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. Common sources include industry, vehicles and equipment, gas stations, paint, and biogenic emissions from trees and plants. Because VOC sources vary greatly, and are therefore more difficult to control, NOx has been labeled the limiting factor in ozone formation and is the focus of the most recent ozone reduction strategies. Ozone exceedances can occur quite far from NOx and VOC sources, depending upon weather and wind patterns and mixing heights. Health effects of ground-level ozone Ozone can cause or aggravate respiratory problems.