Numerous public and private power plants also contribute heavily to area air pollution, as do the more than three million vehicles currently on the road in this region. Add the activity of the Port of Houston, our two major airports, and many smaller air pollution sources associated with the bustle and boom of urban life, and the result is a many-dimensioned problem. For each of these pollutants, the EPA has established primary standards intended to protect public health and secondary standards intended to protect public welfare by preventing plant and property damage. The primary and secondary standard for ozone is 0.12 parts per million averaged over one hour. In 2000, the Houston region, for the second year in a row, not only led the country in the sheer number of days on which at least one monitor exceeded the federal standard, but also had the single highest one-hour ozone reading in the United States.
Confusion sometimes arises because ozone is found at two levels of the earth's atmosphere.
It is this protective layer of ozone that is being damaged by human activity. Ground-level ozone is a corrosive gas and a principal component of smog. But at the higher concentrations resulting from human activity, ground-level ozone is known to harm humans, animals, and plants. Nitrogen oxides are largely a product of high-temperature combustion. Volatile organic compounds are a wide range of chemicals that vaporize easily.