There are a number of health and environmental issues attributed to NOx and derivatives in the family of nitrogen oxides, including smog, acid rain, and possibly global warming. The NOx family includes various compounds including nitrogen dioxide, nitric acid, nitrous oxide, nitrates, and nitric oxide. NOx and associated pollutants can be transported over great distances by the prevailing winds in the U.S. and the world.

NOx can react with other compounds and form nitrate particulate, acid aerosols, and toxic chemicals. A major health concern of these pollutants is the damage they can cause to the human respiratory system. If small particulates penetrate sensitive parts of the lungs, they can damage the tissue and consequentially cause or worsen respiratory diseases, such as emphysema and bronchitis.

	This diagram shows how anthropogenic (human caused) sources of emissions flow into the atmosphere and combine to form acid rain, which then returns to earth.

Ground-level ozone, or smog, forms when volatile organic compounds (VOCs - organic chemicals containing carbon, oxygen, hydrogen, chlorine, and other atoms) and NOx react in sunlight. Smog can have adverse effects on people with lung disease or can cause damage to the lungs of anyone who has long-term exposure. Smog, like many pollutants, can be carried great distances on the earth’s air currents.

Acid rain occurs when NOx and sulfur dioxide (SO₂), another emission, react with other substances to form acids. These acids form in the air and travel on air currents before falling to the earth captured in rain, fog, snow, and even dry particulate. In addition to the damage caused to physical structures, acid rain can cause lakes and streams to become acidic.

Global Warming
One member of the NOx family, nitrous oxide, is reported to be a greenhouse gas. You have, no doubt, heard of the “greenhouse effect” in the news. So, what is the greenhouse gas effect? Well, here is how it works.

	The Earth's Greenhouse Effect Photo courtesy U.S. Global Change Research Program

Energy from the sun (solar radiation) travels to the earth where some of the energy is reflected back into space by the earth’s atmosphere, but most passes through the atmosphere and warms the earth’s surface (water, land, etc.). However, if it weren’t for the earth's atmosphere trapping the energy, the earth's surface would still be cold. Some of the absorbed energy radiates back from the surface to warm the earth’s lower atmosphere. Half of the energy absorbed by the earth’s surface evaporates water – causing a vapor. When this water vapor condenses in the atmosphere, we get rain and snow. Only a small amount of heat radiated back from the earth’s surface escapes out to space. Most of the heat captured by the atmosphere is attributed to greenhouse gases, hence the term “greenhouse effect.” While it is true that the amount of greenhouse gas in the atmosphere fluctuates and also true that manmade causes contribute a small amount of greenhouse gas, man could not exist on the earth if it wasn’t for the atmosphere containing greenhouse gasses.

	Water Vapor Surrounding the Earth.
	The image above is a work of the National Oceanic and Atmospheric Administration, taken or made during the course of an employee's official duties. As works of the U.S. federal government, all NOAA images are in the public domain. Subject to disclaimers.
	The image above shows how the earth and its almost perfectly balanced atmosphere appears from space.

Greenhouse gases accumulate in the atmosphere and possibly cause a rise in the earth's temperature. It is difficult to ascertain that any one gas attributes more to the greenhouse effect than another. But it is useful to note that the list of greenhouse gases is long and includes carbon dioxide, sulfur hexaflouride, hydroflourocarbons, perfluorocarbons, chlorofluorocarbons … and water vapor. That’s right, water vapor is a natural greenhouse gas that could contribute as much as 70% of the greenhouse effect on the earth, and that does not include clouds.

The image to the upper-right shows water vapor surrounding the earth from a Geostationary Operational Environmental Satellite (GOES). This image shows the distribution of water vapor that surrounds the planet, relative to the oceans, clouds, and continents of the Earth. Note that the vapor is unevenly distributed.

Keep in mind, trapped heat within our atmosphere is a critical part of the earth’s climate. Under ideal, balanced conditions, the amount of energy entering the system from solar radiation will exactly match the amount of energy being radiated into space. In this scenario, the earth would maintain a constant climate. However, because no system is ever perfectly balanced, we can assume that the earth will periodically experience fluctuations in the amount of retained heat and the amount of greenhouse gases in the atmosphere. These fluctuations would include periods of global warming and global cooling. Now add a human element generating additional greenhouse gases and you can understand the concern surrounding the issue.