Air quality Meaning, Measurement, Types, and Factors

Air Quality is the degree of purity or contamination of air. It is important because it affects living organisms and the environment.

With regards to environmental sustainability and conservation, air quality is an important concept. This article discusses air quality, as outlined below;

-Air Quality Meaning

-Measurement and Assessment of Air Quality: How is Air Quality Measured?

-Types/Levels of Air Quality

-Common Air Contaminants

-Factors which Affect Air Quality





Air Quality Meaning

Air quality is simply a measure, or assessment, of the degree of purity or contamination, of air.

Based on the above definition, air quality can be described with adjectives like good, fair, moderate, poor, and extremely poor. These adjectives represent different levels of contamination of the air, by foreign materials.

Another term which is associated with air quality, is air pollution. This term is basically the opposite of good air quality, and represents a scenario where the air contains a significant amount of contaminants, which have rendered it unsafe for living organisms and natural processes on Earth.

Within the context, of the theme of air quality, some concept which are usually given consideration include population growth, public health, food insecurity, heat waves, climate change, and global warming.


Measurement and Assessment of Air Quality: How is Air Quality Measured?

Air quality is measured using a set of units and instruments that detect and quantify foreign constituents in an air sample.

In order to adequately define the air quality in any given location, at any given time, it is essential to have a mechanism by which air quality can be assessed, in terms of precise measurement.

A quantity used in measurement of air quality, is known as the  Air Quality Index; AQI [18]. This quantity represents the degree of purity or contamination of the air, in numerical terms.

It may be thought of as a part of an elaborate measurement system, which is composed of numbers that indicate the amount of contaminants present (and identified) in a sample of air. Generally, the AQI value may fall between 0 and 500 (often expressed in degrees) [14].

Aside the use of degrees as a unit of measurement, air quality may also be expressed by volume, in parts per billion (ppb). However, this unit is applicable only in cases where the air quality has been measured to a high degree of precision.

With regards to measuring air quality with precision, there are a number of methods, instruments and technologies used in the effort to achieve this purpose. Some of these are discussed briefly below;

-Air Quality Sensors

The main advantage of this measurement technology is the fact that it is portable.

Air quality sensors provide a compact, (relatively) low-cost means for assessing air quality, especially in comparison to traditional instruments [6].

Most often, air quality sensors are used for outdoor measurements, although they may also apply in indoor measurements. They are also generally equipped with smart capabilities and functions, which enable them to share data with other devices. This increases the ease with which air pollutant levels can be tracked.

Considering the bulky and relatively immobile characteristics of other air quality measurement devices, the sensors help to meet the inadequacies created by these alternatives in terms of versatility and scale.

However, air quality sensors possess one main disadvantage, which is related to their level of accuracy. Because of the simple and portable design of these instruments, they are not always equipped to carry out highly-precise measurements of contaminant levels.

The accuracy of air quality sensors may also decline with age and conditions of use. At the sane time, sensors from different manufacturers often differ in their accuracy. While these limitations can be fairly addressed through measures like repetitive calibration, it is often necessary to carry out crosschecks with other measurement instruments.

air quality, AQI, air quality sensor, measurement
Air Quality Sensor (Credit: Intel Free Press 2013 .CC BY-SA 2.0.)


– Mobile Monitoring Devices

As the term implies, these devices enable air quality measurements to be carried out with a greater degree of flexibility, especially with regards to the geographic location of measurement [17].  

Mobile monitoring devices are differentiated from air quality sensors, based on two main factors. One of these is the smart capability of the two categories of measurement instruments. Air quality sensors are generally equipped with more smart capabilities than mobile monitoring devices.

These capabilities can be exemplified by the relative ease with which air quality sensors are able to share data with other devices, through wireless servers.

The second main difference between air quality sensors and mobile monitoring devices, is the degree of accuracy of both categories of measurement instruments. While mobile monitoring devices are not the most accurate category, they typically perform measurements that are much more accurate than those which can be carried out by air quality sensors.

In fact, the accuracy of mobile monitoring devices can be compared to that of the more sophisticated air quality measurement stations, under favorable circumstances. However, relative to air quality sensors, mobile monitoring devices are expensive and require much more complex maintenance schemes.

