Above certain concentrations, ozone, PM, and CO can cause or exacerbate health problems and/or increase mortality rates, making their control an important goal under the CAA.
Ground-level ozone is the major component of smog. While ozone in the upper atmosphere (the ozone layer) occurs naturally and protects life on earth from harmful ultraviolet radiation, ozone at ground level is a noxious pollutant. Ground- level ozone is not directly emitted, but is formed by the reaction of NOx and VOCs in the presence of sunlight.
Ozone is a severe irritant, responsible for the choking, coughing, and stinging eyes associated with smog. Ozone damages lung tissue, aggravates respiratory disease, and makes people more susceptible to respiratory infections. Children are especially vulnerable to ozone's harmful effects, as are adults with existing disease. Even healthy individuals may experience impaired lung function from breathing ozone-polluted air. In addition to affecting human health, ozone harms vegetation, resulting in reduced agricultural and commercial forest yields, increased tree and plant susceptibility to disease and other environmental stresses, and potential long-term effects on forests and ecosystems. Peak concentrations typically occur in summer.
PM is the term used for a mixture of solid particles and liquid droplets found in the air. These particles come in a wide range of sizes and can remain suspended in the air for extended periods. PM can be emitted directly by a source or formed in the atmosphere by the transformation of gaseous emissions, such as SO2, NOx, and VOCs.
Fine particles, fewer than 2.5 microns in diameter (PM2.5), result from fuel combustion by motor vehicles and other sources, as well as transformation of gaseous emissions. Coarser particles up to 10 microns in diameter (PM10 ) generally consist of windblown dust and are released from agriculture, and crushing and grinding operations.
Particulate matter irritates the membranes of the respiratory system, causing increased respiratory problems and disease, decreased lung function, alterations of the body's defense systems, and premature mortality. Sensitive groups include the elderly, individuals with cardiopulmonary disease such as asthma, and children. In addition to health problems, airborne particles cause soiling and damage to materials and reduce visibility in many parts of the United States.
CO is an odorless, colorless gas that interferes with the delivery of oxygen to the body's organs and tissues. Effects of CO include dizziness, headaches, fatigue, visual impairment, reduced work capacity, reduced manual dexterity, and poor learning ability. The health effects of CO vary depending on the length and intensity of exposure and the health of the individual, and are most serious for those who suffer from cardiovascular disease.
The incomplete burning of carbon in fuels such as gasoline produces CO. High concentrations of CO occur along side roads with heavy traffic, particularly at major intersections, and in enclosed areas, such as garages and poorly ventilated tunnels. Peak concentrations typically occur during the colder months of the year when vehicular emissions of CO are greater and nighttime inversion conditions are more frequent.
NO2 is one of a group of highly reactive gasses known as “oxides of nitrogen,” or “nitrogen oxides (NOx).” NO2 forms quickly from emissions from cars, trucks and buses, power plants, and off-road equipment. In addition to contributing to the formation of ground-level ozone and fine particle pollution, NO2 is linked with a number of adverse effects on the respiratory system.