Airvoice Publishes Updated Guide to the Indoor Air Quality Index (IAQI)

This document explains the logic, structure, and methodology behind the Airvoice Indoor Air Quality Index (IAQI). It is intended for:

• Facility managers and operators
• Green building and HVAC consultants
• Health & safety officers
• Researchers, regulators, and other professionals working with indoor environmental data

The IAQI helps users translate complex indoor air quality measurements into clear, actionable categories. It is built on internationally recognized health guidelines, scientific research, and practical insights from real-world building operations.

The Airvoice Indoor Air Quality Index (IAQI) is a numerical indicator designed to help users understand the current state of indoor air quality and whether any action is needed. It translates measured concentrations of key air pollutants into an intuitive, categorical assessment:

Good, Fair, or Poor.

This categorization is determined based on scientifically grounded thresholds for the following core air quality parameters:

• PM2.5 (fine particulate matter),
• PM10 (coarse particulate matter),
• CO₂ (сarbon dioxide).

To provide a more detailed view, Airvoice also uses a 10-point IAQI scale, which assigns a score from 1 (Good) to 10 (Hazardous) based on the levels of these same pollutants. The exact breakpoints used in this extended scale are provided in the How We Calculate IAQI section.

The IAQI is not just a passive measurement. It serves as a practical guide indicating when air quality is acceptable and when it may pose risks to health, comfort, or cognitive performance. While not all values represent immediate danger, the system is designed to make complex air quality data understandable and actionable for building operators, facility managers, and everyday users.

Airvoice interprets air quality using three universally relevant parameters: PM2.5 (particulate matter with diameter ≤ 2.5 microns), PM10 (particulate matter with diameter ≤ 10 microns), and CO₂ (carbon dioxide).

These are selected for their direct links to human health and cognitive performance. Unlike thermal comfort parameters, which vary by region, the thresholds for these pollutants are consistent across all locations where Airvoice systems are deployed.

Each parameter is evaluated within three bands:

• Good – No action required
• Fair – Acceptable, but improvement is recommended
• Poor – May impact health or performance; action should be considered

The scientific rationale behind these breakpoints is detailed in the Scientific Foundations and Methodology section below.

In addition to air quality, indoor thermal and humidity comfort plays a critical role in well-being, productivity, and health. Airvoice monitors two key comfort-related parameters:

• Temperature (°C)
• Relative Humidity (RH, %)

Unlike air quality thresholds, which are based on physiological limits and are largely universal, comfort parameters vary by region. Airvoice adapts these breakpoints based on regional standards and recommendations from local environmental and health agencies, while still referencing globally recognized research.

Total Volatile Organic Compounds (TVOC) are monitored by Airvoice as a supplementary air parameter. While TVOC is not included in the core IAQI calculation, it is displayed in the user interface and can be potentially referenced in air quality reports when required by certification standards or customer expectations.

TVOC readings are widely available in low-cost indoor monitoring devices, but the metrics remain poorly standardized, and its health relevance is unclear. For this reason, Airvoice treats TVOC separately and transparently communicates its measurement limitations.

While developing Airvoice breakpoints, we aimed to base them on scientific findings related to human physiology wherever possible. This approach contrasts with the more common method of adopting thresholds directly from national regulations or green building certification frameworks.

This science-based method, however, is complicated by two main factors:

• Lack of clearly defined threshold effects: In real-world conditions, there are no abrupt step changes in health risk as pollutant concentrations increase. This makes the task of defining discrete breakpoints inherently abstract.

• Limited scientific data for certain pollutants: For some compounds, especially particulate matter, scientific evidence on health and cognitive effects remains insufficient. At present, Airvoice uses only mass concentration as the basis for its PM-related thresholds, without factoring in other parameters such as chemical composition.

Although we recognize that assigning categorical labels to values along a continuous scale may introduce ambiguity, particularly near category boundaries, we believe this remains the clearest and most usable way to interpret indoor air quality data.

Moreover, the interpretation of these categories continues to offer statistically meaningful insights for assessing air quality and informing action.

