What is Air Quality Index?

Air quality index (AQI) is a numerical scale used for reporting day to day air quality with regard to human health and the environment. The daily results of the index are used to convey to the public an estimate of air pollution level. An increase in air quality index signifies increased air pollution and severe threats to human health. In most cases, AQI indicates how clear or polluted the air in our surrounding is, and the associated health risks it might present. The AQI centers on the health effects that may be experienced within a few days or hours after breathing polluted air.

AQI calculations focus on major air pollutants including: particulate matter, ground-level ozone, sulfur dioxide (SO2), nitrogen dioxide (NO2), and carbon monoxide (CO). Particulate matter and ozone pollutants pose the highest risks to human health and the environment. For each of these air pollutant categories, different countries have their own established air quality indices in relation to other nationally set air quality standards for public health protection.

According to Wikipedia,

An air quality index (AQI) is a number used by government agencies to communicate to the public how polluted the air currently is or how polluted it is forecast to become. As the AQI increases, an increasingly large percentage of the population is likely to experience increasingly severe adverse health effects.

How Does Air Quality Work/Determined?

On an hourly basis, the concentration of each pollutant in the air is measured and converted into a number running from zero upwards by using a standard index or scale. Calculated number for every pollutant is termed as a sub-index. The highest sub-index for any given hour is recorded as the AQI for that hour. In simple terms, AQI is like a yardstick that ranges from zero to five hundred (0-500). The index is a relative scale, meaning, the lower the index the better the quality of air and the lesser the health concern, and vice versa. The concentration of each pollutant varies, therefore, AQI values are grouped into ranges assigned to a standardized public health warnings and color code.

For instance, an AQI value of 0 to 50 means good air quality with limited possibility of affecting public and environmental health. On the other hand, an AQI value of 300 to 500 represents hazardous air quality with greater potential to affect public and environment health. The commonly accepted value is 100 as it corresponds to the generally approved air quality standards as set to safeguard public health. AQI levels below 100 are highly satisfactory while values beyond 100 are regarded to harm human health. As the AQI values get higher, it even poses more serious health concerns.

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During days that the AQI is recorded to be elevated, the institution of public health might: offer advice to sensitive groups such as the children, those with respiratory problems, and the elderly to keep away from outdoor activities; take action to improve air quality by introducing measures for reducing emissions; or recommend the use of air pollution masks, especially in severe cases of air pollution.

Formula for Calculating Air Quality Index

Air quality index is calculated by a linear function by determining the concentration of the pollutant. The equation below is used to compute AQI.


I = the (Air Quality) index,

C = the pollutant concentration,

C {low} = the concentration breakpoint that is ≤ C,

C {high} = the concentration breakpoint that is ≥ C,

I {low} = the index breakpoint corresponding to C {low},

I {high} = the index breakpoint corresponding to C {high}.

Air Quality Index Categories

The AQI is divided in six categories and each category is meant to correspond to different health concern levels. Below is an explanation of the categories and their meanings.

  • 0 – 50 indicates “Good” AQI. At this level the quality of air is deemed to be satisfactory, and air pollution poses little or no risk.
  • 51 – 100 indicate “Moderate” AQI. This means acceptable Air quality. However, some pollutants might arouse modest health concern for a limited number of people. For instance, persons who are remarkably sensitive to ozone may experience respiratory symptoms.
  • 101 – 150 indicate “Unhealthy for Sensitive Groups” AQI. This category may not be able to affect the general health of the public. However, children, older adults, and persons with lung disease are at a greater risk from ozone exposure. Older children, adults and people with lung and heart disease are at greater risk from exposure to particulate matter.
  • 151 – 200 indicate “Unhealthy” AQI. In this category, every person might experience some adverse health effects. Sensitive group members may experience more serious effects. People with heart or lung disease, older adults and children should cut back or reschedule strenuous activities.
  • 201 – 300 indicate “Very Unhealthy” AQI. This would issue a health alert to mean that everybody may experience very serious health implications. People with heart or lung disease, older adults and children should significantly cut back or reschedule strenuous activities.
  • Greater than 300 indicate “Hazardous” AQI. Air quality at this level is life-threatening and would issue warnings of emergency conditions for the entire population.
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At a glance, the table below provides the Air Quality Index (AQI) categories

