The AQI & VSI Air Quality Reporting Systems
The following topic links address:
- How air quality conditions are reported
- When they are reported
- How you can use the reporting systems to protect your health
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The Colorado Department of Public Health and Environment's Air Pollution Control Division uses two reporting systems to inform the public about air quality conditions - the AQI and VSI reporting systems.
The AQI system, using the Air Quality Index, reports levels of carbon monoxide, ozone and PM10 visibility conditions in the Denver metro area.
(particulate matter smaller than 10 microns). The VSI system, using the Visibility Standard Index, reports
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The AQI reporting system provides a simple, uniform way to report daily levels of air pollution to residents of the Denver metro area, Colorado Springs and some other Colorado communities.
Year round, the Division's continuous monitoring system provides hourly levels of carbon monoxide, ozone and PM10. These are the three pollutants of greatest concern in Colorado. All three pollutants can harm human health. The AQI is not reported for Lead, nitrogen dioxide, or PM2.5. A federal National Ambient Air Quality Standard defines the maximum levels each pollutant can reach before unhealthy conditions exist.
National Ambient Air Quality Standards
| POLLUTANT | STANDARD VALUE | STANDARD TYPE | |
| Carbon Monoxide (CO) | |||
| 8-hour Average | 9 ppm | (10 mg/m3)[2] | Primary |
| 1-hour Average | 35 ppm | (40 mg/m3)[2] | Primary |
| Nitrogen Dioxide (NO2) | |||
| Annual Arithmetic Mean | 0.053 ppm | (100 µg/m3)[2] | Primary & Secondary |
| Ozone (O3) | |||
| 1-hour Average[1] | 0.12 ppm | (235 µg/m3)[2] | Primary & Secondary |
| 8-hour Average | 0.075 ppm (2008 std) | (147 µg/m3)[2] | Primary & Secondary |
| 8-hour Average | 0.08 ppm (1997 std) | (157 µg/m3)[2] | Primary & Secondary |
| Lead (Pb) | |||
| Quarterly Average | 1.5 µg/m3 | Primary & Secondary | |
| Particulate < 10 micrometers (PM-10) | |||
| 24-hour Average | 150 µg/m3 | Primary & Secondary | |
| Particulate < 2.5 micrometers (PM-2.5) | |||
| Annual Arithmetic Mean | 15 µg/m3 | Primary & Secondary | |
| 24-hour Average | 35 µg/m3 | Primary & Secondary | |
| Sulfur Dioxide (SO2) | |||
| Annual Arithmetic Mean | 0.03 ppm | (80 µg/m3)[2] | Primary |
| 24-hour Average | 0.14 ppm | (365 µg/m3)[2] | Primary |
| 3-hour Average | 0.50 ppm | (1300 µg/m3)[2] | Secondary |
- The standard is attained when the expected number of days per calendar year with maximum hourly average concentrations above 0.12 ppm is < 1.
- As of June 15, 2005 EPA revoked the 1-hour ozone standard in all areas except the 8-hour ozone nonattainment Early Action Compact (EAC) Areas. In Colorado, the 1-hour ozone standard was revoked effective June 15, 2005 for all areas of the state except the Denver (Denver-Boulder-Greeley-Ft.Collins-Loveland) area.
- Parenthetical value is an approximately equivalent concentration.
After analyzing current data, Division staff convert the information about the individual pollutant concentrations into numbers on a scale - the AQI scale. The National Ambient Air Quality Standard for each pollutant equals 101 on the AQI scale. AQI reports greater than 100 generally indicate exceedances of a pollutant's standard. The higher the AQI reading, the worse the air quality.
The AQI scale is described by six air quality categories. These are good, moderate, unhealthy for sensitive groups, unhealthy, very unhealthy, and hazardous.
| AQI | Descriptor | Health Messages | ||
| Ozone (O3) | Carbon Monoxide (CO) | Particulate Matter (PM) | ||
| 0 to 50 | Good | OZONE: None | CO: None | PM: None |
| 51 to 100 | Moderate | OZONE: Unusually sensitive individuals may experience respiratory symptoms. Unusually sensitive people should consider limiting prolonged outdoor exertion. |
CO: None | PM: None |
| 101 to 150 | Unhealthy for Sensitive Groups | OZONE: Increasing likelihood of respiratory symptoms and breathing discomfort in active children and adults and people with respiratory disease, such as asthma. Active children and adults and people with respiratory disease, such as asthma should limit prolonged outdoor exertion. |
CO: Increasing likelihood of reduced exercise tolerance due to increased cardiovascular symptoms, such as chest pain, in people with cardiovascular disease. People with cardiovascular disease, such as angina, should limit heavy exertion and avoid sources of carbon monoxide, such as heavy traffic. |
PM: Increasing likelihood of respiratory symptoms in sensitive individuals, aggravation of heart or lung disease and premature mortality in persons with cardiopulmonary disease and the elderly. People with respiratory or heart disease, the elderly and children should limit prolonged exertion. |
| 151 to 200 | Unhealthy | OZONE: Greater likelihood of respiratory symptoms and breathing difficulty in active children and adults and people with respiratory disease, such as asthma; possible respiratory effects in general population. Active children and adults, and people with respiratory disease, such as asthma, should avoid prolonged outdoor exertion; everyone else, especially children, should limit prolonged outdoor exertion. |
CO: Reduced exercise tolerance due to increased cardiovascular symptoms, such as chest pain, in people with cardiovascular disease. People with cardiovascular disease, such as angina, should limit moderate exertion and avoid sources of carbon monoxide, such as heavy traffic. |
PM: Increased aggravation of heart or lung disease and premature mortality in persons with cardiopulmonary disease and the elderly; increased respiratory effects in general population. People with respiratory or heart disease, the elderly and children should avoid prolonged exertion; everyone else should limit prolonged exertion. |
| 201 to 300 | Very Unhealthy | OZONE: Increasingly severe symptoms and impaired breathing likely in active children and adults and people with respiratory disease, such as asthma; increasing likelihood of respiratory effects in general population. Active children and adults, and people with respiratory disease, such as asthma, should avoid all outdoor exertion; everyone else, especially children, should limit outdoor exertion. |
