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Indoor Air Quality Index - IAQI


In our fast-paced lives, the importance of indoor environments often goes unnoticed. However, considering that Americans spend about ninety percent of their time indoors, as revealed by the United States Environmental Protection Agency (EPA), the quality of the air we breathe while indoors is of paramount importance. The Indoor Air Quality Index (IAQI) is a tool designed to evaluate and ensure the healthiness of our indoor surroundings.

Indoor Air Quality, often abbreviated as IAQ, refers to the quality of air within buildings and structures, directly impacting the health and comfort of their occupants. Numerous pollutants can degrade IAQ. Common culprits include factors like poor ventilation, fluctuating temperatures, and humidity, which can exacerbate the situation by amplifying the effects of these pollutants.

To set the stage, this article will introduce the Indoor Air Quality Index (IAQI) and explore how it's calculated.

The Birth of the IAQI

While outdoor air quality monitoring systems have been in place for a while, with indexes like the Air Quality Index (AQI) by the US EPA measuring common pollutants, the focus on indoor air has been relatively recent.

Recognizing the pressing need for a similar system tailored for indoor environments, ATMO used existing research to develop the IAQI.

The IAQI is not just about monitoring pollutants. It offers a comprehensive view of air quality by transforming raw data into a user-friendly index. This index categorizes the indoor air quality status into the following main categories: Good, Moderate, Polluted, Very Polluted, and Severely Polluted.

This classification empowers individuals with real-time insights, enabling them to make informed decisions about their environment.

Decoding the IAQI


To better understand the IAQI, let's delve into its calculation methodology.

The IAQI was developed based on the existing ratio method, which was introduced by the U.S. EPA in 2006 to calculate the outdoor air quality index (AQI). The U.S. EPA defines the national outdoor air quality standards based on the six common air pollutants, and the index was calculated using the following linear interpolation formula to convert from concentration to AQI:

To calculate the AQI, the EPA measured outdoor pollutants such as ozone (O3), particulate matter (PM10 and PM2.5), and carbon monoxide (CO) based on an hourly to 24-hour average, depending on the type of pollutant. Then, the average value for each pollutant was converted into an individual index based on the EPA’s breakpoint table

The EPA's breakpoint table primarily focuses on outdoor air quality and might not capture rapid shifts in indoor environments. Additionally, it doesn't cover certain indoor pollutants like CO2, VOCs, and formaldehyde. For indoor spaces, it's essential to get instant updates on air quality to ensure safety and prompt action.

IAQI Categories and Breakpoint Table

Using the same methodology, we created a breakpoint table for IAQI:





VOC (Index)

NOx (Index)














81 - 100

400 - 599

1 - 199

1 - 49

0 - 0.05

0 - 14

0 - 20

0 - 30

0 - 1.7

0 - 0.025


61 - 80

600- 999

200 - 249

50 - 99

0.06 - 0.1

15 - 34

21 - 50

31 - 75

1.8 - 8.7

0.026 - 0.06


41 - 60

1000 - 1499

250 - 349

100 - 299

0.11 - 0.3

35 - 61

51 - 90

76 - 125

8.8 - 10

0.061 - 0.075

Very Polluted

21 - 40

1500 - 2499

350 - 399

300 - 349

0.31 - 0.75

62 - 95

91 - 140

126 - 200

10.1 - 15

0.076 - 0.1

Severely Polluted

0 - 20

2500 - 4000

400 - 500

350 - 500

0.76 - 1

96 - 150

141 - 200

201 - 300

15.1 - 30

0.101 - 0.3

When comparing the IAQI to the EPA's AQI, there are some key differences:

Scale and Categorization: The IAQI categorizes air quality into five levels, each with its own color and range:

  • Good (81-100)
  • Moderate (61-80)
  • Polluted (41-60)
  • Very Polluted (21-40)
  • Severely Polluted (0-20)

Contrary to the U.S. EPA's scale, a higher IAQI value means better air quality.

Time Averages for Index Calculation: The EPA determines its index based on 1 or 8-hour pollutant averages. In contrast, the IAQI uses a one-minute average to provide more immediate readings.

To determine the real-time IAQI value, the following formula is used:

Let’s say the individual index for CO2 is 81; the IAQI status for CO2 is therefore “Good”. Similarly, the index for PM2.5 is 70, or “Moderate” and for VOC, it is 60, or “Polluted”. 

Each individual index is then compared, and the lowest index value becomes the final value for IAQI. Thus, the final AQI value is 60, and the status is “Polluted”.

Practical Usage of the IAQI

Instead of parsing through fragmented data from individual pollutants like Particulate Matter or Volatile Organic Compounds, the IAQI provides a quick snapshot of overall air quality. This unified metric simplifies monitoring across various settings, from schools to offices and homes, turning complex data into a clear and actionable metric and promoting healthier indoor spaces. Here are some advantages of IAQI:

Holistic Overview: Unlike individual pollutant readings, the IAQI amalgamates data from various sources, providing a comprehensive snapshot of overall indoor air quality.

Quick Interpretation: With a singular value, the IAQI eliminates the need to sift through multiple pollutant metrics, allowing for an immediate understanding of the air quality status.

Prompt Actions: This rapid assessment capability of the IAQI ensures timely interventions. For instance:

  • Schools: If the IAQI indicates deteriorating air quality, administrators can swiftly adjust ventilation or modify outdoor activity schedules.
  • Offices: Management can instantly gauge if indoor conditions are conducive to productivity and well-being and take corrective measures if needed.
  • Residential Settings: Homeowners can rely on the IAQI to decide when to use air purifiers or ventilate without being overwhelmed by multiple readings.

Informed Decisions: The simplified metric of the IAQI empowers individuals and institutions to make informed choices about indoor environments without the complications of tracking individual pollutants.

By offering a clear and rapid overview of indoor air quality, the IAQI stands out as an indispensable tool, driving immediate and effective actions to ensure optimal indoor environments.