Air Quality Index
(AQI) Values
Levels of Health Concern Colors
When the AQI is in this range: ..air quality conditions are: …as symbolized by this color:
0-50 Good Green
51-100 Moderate Yellow
101-150 Unhealthy for Sensitive Groups Orange
151 to 200 Unhealthy Red
201 to 300 Very Unhealthy Purple
301 to 500 Hazardous Maroon

How Can I Avoid Exposure to Unhealthy Air?

The AQI is calculated for four major air pollutants regulated by the Clean Air Act: ground-level ozone, particle pollution, carbon monoxide, and sulfur dioxide. You need to take following simple steps to avoid exposure to unhealthy air:

Prolonged Exertion: Prolonged exertion is any outdoor activity that you do intermittently for several hours and may cause you to breather slightly faster than normal. When air is unhealthy outside, you can reduce intake of unhealthy air by reducing how much time you spend on this type of activity.

Heavy Exertion: Heavy exertion means intense outdoor activities that cause you to breathe hard. When air quality is bad outside, you can protect your health by reducing the amount of time you spend on this activity or by substituting it with less intense activity.

Four Major Types of Air Pollutants

Ozone: Ozone is a gas found in the air we breathe. Several people who are active outdoor are sensitive to ozone as ozone levels are high outdoors. When you do physical activity, it causes faster and deep breathing which draws more ozone into the body. Good ozone is present naturally in the Earth’s upper atmosphere. Bad ozone forms near the ground when pollutants react chemically in sunlight.

Particulate Pollution: Particle pollution (also known as “particulate matter”) consists of a mixture of solids and liquid droplets. Some particles are emitted directly; others form when pollutants emitted by various sources react in the atmosphere. People with heart or lung disease are more sensitive to particulate pollution. Particulate pollution comes in 2 sizes: Fine particles (those 2.5 micrometers or less in diameter) and Coarse particles (particles between 2.5 and 10 micrometers).

Carbon Monoxide: Carbon monoxide is an odorless, colorless gas. It forms when the carbon in fuels does not completely burn. The major sources of carbon monoxide are vehicles exhaust, industrial processes, natural sources such as wildfires. Carbon monoxide can enter into the body through lungs and binds to hemoglobin. It is a substance in blood that carries oxygen to cells. In the body, it (carbon monoxide) reduces the amount of oxygen reaching the body’s organs and tissues.

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Sulfur Dioxide: Sulfur dioxide, a colorless, reactive gas, is produced when sulfur-containing fuels such as coal and oil are burned. Major sources of sulfur dioxide include power plants, refineries, and industrial boilers. It is more commonly found near industrial complexes.

AQI colors

Each AQI category is assigned a specific color to make it easier for people to understand the unhealthy levels of air pollution. For example, the color red means that conditions are “unhealthy for everyone.” Please see the table for various color meanings of each AQI category.

Air Quality Index Levels of Health Concern Numerical
Good 0 to 50 Air quality is considered satisfactory, and air pollution poses little or no risk
Moderate 51 to 100 Air quality is acceptable; however, for some pollutants there may be a moderate health concern for a very small number of people who are unusually sensitive to air pollution.
Unhealthy for Sensitive Groups 101 to 150 Members of sensitive groups may experience health effects. The general public is not likely to be affected.
Unhealthy 151 to 200 Everyone may begin to experience health effects; members of sensitive groups may experience more serious health effects.
Very Unhealthy 201 to 300 Health warnings of emergency conditions. The entire population is more likely to be affected.
Hazardous 301 to 500 Health alert: everyone may experience more serious health effects

Source: http://airnow.gov/index.cfm?action=aqibasics.aqi

 Image credit: flickr , Roman
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