The advantage of these devices stems from the fact that (due to their high accuracy) they can easily replace the fixed measuring devices, in places where they are not installed. They also offer a cost-effective and flexible alternative in such cases.

Mobile monitoring devices can be attached to moving automobiles like vehicles and airplanes [12], making it possible to measure air quality across a wide range of locations, in a short time.

air quality, mobile monitoring, air quality monitoring, measurement, AQI
Mobile Monitoring Devices for Air Quality Measurement (Credit: Cjp24 2014 .CC BY-SA 4.0.)


-Fixed Monitoring Stations

Otherwise referred to as “Fixed Automated Monitoring Stations,” these instruments obviously differ from mobile monitoring devices in terms of portability.

Fixed monitoring stations have a vast array of advantages. One of these is their versatility. Unlike most other categories of measurement devices, which can access air quality based on a predefined set of criteria; fixed monitoring stations can be used to perform a much-more holistic assessment of air quality.

In addition to being able to detect and measure basically all major pollutants in air, some fixed monitoring stations can assess other criteria that influence air quality, such as temperature.

Fixed monitoring stations generally set the standard for air quality assessment, and carry out measurements in terms of AQI (Air Quality Index), which has been earlier identified as the universal and widely-accepted system of evaluation of air quality.

This makes it relatively easy to assess and interpret air quality using fixed monitoring stations. The ability of these systems to detect all major air pollutants, also makes them relevant to health applications, such as the evaluation of adherence to WHO air quality standards.

The most prominent advantage of fixed monitoring stations is their accuracy, which is higher than that of all other categories of air quality measuring instruments.

They can also be used for large-scale measurements, and are very reliable for real-time study of air contaminants. As the term ‘automated’ implies; these stations are usually self-driven.

Fixed monitoring stations are generally the most expensive to install and operate, requiring regular sessions of high-quality maintenance by professionals.



In cases of extremely-poor air quality, satellites can be used for assessment.

Satellite usage is restricted to poor air quality assessment, because the observations which can be made using these implements are mainly visual.

This means that satellites cannot directly measure the level of pollutants in the air, but can rather observe the visual (or aesthetic) effects of poor air quality (that is; of air pollution) such as smog, which usually occurs in the troposphere in regions where air pollution has occurred.

Extremely-poor air quality often goes along with visibility degradation, which occurs when the visibility of distant objects is affected or obscured by air contaminants like aerosols and other forms of suspended particulate matter [10].

Stationed in space, satellites are capable of detecting poor air quality based on visual observations.

The nature of these observations can be used to assess the level of severity of air pollution, and satellites also provide means by which real time changes in air quality across a large area can be evaluated over a given time period.

satellite, satellite imagery, satellite monitoring, air quality, AQI
Satellite Imagery showing Poor Air Quality (Smog from Aerosols) Over a Given Area


Examples of satellites used for air quality measurement include the Sentinel-5 Precursor (Sentinel-5P); launched in 2017, by the ESA, and equipped with an air quality observation instrument known as the TROPOMI [16].

The TROPOMI (TROPOspheric Monitoring Instrument) is a spectrometric instrument which is capable of monitoring air quality in terms of the concentrations of contaminants including methane, aerosol, formaldehyde, Sulfuric Oxide (SO2), Nitrous Oxide (NO2) and Carbon Monoxide (CO).  

The TROPOMI (which is a component of the Sentinel-5P satellite), can detect air quality conditions across a wide range of spectroscopic wavelengths including Visible Light, Ultraviolet, Short-Wavelength Infrared, and Near infrared [4]. .

air quality, TROPOMI, satellite, se
The Sentinel-5P as an Air-Monitoring Satellite (Credit: SkywalkerPL .CC BY 3.0.)


With regards to air quality assessment, satellites have the advantage of being able to provide freely-accessible air quality data for a wide geographical area. However, their major disadvantage is the low quality, low resolution and low accuracy of air quality measurements.


-Passive Samplers

Also known as passive collectors, these instruments are used to measure air quality on the basis of measurement of the concentration of certain contaminants in a sample of air.

Through repetitive sampling, passive samplers can be used to evaluate cumulative or average air quality, over a given duration of time.

As the name implies, passive samplers depend on air samples to carry out their measurements. The mechanisms by which these samples are trapped, may be either physical or chemical, and include absorption and adsorption [13].

