How to Understand AQI (Air Quality Index)

Understanding Air Quality Index

Checking the air quality isn’t a hard task. Open your preferred app or website, check the AQI number (or colour if you prefer) and sigh at another day of polluted air (at least if you live in Seoul!).

But what do those numbers and colours really mean? While the colours make the system easy to follow (we all know red means bad!), there are often many questions left. What exactly are the dangers represented by each colour? When should you stop outdoor activities? And perhaps most importantly, when do you need to wear a respirator?

In this post I want to give a comprehensive overview of the air quality index, often shortened to AQI. This article will cover everything that you need to know to understand the AQI data and what it means.

This post may contain affiliate links. For more information, please refer to my affiliate disclaimer.

Why Air Quality is Important

Even with the dangers of low air quality becoming more apparent, there still seems to be a weak reaction to this dangerous phenomenon. ‘The GBD study estimates that pollution-related disease was responsible for 9 million premature deaths in 2015—16% of total global mortality’ (The Lancet).

While not all of these deaths are attributable to air pollution, it is estimated that 6.5 million of them are caused by air pollution. Air pollution kills three times more people every year than AIDS, HIV and Malaria combined (NPR).

Despite this, air quality is often overlooked. With more and more health conditions due to air pollution being uncovered every day, it’s more important than ever to know the dangers and how to act accordingly.

Want to learn more about air quality? You can find more on this post about 10 facts about air pollution that you should know.

Dangers of Bad Air Quality

For an ever-increasing amount of countries, air quality is becoming a more serious problem by the day. Since it is something that is so gradual and consistent in many people’s lives, the dangers are often overlooked.

I live in South Korea, a country with bad air quality, but by no means as bad as some countries. The general lack of awareness around air quality makes me feel very sad sometimes though, as many people have just learned to live with the bad air.

Realistically, however, polluted air and fine dust are incredibly harmful. We should be doing everything that we can to prevent exposure to these extremely harmful particles that damage virtually every cell in the human body (TheGuardian).

Further, 91% of the world’s population live in locations where the air quality exceeds the WHO guidelines. This accounts for as low as 4 million deaths per year (WHO) but could be far higher, with another estimate being 9 million (TheGuardian).

Air pollution causes a loss of life expectancy similar to that of tobacco smoking (Oxford Academic). This means that simply be living in a country with a high AQI, you are getting the same health damage that you would from smoking.

At this point, the number of estimated deaths are increased gradually in every report that gets released. Further, the health dangers are consistently being constantly increased as we do more and more research into the matter.

This isn’t meant to seem like fear-mongering. I don’t intend that at all. However, after living in a country with air quality issues I do believe that this is something that is often overlooked. Air pollution is something that must be considered and that measures should be taken against. Luckily, our exposure to bad air pollution can be limited.

Read more: How to find cheap fine dust respirators.

What Is An Air Quality Index?

Understanding Air Quality Index

East-Asia AQI as seen on

An air quality index is a measurement that a government or organisation uses to monitor the quality of the air in a city or country. These indexes are used to communicate the quality of the air to the citizens so that they can react accordingly.

Many countries use different systems, and although many use a similar scale, some can also be vastly different. Airnow has a fantastic list of many of the different governments and organisations that report on air quality in different countries. Wikipedia also has an article which compares many of the different air quality indexes used globally.

Air quality is reported in tiers. Although countries use different systems, the danger level is generally represented by a colour. Green > yellow > orange > red > purple > brown is the usual colour-coding used, however, some systems differ a little.

The air quality index is usually made up of a few different environmental factors. Fine particles such as PM2.5 and PM10 are reported as well as other harmful gases in the air. Ozone, carbon monoxide, sulphur dioxide and nitrogen dioxide are all usually measured within an AQI rating.

AQI levels

Although every country measures air quality levels differently, there are some commonalities between the systems. The biggest similarity is that every system divides the air quality into coloured tiers. Although the exact numbers of each tier differ per system, the colours usually represent the same. Air quality index scale. This scale is available in many international locations.

The colours usually follow the above system. Green colour represents good air quality, and no need to worry. Yellow and orange can be harmful to sensitive groups but are not likely to affect the general public. Red is unhealthy and can have a negative health affect on all. Purple and brown both represent danger to all citizens and outdoor activities should be avoided if possible.

For more detailed information, I recommend referring to your local air quality indexes. For some of the most commonly used air quality indexes, please feel free to refer to these:

Many countries also have further differences between states. For example, Australia has different guidelines for each state, and recommendations vary. It’s impossible to list every AQI here, so please make sure to refer to your local government information for the best data.

Singapore Air Advisory

Haze advisories from the Singapore Ministry of Health.

Components of AQI

There are 6 components that are important for finding an overall AQI. These elements can be divided into two categories – pollution particles and gases.

