Carbon monoxide (CO) is a poisonous gas that forms when the carbon in fuels such as gasoline, heating oil, natural gas, wood and charcoal does not burn completely. Carbon monoxide cannot be seen or smelled, but it can be dangerous to our health and in high concentrations, even deadly.
Motor vehicle exhaust contributes roughly 60 percent of all carbon monoxide emissions nationwide, and up to 95 percent in cities. Air concentrations of carbon monoxide can be particularly high in areas with heavy traffic congestion. Other sources include industrial boilers, waste incinerators and natural events, such as wildfires.
Levels of carbon monoxide in the air are typically highest during the winter, because vehicles work harder and burn fuel less efficiently in cold weather, and on winter nights, a strong inversion layer develops in the atmosphere, trapping pollution near the ground and preventing it from mixing with cleaner air above.
Health Effects of Carbon Monoxide & Who Is Most at Risk
Carbon monoxide enters the bloodstream through the lungs and binds to hemoglobin, the substance in blood that carries oxygen to cells. This reduces the amount of oxygen that reaches our body's tissues and organs, especially the heart and brain, as well as its central nervous system. Tests of automobile drivers show exposure to carbon monoxide can impair their judgment and ability to respond rapidly in traffic.
The health threat from exposure to carbon monoxide is most serious for people who suffer from cardiovascular diseases, such as angina. They may experience chest pain and other cardiovascular symptoms if they are exposed to the gas, particularly while exercising.
People with cardiovascular and respiratory problems such as cerebrovascular disease, chronic obstructive lung disease, congestive heart failure and anemia also are at greater risk from carbon monoxide exposure, as are young infants and developing fetuses.
Even in healthy individuals, exposure to higher levels of carbon monoxide can lead to headaches and affect manual dexterity, mental alertness, work capacity and vision.
Environmental Effects of Carbon Monoxide
Carbon monoxide is one of several pollutants that can interact in the presence of sunlight to produce ground-level ozone or "smog," particularly on hot summer days. In addition to posing a health risk, ozone can damage buildings and harm crops. Carbon monoxide also can have health impacts on wildlife similar to those experienced by people: decreased supply of oxygen to the organs, tissues and central nervous system.
Carbon Monoxide Standards
The National Ambient Air Quality Standards (NAAQSs) for carbon monoxide, established by the U.S. Environmental Protection Agency (EPA), are 9 parts per million (ppm) measured as an annual second-maximum eight-hour average concentration and 35 ppm averaged over a one-hour period.
Although these are very protective standards - most people do not begin to feel the effects of carbon monoxide poisoning until they have been exposed to levels of at least 200 ppm for several hours or more - they underscore the degree of public health concern about carbon monoxide exposure.
Long-Term Carbon Monoxide Trends
Before the mid 1980s, carbon monoxide levels in some Massachusetts cities occasionally exceeded the levels allowed under the NAAQS, prompting EPA to consider the state in violation of the standard. A variety of air pollution control initiatives since then, however, have dramatically reduced carbon monoxide levels statewide. Today, all of Massachusetts meets the EPA standard. Our state's last violation of the NAAQS for carbon monoxide was recorded in 1986.
Nationwide, carbon monoxide emissions declined by 16 percent and average air concentrations of the poisonous gas dropped by 37 percent during the decade ending in 1995.