However, in most passive samplers, instead of capturing air samples, specific contaminants in the air, such as sulfuric oxide (SO2) are captured. The concentration of these contaminants is then measured, and used to evaluate the quality of the air.

The advantages of passive samplers include the fact that they are relatively cheap and portable. However, most passive collectors are unable to make comparative measurements, and provide only cumulative and mean values which can hardly be used to assess spatial and temporal variations in air quality.


Types/Levels of Air Quality

As stated earlier, there are various adjectives which can be used to describe air quality. These adjectives can be specified into six levels, each of which is also represented by distinct color code.

These levels are defined by a range of AQI values, which indicate the level of purity or pollution of air.

The table below outlines the six levels of air quality, in terms of their description, color code and implication for health and the environment;

Table Showing the Six Levels of Air Quality

Level Color Code AQI Range Interpretation Description
1 Green 0-50 Good Air Quality Fairly safe for the ecosystem and environment, poses no notable health implications
2 Yellow 51-100 Moderate Air Quality Generally safe and acceptable for the ecosystem and environment. However, highly-sensitive organisms may be affected
3 Orange 101-150 Unhealthy for Sensitive Organisms Affects sensitive groups, less likely to affect the general public
4 Red 151-200 Unhealthy A great percentage of the living population is likely to be affected. Sensitive groups experience serious health effects
5 Purple 201-300 Very Unhealthy High risk of negative health effects, exists for all groups (sensitive and relatively-insensitive)
6 Amber/Maroon 301 and above Hazardous Emergency health conditions likely to arise. Unsafe for the ecosystem and environment


Common Air Contaminants

With regards to the measurement of air quality, there are some contaminants which serve as major yardsticks for assessment (note that contaminants become ‘pollutants’ only when they have reached a high level of concentration, so as to cause pollution of a medium).

The five main contaminants used to assess air quality [8], are as follows;

1). Carbon Monoxide (CO)

2). Ground Level Ozone

3). Nitrogen Dioxide

4). Aerosols/Particulate Matter

5). Sulfur Dioxide


Most of these contaminants are released as a part of greenhouse emissions, which result from a broad range of human activity including deforestation, mining, and fossil fuel combustion.

Although the five contaminants listed above, are used as criteria for the AQI assessment of air quality, two of them have been identified as major (potential) causes of health problems. These two are particulate matter and ground-level ozone.

Both ground level ozone and particulate matter, can coexist to form what is referred to as smog. In most cases, these gases occur alongside others like nitrous oxide and volatile organic compounds (VOCs). This smog can be produced from nearly all forms of industrial activity that involve energy consumption.


Factors which Affect Air Quality

1). Natural Factors

Wind is a good example of a natural factor that affects air quality. This is because wind currents can affect the distribution of contaminants in air, thereby affecting their concentration at any given location, and hence the air quality at the same location.

Contaminant transport by wind, may either increase or decrease the severity of air pollution, at a given location.


Solar Radiation affects air quality, by altering the density of the gases which make up air.

When solar radiation (or solar energy) heats up air, it makes it less dense, so that heated air rises and cool air sinks, in a fluid-flow pattern which can be described as convection.

This movement of gaseous molecules in the air, causes contaminants which are close to the Earth’s surface to rise into the troposphere. One of the potential results of the scenario thus described, is an increase in ground-level ozone, which is a harmful contaminant.

Conversely, in the absence of intense solar radiation (such as in winter) contaminants tend to remain trapped close to the surface of the Earth. This phenomenon, where cold air is close to the ground and warm air is in the upper atmosphere, can be referred to as thermal inversion or temperature inversion [1].  


Although the vast majority of recorded incidents have resulted from human activity, Wildfires may occur without any human influence. Lightning is an example of a natural cause of wildfires [3].

When vegetation burns, in such events, a number of contaminants are often released into the atmosphere, including greenhouse gases like carbon monoxide (CO) and methane.

Enormous amounts of particulate matter are also likely to be released into the atmosphere, as the plant biomass disintegrates.


Weathering and Dust Storms are distinct natural phenomena, which usually occur co-dependently.

This is because weathering is usually required in order for the fine particles that constitute a dust storm, to be produced. Similarly, a dust storm is one of the natural events that lead to weathering of earth materials.