  • Fine dust particles – PM2.5 and PM10. The number represents the size of the particle.
  • Gases – carbon monoxide, ozone, sulphur dioxide, and nitrogen dioxide.

The concentrations of all of the above particles and gases are put into a calculation that finds the overall air quality index rating. For more details on the calculation, please read the next section.

How Is An AQI Rating Found?

Due to the differing global systems, there is no universal AQI calculation. However, many systems do act very similarly and use only slightly different calculations.

An air quality index is usually calculated using a relatively complex calculation (shown below). If you want further details, please refer to this page from AirKorea. However, the most important aspect to understand is that the overall AQI rating is usually found by comparing the current concentration of each particle/gas to the category maximum and minimum concentrations.

Calculating AQI

Calculation from AirKorea.

Although the calculation looks complicated, the key is to understand that AQI measures a range of particles and gases considered to be harmful in high concentrations. If you are particularly sensitive to a specific element of the air quality, you can view the individual levels of gases and particles. I will discuss this more below.

What Air Quality Level Is ‘Safe’?

What exactly is a ‘safe’ level of air quality? Some countries have different levels that they consider safe. However, the most commonly followed global guideline is that issued by the WHO.

WHO Safe Air Quality

WHO Air Quality Guidelines. PM2.5 and PM10 guidelines.

The WHO rates the above PM2.5 and PM10 means as the highest levels considered safe. Although PM2.5 and PM10 levels are the most commonly referred to in regards to air pollution, there are also other measurements worth considering. All data is from the WHO Air Quality Guidelines.

  • Ozone (O3) – 100 μg/m3 8-hour mean
  • Nitrogen Dioxide (NO2) – 40 μg/m3 annual mean, 200 μg/m3 1-hour mean
  • Sulphur Dioxide (SO2) – 20 μg/m3 24-hour mean, 500 μg/m3 10-minute mean

Sites such as will calculate the overall AQI using a calculation which accounts for all of these particles and gases in the air. However, it also makes it far easier to understand by showing the concentration of everything separately.

Understanding AQI.

On the left of the above image from Aqicn, we can see all of the above particle and gas concentration as well as carbon monoxide (CO). All of these readings are separated into colours (and for more detail, numbers) to show the exact concentrations of each in the air.

Generally, however, if the overall number is green, then the air quality is within a range that is considered safe. Yellow and orange are moderate air quality, with some individuals who are especially sensitive beginning to be affected, and red and above are considered unhealthy for everyone.

Following the WHO guidelines, however, the vast majority of the world’s population are living in unsafe air. Something which has been proven to have a wide range of harmful health effects.

AQI Level Dangers

While it’s easy to see the colours and what air quality conditions they represent, it’s more difficult to find out exactly what levels are considered dangerous and what the health impacts of each are.

Particle Pollution

The United States Environmental Protection Agency. Particle Pollution.

The most common number to refer to when it comes to AQI are the particle pollution (PM2.5 and PM10) numbers.

The above table shows what actions should be taken to protect your health from particle pollution. The health dangers don’t appear to change based on the level of pollution, but rather the dangers can be exasperated at higher levels of pollution.

The dangers of exposure to air pollution as listed by the U.S Environmental Protection Agency are as follows:

  • When exposed to particle pollution, people with heart or lung diseases and older adults are more likely to visit emergency rooms, be admitted to hospitals, or in some cases, even die.
  • Exposure to particle pollution may cause people with heart disease to experience chest pain, palpitations, shortness of breath, and fatigue. Particle pollution has also been associated with cardiac arrhythmias and heart attacks.
  • When exposed to high levels of particle pollution, people with existing lung disease may not be able to breathe as deeply or vigorously as they normally would. They may experience symptoms such as coughing and shortness of breath. Healthy people also may experience these effects, although they are unlikely to experience more serious effects.
  • Particle pollution also can increase susceptibility to respiratory infections and can aggravate existing respiratory diseases, such as asthma and chronic bronchitis, causing more use of medication and more doctor visits. (U.S EPA)

It’s important to remember that these are only the dangers of particle pollution (PM2.5 and PM10). There are also dangers associated with other gases in the air.

Carbon monoxide and ozone actions to protect your health. (U.S EPA)

There are separate health conditions related to each different gas. If you are interested in reading more about the different health impacts, please refer to this document.

Where can I see the AQI?

Due to the differing AQI systems, this differs from country to country. Most countries, states, and larger cities will have a website where they report on air quality.

However, there are also a few sites which aggregate data from these different governments. Although they don’t have data for every location, they will have accurate data for the majority of people.

My favourite tool to use is This website is where most of the images in this post have come from, and I prefer it because it is easy to follow. Although being easy to follow, it also has a lot of information, and it’s possible to see all of the individual readings as well as the overall AQI.

