Ozone is formed by chemical reactions between volatile organic compounds (VOCs) and oxides of nitrogen in the presence of sunlight and elevated temperatures. The primary human sources of VOCs and nitrogen oxides are industrial and automobile emissions. Ozone is a colorless gas that exists naturally in the upper atmosphere. It shields the earth from the sun’s harmful ultraviolet rays. Ozone close to the earth’s surface is an air pollutant. Ozone concentrations can be transported hundreds of miles to even the most remote areas of the country.

Human Health Effects of Ozone

High concentrations of ozone can cause inflammation and irritation of the respiratory tract, particularly during physical activity. The resulting symptoms may include pain when taking a deep breath, coughing, throat irritation, and breathing difficulties. High concentrations of ozone can affect lung function and worsen asthma attacks. Exposure to ozone can damage lung tissue and increase the susceptibility of the lungs to infections, allergens, and other air pollutants. Medical studies have shown that health problems caused by ozone may continue long after exposure has ended.

In some National Forests in the Southeast, Northeast and California, ozone concentrations have exceeded standards set by EPA to protect human health. And while the Pacific Northwest and Intermountain West experience lower levels of ozone pollution than other regions of the country, but is increasing in the Colorado Plateau and Rocky Mountains regions.

Natural Resource Effects of Ozone

Ozone is one of the most phytotoxic air pollutants, and causes considerable damage to vegetation throughout the world. Plants are generally more sensitive to ozone than humans. Most ozone effects research has concentrated on crops and large economic losses have been documented.

Many native plants in natural ecosystems are sensitive to ozone. The effects of ozone range from visible injury to the leaves and needles of deciduous trees and conifers to premature leaf loss, reduced photosynthesis, and reduced growth in sensitive plant species. These physiological changes can occur in the absence of foliar injury, and vice versa. In a natural ecosystem, many other factors, such as soil moisture, presence of other air pollutants, insects or diseases, genetic make-up, topographical locations, and other environmental stresses, can lessen or magnify the extent of ozone injury. For example, higher ozone exposure levels occur at higher elevations, thus, high elevation vegetation is more at risk to injury.

In July 1997, the Environmental Protection Agency (EPA) established a new federal public health standard for ozone. The new standard is based on an 8-hour average concentration set at 0.08 parts per million (ppm), which is significantly more stringent than the old standard based on a 1-hour concentration set at 0.12 ppm. (Note: due to rounding, EPA considers an exceedance of the 8-hour standard when the ozone concentration reaches 0.085 ppm or 85 parts per billion (ppb).) The EPA has also proposed rules for air quality index reporting (Federal Register Notice Volume 63, Number 236, 12/9/98).