Nature and Sources of the Pollutant:
Carbon monoxide is a colorless, odorless, poisonous gas formed when carbon
in fuels is not burned completely. It is a byproduct of highway vehicle
exhaust, which contributes about 60 percent of all CO emissions nationwide.
In cities, automobile exhaust can cause as much as 95 percent of all CO
emissions. These emissions can result in high concentrations of CO, particularly
in local areas with heavy traffic congestion. Other sources of CO emissions
include industrial processes and fuel combustion in sources such as boilers
and incinerators. Despite an overall downward trend in concentrations and
emissions of CO, some metropolitan areas still experience high levels of
CO.
Health and Environmental Effects:
Carbon monoxide enters the bloodstream and reduces oxygen delivery to the
body's organs and tissues. The health threat from exposure to CO is most
serious for those who suffer from cardiovascular disease. Healthy individuals
are also affected, but only at higher levels of exposure. Exposure to elevated
CO levels is associated with visual impairment, reduced work capacity,
reduced manual dexterity, poor learning ability, and difficulty in performing
complex tasks. EPA's health-based national air quality standard for CO
is 9 parts per million (ppm) measured as an annual second-maximum 8-hour
average concentration.
Trends in Carbon Monoxide Levels:
Long-term improvements continued between 1986 and 1995. National average
CO concentrations decreased 37 percent while CO emissions decreased 16
percent. Long-term air quality improvement in CO occurred despite a 31
percent increase in vehicle miles traveled in the U.S. during the past
10 years. Between 1994 and 1995, national average CO concentrations decreased
10 percent, while total CO emissions decreased 7 percent. Transportation
sources (includes highway and off-highway vehicles) now account for 81
percent of national total CO emissions.
![CO Concentrations](https://webarchive.library.unt.edu/eot2008/20090511180459im_/http://www.epa.gov/air/airtrends/aqtrnd95/image5b1.gif)
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