Carbon monoxide (CO) is a colorless, odorless and poisonous gas produced by incomplete burning of carbon in fuels. When CO enters the bloodstream, it reduces the delivery of oxygen to the body's organs and tissues. Health threats are most serious for those who suffer from cardiovascular disease. Exposure to elevated CO levels can cause impairment of visual perception, manual dexterity, learning ability and performance of complex tasks.
77% of the nationwide CO emissions are from transportation sources. The largest emissions contribution comes from highway motor vehicles. Thus, the focus of CO monitoring has been at traffic-oriented sites in urban areas where the main source of CO is motor vehicle exhaust.
NAAQS: 9 ppm (8-hr average) and 35 ppm (1-hr average)
Lead (Pb) is a widely used metal that, once released to the environment, can contaminate air, food, water, or soil. Exposures to even small amounts of lead over a long time can accumulate to reach harmful levels. Harmful effects may therefore develop gradually without warning. Short-term exposure to high levels of lead may also cause harm. Lead can adversely affect the nervous, reproductive, digestive, cardiovascular blood-forming systems, and the kidney. In men, adverse reproductive effects include reduced sperm count and abnormal sperm. In women, adverse reproductive effects include reduced fertility, still-birth, or miscarriage. Children are a sensitive population as they absorb lead more readily and their developing nervous system puts them at increased risk for lead-related harm, including learning disabilities.
Lead gasoline additives, non-ferrous smelters, and battery plants are the most significant contributors to Pb emissions into the atmosphere. In 1993 transportation sources contributed 33% of the annual emissions, down substantially from 81% in 1985. Total Pb emissions from all sources dropped from 20,100 tons in 1985 to 4,900 tons in 1993. The decrease in Pb emissions from cars and trucks shifting to lead-free gasoline accounts for essentially all of this decline.
NAAQS: 1.5 ug/m3 (quarterly average)
NITROGEN DIOXIDE (NO2)
Nitrogen dioxide (NO2) is a brownish, highly reactive gas that is present in all urban atmospheres. NO2 can irritate the lungs, cause bronchitis and pneumonia, and lower resistance to respiratory infections.
The major mechanism for the formation of NO2 in the atmosphere is the oxidation of nitric oxide (NO), which is produced by most combustion processes.
NAAQS: 0.053 ppm (annual mean)
NITROGEN OXIDES (NOx)
Nitrogen oxides (NOx) include various nitrogen compounds like nitrogen dioxide (NO2) and nitric oxide (NO). These compounds play an important role in the atmospheric reactions that create ozone (O3) and acid rain. Individually, they may affect ecosystems, both on land and in water.
NOx forms when fuels are burned at high temperatures. The two major emissions sources are transportation vehicles and stationary combustion sources such as electric utility and industrial boilers.
Ozone (O3) is the major component of smog. Although O3 in the upper atmosphere is beneficial because it shields the earth from the sun's harmful ultraviolet radiation, high concentrations of O3 at ground level are a major health and environmental concern. The reactivity of O3 causes health problems because it damages lung tissue, reduces lung function and sensitizes the lungs to other irritants. Scientific evidence indicates that ambient levels of O3 not only affect people with impaired respiratory systems, such as asthmatics, but healthy adults and children as well. Exposure to O3 for several hours at relatively low concentrations has been found to significantly reduce lung function and induce respiratory inflammation in normal, healthy people during exercise.
O3 is not usually emitted directly but is formed through complex chemical reactions in the atmosphere. Precursor compounds like volatile organic compounds (VOC) and oxides of nitrogen (NOx) react to form O3 in the presence of sunlight. These reactions are stimulated by ultraviolet radiation and temperature, so peak O3 levels typically occur during the warmer times of the day and year.
NAAQS: 0.12 ppm (1-hr average) and 0.08 ppm (8-hr average)
PARTICULATE MATTER (PM)