• What Is the Air Quality Index (AQI)?

Air Monitoring

What Is the Air Quality Index (AQI)?

May 07 2022

Air pollution is a significant problem affecting countries all over the world. Not only is poor quality air responsible for causing and exacerbating a variety of health complications and leading to seven million premature deaths per annum, but it also contributes to global warming, climate change and extreme weather events.

Of course, the first step in overcoming any issue of this kind is fully understanding it. As our awareness of the dangers of inhaling contaminated air have grown, so too have our capabilities in measuring, quantifying and reporting on specific pollutants themselves. To this end, the idea of an Air Quality Index (AQI) was conceived to allow governments, businesses and members of the public round-the-clock access to real-time information about the air quality in their local area.

Today, many countries have implemented their own AQIs which may vary from one another in subtle ways. For simplicity’s sake, this introductory article will focus largely on the US AQI, which is one of the first and most well-respected systems in the world. However, those keen to learn more about the subject are directed to attend this year’s Air Quality and Emissions (AQE) Show, which is scheduled to take place this October in Telford in the UK.

What is the AQI?

First launched in 1999, the AQI was designed to collect, assimilate and present up-to-date information about localised air quality across the United States. Many other countries around the globe have followed America’s lead by implementing their own AQI system in subsequent years.

The basic objective of the service is to allow citizens to see, at a glance, the quality of air in their area. This will allow them to avoid pollution hotspots and tailor their daily routines to minimise exposure to harmful contaminants as much as is feasibly possible.

How is the AQI calculated?

The AQI is calculated by amassing data on a number of different pollutants in the air. This information is gathered via the use of an innovative new air quality research network, which consists of a plethora of ground-level sensors. Many of these are installed at government buildings and industrial sites, while others are attached to moving vehicles and even carried around manually by teams of volunteers. Finally, the data is supplemented by satellite images which give a broader overview of the pollution picture.

That data is then assessed by the AQI’s advanced computing system, which assigns it a figure of between 0 and 500 based upon the cleanliness of the air samples. The closer to zero a sample is, the higher quality the air, and vice versa. That 500-figure scale is then further broken down into six different categories, each of which is colour-coded to allow citizens to understand the data more easily.

Which pollutants are measured by the AQI?

The US AQI measures five pollutants which are generally considered to pose the greatest risk to human health. These are as follows:

Particulate matter (PM)

This assignation includes both PM10 (particles of pollution that are 10 microns or less in diameter) and PM2.5 (particles of pollution that are a quarter of that size or smaller, or roughly 30 times thinner than a human hair). Because PM can be inhaled into the lungs and even absorbed into the bloodstream, it’s believed to be highly dangerous to human health over the long term.


Not to be confused with the layer of ozone which protects the Earth from the strongest rays of the sun, tropospheric ozone – better known as ground-level ozone – is a damaging pollutant that contributes to the formation of smog and causes respiratory problems among those exposed to it. It is not emitted into the air directly, but is rather created when volatile organic compounds (VOCs) and nitrogen oxides (NOx) react in sunlight.

Carbon monoxide

Sometimes dubbed “the silent killer”, carbon monoxide is highly dangerous due to the fact that it’s odourless, colourless and produces no immediately noticeable symptoms when inhaled – but it can be deadly in a confined area. In an outdoor environment, it’s more responsible for trapping heat in the air and contributing to environmental issues.

Sulphur dioxide

Sulphur dioxide is a naturally occurring element which is often spewed into the atmosphere during volcanic eruptions. However, the proliferation of industrial activity over the last couple of centuries has seen sulphur dioxide concentrations in our atmosphere skyrocket, meaning it’s now of grave danger to humans as well as animals, plants and the wider ecosystem.

Nitrogen dioxide

Although nitrogen dioxide is not actually combustible itself, it can serve as a catalyst for the combustion of other materials. It’s most commonly emitted into the atmosphere via the exhaust pipes of road traffic vehicles, which is why the use of mobile measuring equipment attached to vehicles is providing new insights into NO2 concentrations and their effects.

What are the different levels of the AQI?

As mentioned above, the air quality in a given area is attributed a numeric value between 0 and 500, which is then further divided into six subcategories. These are comprised of:

  • Good. This refers to air quality of between 0 and 50 and does not pose any risk to any member of the general public.
  • Moderate. This refers to air quality of between 51 and 100 and is unlikely to adversely affect members of the general public, though those with health conditions may experience some deleterious effects.
  • Unhealthy for sensitive groups. This refers to air quality of between 101 and 150 and could potentially exacerbate respiratory conditions for those who suffer from them already.
  • Unhealthy. This refers to air quality of between 151 and 200 and could potentially damage the health of anyone, though vulnerable groups are obviously more at risk.
  • Very unhealthy. This refers to air quality of between 201 and 300 and all members of the general public are advised to limit their exposure to it, since they could develop or exacerbate health complications as a result.
  • Hazardous. This refers to air quality of between 301 and 500 and is regarded as the most dangerous category of air quality. As such, all members of the general public are strongly advised to avoid exposure.

To make the system even easier to use, every level is colour coded as well. In order of highest air quality to lowest air quality, the colours are as follows: Green; Yellow; Orange; Red; Purple; Maroon. This allows people to see at a glance if the air quality in their area is safe to breathe.

Which countries are home to the highest AQI rating?

The cleanest airways are found in small island archipelago nations which do not host much in the way of industry, or else forward-thinking wealthy nations which have been able to implement the requisite technology to clean up their emissions.

It should not come as a surprise, then, to learn that the Nordic nations of Sweden, Finland and Denmark all enjoy good air quality, as does Zurich in Switzerland. Elsewhere, Honolulu in Hawaii and Caribbean oases like the Bahamas and Puerto Rico also score highly on the AQI.

Which countries are home to the lowest AQI rating?

At the same time, it’s hardly a shock that the poorest quality air in the world can be found in some of its most densely populated countries. China and India are both home to over a billion people and have relied heavily on coal and other damaging fossil fuels to provide energy for their ballooning population and power their rampant industrialisation, which is why they have some of the dirtiest air in the world.

Meanwhile, Chad, Bangladesh, Nigeria and Pakistan are other nations where poor-quality air is a daily crisis for the local inhabitants. While it’s certainly true that the lowest AQI ratings are generally concentrated in the developing world, there are parts of the UK and the USA which also suffer abysmal AQI readings on a regular basis.

What is the DAQI?

Speaking of the UK, the government recently performed a strategic overview of the national air monitoring system to produce better, more accurate and more helpful results. The result of this is the Daily Air Quality Index (DAQI), which measures the same pollutants as the USA with the exception of carbon monoxide, which was removed from its monitoring system after the last update.

Instead of a scale of 0 to 500 and six categories of air quality, the DAQI favours numeric values of between 1 and 10 and four subdivisions of that scale. The first, accounting for values of 1 to 3, is deemed “Low”; the second comprises values of 4 to 6 and is termed “Moderate”; the third covers 7 to 9 and is called “High”; while the last refers to a value of 10 and is named “Very High”.

How to use the AQI to protect yourself

Whether using the AQI, DAQI or some other monitoring system in the country of your residence, it’s imperative that you first determine whether you belong to a vulnerable group. This includes the very young, the elderly, the infirm and those diagnosed with pre-existing lung or heart conditions.

Next, you should consult the AQI in your area to see what its score is at the present time, as well as reading carefully the guidance provided by your government based upon that score. Equipped with that knowledge, you can then make an informed decision about how best to go about your daily activities while still minimising your exposure to harmful pollutants.

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