IQ Air’s AirVisual is another popular platform and also has a fantastic application which can be used. Although AirVisual doesn’t provide the same, detailed information that aqicn does, it is a fantastic app to use for quickly checking the air quality. Due to the ease of use, this is my preferred application for checking air quality.

Breezometer is another popular option. This website has a slightly different system for monitoring air quality, which can make it confusing at first. However, it is visually attractive and makes understanding the AQI easy. If you want a more detailed analysis, however, you will be better off using aqicn.

Of course, there are many more options that aren’t listed here. However, these are some of the most widely-used websites and apps due to their abundant data and global coverage.

How to Stay Safe From Polluted Air

Staying safe from polluted air is a difficult task. Unfortunately, it’s also not the cheapest. While many people know about the filtration effects of masks, not many people consider the dangers within their own home.

PM2.5 and PM10 particles are both ultra-fine and are capable of getting inside – even if you keep your windows and doors closed. Although it’s easy to feel safer inside, on days of high pollution, indoor air must be filtered also.

‘People often think the answer is to escape indoors – but that’s not true. According to the US Environmental Protection Agency (EPA), air pollution indoors is often between two and five times greater than outdoors – and can get at its extreme up to 100 times worse than the open air.’ (BBC).

This is because not only will the outdoor air pollution get inside, but there is also air pollution that is created inside. Think of cooking and cleaning fumes, fungal spores, and other dangerous particles that can be created inside.

All of this is to say, indoor areas are not safe from air pollution. In fact, they often have significantly worse air pollution.

Check out my Totobobo review for some of the best fine dust masks.

Staying Safe Indoors

Kaiterra Laser Egg Review

Laser Egg indoor air pollution monitor.

With this being said, what is the best way to make sure that you are breathing clean air when you are indoors? The best way is to purchase an air purifier that is capable of filtering out ultra-fine particles, and an air quality monitor that can let you know exactly what you are breathing.

Although there are many different air purifiers available, you want to make sure that you purchase one capable of filtering out very fine particles such as PM2.5 particles. Although I have no personal experience with these air filters currently, these are some of the most highly rated that are capable of such filtration.

Honeywell HPA300 Air Purifier | Levoit Air Purifier | Whirlpool Whispure Air Purifier

Staying Safe Outdoors

When it comes to being outdoors, a respirator is often necessary. I have already covered respirators and masks extensively on this blog, so please refer to those articles for more information. However, there are a few key things to look for when selecting the right respirator.

  • Make sure to pick a rated respirator (such as N95) rather than a mask. Read more about respirator ratings.
  • Make sure that the mask fits correctly and creates a seal with your face. Read more about fitting a respirator.
  • Make sure to dispose of, and replace, a respirator when breathing becomes difficult.

If you are feeling overwhelmed while picking a respirator, or have more questions about choosing the right one, please refer to this respirator FAQ.

Purchase N95 respirators3M 8511 N95 | 3M 8210 N95

Purchase FFP2 respiratorsFFP2 Mask Anti-Pollution | FFP3 Respirator Mask

Read more: Outdoor air quality monitor, Flow 2.


At What AQI Should I Wear a Mask?

Unfortunately, there is no single answer to this question. Masks should be worn by all in an AQI above 150. For sensitive groups, masks may be needed above 50 or 100. If you are unsure, contact your doctor.

Is Air Pollution Dangerous?

Yes. Air pollution is currently the largest cause of deaths worldwide. It has recently been found to damage every organ in the human body. For more information, refer to sources such as the WHO.

Where Can I Find My AQI?

AQI sites differ from country to country. However, some good resources are AirVisual and These sites/apps are fantastic and collect data from local governments and organisations.

What Is the Difference Between PM2.5 and PM10?

The number after PM refers to the size of the particle. PM2.5 particles have a diameter of less than 2.5 micrometers, while PM10 particles have a diameter of less than 10 micrometers.

What Pollution Level Is Considered ‘Safe’?

The WHO guidelines the following as the highest particulate matter levels considered safe. PM2.5: 10 μg/m³ annual mean, or 25 μg/m³ 24-hour mean. PM10: 20 μg/m³ annual mean, or 50 μg/m³ 24-hour mean.

Do I Need To Wear a Mask Indoors?

Contrary to what most people think, the air indoors is often substantially more polluted than that which is outdoors. However, it’s not always logical to wear a mask indoors. In this case, an air purifier is a good choice.


AQI can be hard to understand, especially if you have never had to refer to it before. When I first moved to South Korea I had almost no understanding of the air quality index and how it worked.

After doing research about the AQI and how to understand it, I wanted to share my findings and I decided to create this post. If you have any further questions or comments please don’t hesitate to reach out to me.

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