among metropolitan areas.
Although there is evidence proving NOx, in itself, is harmful to humans, the main reason NOx is considered an environmental problem is because it initiates reactions that result in the production of ozone (O3) and acid rain. Ozone is created when nitrogen oxides and volatile organic compounds (VOCs) combine in the presence of sunlight. Plenty of sunlight is needed to create smog, which is why we get more of it in summer than in the winter. In some major urban areas, more than 50% of the ozone forming nitrogen oxides come from vehicles.
Ground level ozone is a major component of urban smog. Ozone and acid rain can result in human health problems, damage to vegetation, reduced visibility, damage to buildings, and harm forests and lakes. Children, the elderly, and people with respiratory or heart conditions are the most vulnerable to the effects of smog. Fine particles, the other major component of smog, are emitted directly into the air from many sources. They are also formed in the atmosphere from the chemical reaction of gaseous pollutants. By controlling NOx levels, along with the other pollutants, the levels of acid rain and ozone can be reduced.
Source: Sunil Kumar, last updated on 9-2002