CO: Significant aggravation of cardiovascular symptoms, such as chest pain, in people with cardiovascular disease. People with cardiovascular disease, such as angina, should avoid exertion and sources of carbon monoxide, such as heavy traffic. |
PM: Significant aggravation of heart or lung disease and premature mortality in persons with cardiopulmonary disease and the elderly; significant increase in respiratory effects in general population. People with respiratory or heart disease, the elderly and children should avoid any outdoor activity; everyone else should avoid prolonged exertion. |
| 300 to 500 | Hazardous | OZONE: Severe respiratory effects and impaired breathing likely in active children and adults and people with respiratory disease, such as asthma; increasingly severe respiratory effects in general population. Everyone should avoid all physical activity outdoors. |
CO: Serious aggravation of cardiovascular symptoms, such as chest pain, in people with cardiovascular disease; impairment of strenuous activities in general population. People with heart disease, such as angina should avoid exertion and souces of CO, such as heavy traffic. Everyone else should reduce heavy exertion. |
PM: Serious aggravation of heart or lung disease and premature mortality in people with cardiopulmonary disease and older adults; serious risk of respiratory effects in general population. People with heart or lung disease, older adults, and children should remain indoors and keep activity levels low. Everyone else should avoid all physical activity outdoors. |
According to the index, any reading between 0-50 indicates good air quality, 51-100 moderate air quality, 101-150 unhealthy for sensitive groups, 151-200 unhealthy air quality, 201-300 very unhealthy air quality, and 301 and above hazardous air quality. A table at the end of this document lists the health effects for each AQI category by pollutant.
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In the Denver metro area, the Air Pollution Control Division utilizes a network of gaseous and particulate monitors to provide current air quality data. Each monitor reports to a computer at the Air Pollution Control Division. Hourly pollutant levels and corresponding AQI levels are tabulated and made available to the media and public.
The Visibility Standard Index (VSI) |
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The Visibility Standard Index is a measure of the visual air quality in the Denver metro area. In other words, the Visibility Standard Index corresponds to how clear the air is.
Air Pollution Control staff at the Department of Public Health and Environment use an instrument called a transmissometer to measure the clarity of the air. The instrument measures the amount of light that is able to pass through the atmosphere. The more light that is allowed to pass, the clearer the air.
Data from the transmissometer is converted to a Visibility Standard Index based on a scale much like the AQI scale.
| Visibility Standard Index (VSI) | Descriptor |
| 0 to 50 | Good |
| 51 to 100 | Moderate |
| 101 to 200 | Poor |
| 201 to 300 | Extremely Poor |
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The standard for visual air quality is 0.076 per kilometer of atmospheric extinction, which means that 7.6 percent of a light source's intensity is extinguished over a 1-kilometer path. A violation occurs when the four-hour average extinction exceeds the 0.076 standard. The standard is in effect during the core daylight hours from 8 a.m. to 4 p.m., although extinction is monitored 24 hours a day.
The Denver metro transmissometer instrument is housed atop a building near Cheesman Park. The device, a special telescope and sensor, is aimed at a light source of known brightness atop the Federal Building, one and one-half miles away.
The transmissometer measures the intensity of the light source and then relays the data to a computer that compares the measured intensity against the light's actual intensity. The difference represents how much light the atmosphere is absorbing or scattering. This difference is converted to a number on the VSI scale. Higher VSI values are associated with higher atmospheric extinction, hazier skies, and less clear the air.
Once this information is converted into numbers on the VSI scale, it is made available to the public twice daily via the Air Quality Bulletin. The Air Quality Bulletin is recorded on a 24-hour answering device (303/782-0211). Hourly VSI values can also be found by using the reports section of this web site.
Determining the actual VSI reading for a particular day can often be complicated by the presence of precipitation, relative humidity of 70 percent or greater, and the obscuring phenomena of fog. When such phenomena are present in the atmosphere, the VSI readings are excluded because they are non representative of the visual air quality problem.
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Visibility impairment in Colorado is most often caused by fine particles in the 0.1 to 2.5 micrometer size range. These particles will either scatter or absorb light, impacting a person's view of an object.
While particles of sulfate, nitrate, elemental carbon and organic carbon are the most effective at scattering and/or absorbing light, geologic dust also plays a role in decreasing visibility in Colorado. The man-made sources of these particles include woodburning, electric power generation, industrial combustion of coal or oil, agricultural practices, road sanding, and emissions from cars, trucks and buses.
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Since these three pollutants are most common in Colorado's major urban
areas and are potentially harmful to human health, the Air Pollution Control
Division keeps the public informed about their levels. Below is a description
of the health affects of these pollutants at different levels of the AQI:
For additional information on air quality and the AQI, try this EPA link:
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EnviroFlash is an e-mail and pager notification system that provides subscribers with instant air quality information that can be customized for individual needs. Denver-metropolitan area residents can sign up for this free service at:
https://enviroflash.epa.gov/airnow/subscriber/Subscriber.do?method=start