National Ambient Air Quality Standard – 9 parts per million (ppm) – Not to be exceeded more than once a year.
Carbon monoxide (CO) is an odorless, colorless gas. It forms when the carbon in fuels does not completely burn. Vehicle exhaust contributes roughly 60 percent of all carbon monoxide emissions nationwide, and up to 95 percent in cities. Other sources include fuel combustion in industrial processes and natural sources such as wildfires. Carbon monoxide levels typically are highest during cold weather, because cold temperatures make combustion less complete and cause inversions that trap pollutants close to the ground.
Air Quality Index (AQI): Carbon Monoxide (CO)
Index Values
Levels of Health Concern
Cautionary Statements
0-50
Good
None
51-100*
Moderate
None
101-150
Unhealthy for Sensitive Groups
People with heart disease, such as angina, should reduce heavy exertion and avoid sources of CO, such as heavy traffic.
151-200
Unhealthy
People with heart disease, such as angina, should reduce moderate exertion and avoid sources of CO, such as heavy traffic.
201-300
Very Unhealthy
People with heart disease, such as angina, should avoid exertion and sources of CO, such as heavy traffic.
301-500
Hazardous
People with heart disease, such as angina, should avoid exertion and sources of CO, such as heavy traffic. Everyone else should reduce heavy exertion.
* An AQI of 100 for carbon monoxide corresponds to a CO level of 9 parts per million (averaged over 8 hours).
