Air Quality Index: Categories and Types of Air Pollutants


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 clean 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 is Air Quality 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. The 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 standardized public health warnings and color code.

For instance, an AQI value of 0 to 50 means good air quality with a 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 environmental 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.

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.

1. 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. This category has no health implications. Everyone can continue their outdoor activities normally.

 2. 51 – 100 indicate “Moderate” AQI

This means acceptable Air quality. However, some pollutants might arouse modest health concern for a limited number of hypersensitive people. For instance, persons who are remarkably sensitive to ozone may experience respiratory symptoms such as minor breathing difficulties. Only very few hypersensitive people are recommended to reduce outdoor activities.

3. 101 – 150 indicate “Unhealthy for Sensitive Groups” AQI

This category may not be able to affect the general health of the public. Healthy people may experience slight irritations, and sensitive individuals will be slightly affected to a larger extent. However, children, older adults, and persons with lung disease are at greater risk from ozone exposure.

Older children, adults and people with lung and heart disease are at greater risk from exposure to particulate matter and, therefore, should reduce sustained and high-intensity outdoor exercises.

4. 151 – 200 indicate “Unhealthy” AQI

In this category, every person might experience some adverse health effects. Sensitive group members may experience more serious effects. The hearts and respiratory systems of healthy people may be affected. Children, seniors and people with heart or lung disease should cut back sustained and high-intensity outdoor exercises or reschedule strenuous activities. General population should moderately reduce outdoor activities.

5. 201 – 300 indicate “Very Unhealthy” AQI

This would issue a health alert to mean that everybody may experience very serious health implications. Healthy people will commonly show symptoms. People with heart or lung disease will be significantly affected and will experience reduced endurance in activities.

Older adults and children should stay indoors and avoid strenuous outdoor activities, significantly considering rescheduling or avoiding such activities. General population should reduce outdoor activities.

6. 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. Healthy people will experience reduced endurance in activities and may also show noticeably strong symptoms. Other illnesses may be triggered in healthy people. Healthy individuals should avoid outdoor activities.

Elders and the sick should remain indoors and avoid outdoor exercise. It is recommended that children, seniors and the sick should stay indoors and avoid physical exertion. General population should avoid outdoor activities.

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 the 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 breathe slightly faster than normal. When air is unhealthy outside, you can reduce the 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.

Source: Canva

Eight Major Types of Air Pollutants

The air quality index is composed of 8 pollutants namely particulate matter (PM) 10, PM2.5, Ozone (O3), Sulphur dioxide (SO2), nitrogen dioxide (NO2), carbon monoxide (CO), lead (Pb) and ammonia (NH3) and acts as major parameters in deriving the AQI of an area.

Particulate Pollution (PM 2.5 and PM 10)

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).

1. PM10

It refers to inhalable particles that are less than or equal to 10 micrometers in diameter. Examples include dust, pollen, and mold.PM10 gets filtered at the top of your respiratory system that is nose.

2. PM2.5

These are fine particles that are less than or equal to 2.5 micrometers in diameter. They are about 1/30th of a strand of human hair that means too small for the human eye to see.PM2.5 is hazardous to the lot. PM2.5 causes 70-80 per cent of the health issues only when it crosses body’s natural barriers and enters the lungs. According to the Berkeley Earth science research group, breathing in air with a PM2.5 content of between 950 to 1,000 is like smoking 44 cigarettes a day.

3. Ozone (O3)

Ozone is a gas found in the air we breathe. Ozone is a greenhouse gas and the second most dangerous parameter as it aggravates lung diseases such as asthma, emphysema, and chronic bronchitis. It is created by a chemical reaction that takes place in exhaust and in the presence of sunlight. The gas is acrid-smelling and whitish.

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.

4. Sulfur Dioxide (SO2)

Sulfur dioxide, a colorless, reactive gas, is produced when sulfur-containing fuels such as coal and oil are burned. These gases react in the air to form particulate matter and, in large concentrations, lead to smog.Major sources of sulfur dioxide include power plants, refineries, and industrial boilers. It is more commonly found near industrial complexes.Volcanoes are also a natural source of SO2 emissions.

5. Nitrogen Dioxide (NO2)

Another dangerous type of urban air pollution is a group of gases known as nitrogen oxides. Nitrogen dioxide is a chemical compound. They are both highly reactive and odorless. They react in the air to form particulate matter (PM) and ozone. Especially nitrogen dioxide is emitted from high-temperature combustion. It is responsible for photochemical smog, acid rain etc.

6. Carbon Monoxide (CO)

Carbon monoxide is an odorless, colorless gas. While often thought of as an indoor hazard, it also is major outdoor air pollution as well. It forms when the carbon in fuels does not completely burn. The major sources of carbon monoxide are fossil fuel burning in vehicles exhaust, industrial processes, and natural sources such as wildfires.

Carbon monoxide can enter into the body through the lungs and binds to hemoglobin. It is a substance in the blood that carries oxygen to cells. In the body, it (carbon monoxide) reduces the amount of oxygen reaching the body’s organs and tissues.

7. Ammonia (NH3)

It is emitted from agricultural processes. Ammonia is a compound with the formula NH3. It is normally encountered as a gas with a characteristic pungent odor. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to foodstuffs and fertilizers.

Ammonia is also a building block for the synthesis of many pharmaceuticals, either directly or indirectly. Although in wide use, ammonia is both caustic and hazardous.

8. Lead (Pb)

Lead is a toxic metal that has toxic effect on man. Lead was once widely used in automobile fuels, paint, and pipes. The source of particulate matter Lead halides or lead pollution is the combustion of leaded gasoline products. This heavy metal can cause brain damage or blood poisoning.

Air Quality Index (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


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