Weathering contributes to the formation of particulate matter, which is a contaminant, capable of affecting air quality.

Dust storms mostly occur in arid regions where strong winds and dry climatic conditions are prevalent. In addition to helping form particulate matter, dust storms distribute this contaminant in the air.

dust storm, air quality
Weathering and Dust Storms as Natural Factors Affecting Air Quality


Volcanic Activity naturally affects air quality.

Typically, volcanic eruptions are accompanied by the emission of large quantities of air contaminants.

Examples of such materials released include ash (particulate matter), hydrogen sulfide (H₂S), carbon monoxide (CO), hydrogen chloride (HCl), carbon dioxide (CO₂), sulfuric oxide (SO₂), and bromine oxide (BrO).

The effects of volcanic contaminants on air quality are often severe, leading to hazardous pollution and affecting climatic conditions in some cases.

A good example of the hazardous effect of volcanic eruption on air quality, is the Holuhraun eruption which occurred in Iceland on the 31st of August, 2014, resulting in lava flows and large amounts of toxic gases that spread over several kilometers [2].


Organic processes also affect the quality of air.

This is because gases like methane and carbon dioxide can be produced naturally, by these organic processes.

The transfer of energy from one level of the ecosystem to another (as indicated by the energy pyramid) involves several processes where energy is lost. These include digestion, excretion, and biomass decomposition. Each of these processes involves the release of air contaminants.


2). Anthropogenic (Human-Induced) Factors

Fossil Fuel Combustion is the most common and prominent, anthropogenic factor which affects air quality.

Coal, natural gas and petroleum represent major examples of fossil fuels [5], which are burnt in industrial facilities like manufacturing stations and power plants, as well as in automobile engines, portable generators, furnaces and waste incinerators.

Greenhouse emissions are mostly produced when these fossil fuels are burnt to generate energy.

As of 2019, about 62 percent of all electricity generated in the United States was from fossil fuels [9]. Fossil fuels also produced about 74 percent of all greenhouse emissions in the US, that same year [7].


Waste Management is another anthropogenic factor that affects air quality.

When waste accumulates in dump-sites such as landfills, the organic components of the waste begins to decompose, releasing air contaminants like carbon dioxide and methane [15].

In addition to being unpleasantly odorous, the gases released by waste (which is produced from human activity) can contribute to the greenhouse effect, global warming, air pollution, and climate change. These are all prominent environmental problems.


Agriculture produces methane as well.

The sub-sector of agriculture which is known for producing the most air contaminants is livestock farming. Cattle produce methane as a by-product of digestion [11].

Livestock farming also leads to the loss of plant biomass that would otherwise help to sequester or store carbon dioxide away from the atmosphere.

Other ways in which agriculture affects air quality include food wastage and energy generation for processing of agricultural products.



Air quality is simply a measure of how clean or polluted the air is.

It is a variable which assesses the nature of air, based on the presence and concentration of foreign materials (contaminants). Some important environmental concepts bear relation to air quality, including greenhouse emission, climate change, air pollution and global warming.

Various devices can be used to assess air quality in a location, at a given time. Also, units of parts per billion (ppb) can be used to define air quality.

A more standard approach, however, is the Air Quality Index (AQI); a standard unit which includes values ranging from 0-500 and is applicable for health-related air quality assessments.

Various instruments are also applicable for air quality measurement. These include the air quality sensors, mobile monitoring devices, passive samplers, fixed monitoring stations, and satellites.

The differences between these instruments include size, cost, mobility, precision and accuracy.

Six distinct levels of air quality exist. which are all defined by a range of AQI values. These levels include Good air quality; Moderate air quality; Unhealthy for specific groups, Unhealthy; Very unhealthy, and Hazardous.

For assessing air quality in any location, a number of contaminants may serve as the major criteria. Five of these are sulfur dioxide, carbon monoxide, nitrogen dioxide, particulate matter, and ground-level ozone.

Factors which can affect air quality, can be categorized as natural and anthropogenic.

The natural factors include wind, volcanic eruption, organic processes, solar radiation, wildfires, weathering and dust storms. Anthropogenic factors are those which are basically induced by human activity, including fossil fuel combustion, agriculture, and waste management.



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