While we make every effort to ensure that our breakpoints are scientifically grounded, in some cases, where direct evidence is lacking, we follow recommendations from national agencies. One exception is TVOC, for which Airvoice applies thresholds defined by the Indian Green Building Council, in line with the regional focus of our current products.

Below, we describe the methodology used for each monitored parameter.

Breakpoints for PM2.5 and PM10 are derived from the U.S. EPA National Ambient Air Quality Standards (NAAQS) [1] and supported by the WHO Global Air Quality Guidelines [10]. These are among the most widely used references in environmental health and are also reflected in several green building certification programs.

While originally developed for outdoor air, these standards offer a practical benchmark for indoor air as well. However, their application indoors presents two key challenges:

Despite these limitations, NAAQS-derived values remain the most consistent, scientifically recognized basis available for evaluating indoor particulate concentrations.

CO₂ thresholds are based on findings from controlled exposure studies, including those by Allen et al. (2016) and Satish et al. (2012). These studies demonstrated statistically significant declines in cognitive performance at elevated indoor CO₂ levels.

Though these are not formal health limits, they offer a scientifically meaningful benchmark for interpreting indoor CO₂ levels in relation to cognitive performance and ventilation effectiveness.

In addition to cognitive effects, elevated CO₂ is a strong indicator of insufficient ventilation, which has been directly linked to increased risk of airborne transmission of respiratory infections, including influenza and COVID-19. While CO₂ itself is not infectious, poor ventilation allows airborne pathogens to accumulate, making CO₂ a reliable proxy for evaluating the likelihood of cross-contamination in shared indoor spaces.

As such, maintaining lower CO₂ concentrations is important not only for comfort and productivity, but also for health protection in schools, offices, hospitals, and other high-occupancy environments.

Airvoice groups temperature and humidity under Air Comfort parameters. Unlike pollutant thresholds, these values are regionally adaptable and based on a combination of health, comfort, and productivity factors.

Since temperature and humidity interact, combined thermal comfort models are entering widespread adoption, e.g. models using RH(T) zones in accordance with ASHRAE Standard 55. [11]

TVOC is treated as a supplementary metric, not included in IAQI scoring due to its complex and non-standardized nature.

Airvoice measures TVOC using metal-oxide (MOX) gas sensors, which are broadly sensitive but non-specific. These sensors:

Additionally, TVOC presents two major interpretation challenges:

Despite this, Airvoice displays TVOC levels for users and uses thresholds from the Indian Green Building Council (IGBC) Health & Well-being Rating System, with µg/m3 to ppb conversion based on isobutylene equivalency, as per the the RESET Air Standard v2.0 [12].

TVOC is not factored into IAQI scoring or automated recommendations but is often useful in trend analysis, especially for detecting activities like cooking, cleaning, smoking, or alcohol consumption indoors.

The Airvoice Indoor Air Quality Index (IAQI) is calculated based on three key air quality parameters: PM2.5, PM10, and CO₂.

For each parameter, a partial IAQI score is determined according to its concentration. The final IAQI value is calculated as the highest of the three partial scores. This method ensures that the most critical pollutant at any given time is reflected in the overall IAQI.

A full table of partial IAQI breakpoints is provided below.

While the highest-score approach offers a simple and conservative way to highlight potential risks, it has three main limitations:

To address these challenges, Airvoice is developing a revised IAQI framework. In this updated version, the IAQI will be represented as a three-dimensional vector with distinct components:

This new approach will enable a more nuanced and actionable interpretation of indoor air quality, more accurately reflecting both the health impacts of different pollutants and the interventions they require.

The Airvoice Indoor Air Quality Index (IAQI) is designed to bridge the gap between raw environmental data and practical, health-relevant insights. By combining physiological research, regulatory guidance, and field experience, we aim to make indoor air quality measurable, interpretable, and actionable for building operators, health officers, and individuals alike.

While the current IAQI model already provides clear and conservative assessments of indoor environments, we recognize its limitations and are actively working to improve its precision and responsiveness.

We believe that informed decisions begin with transparent data. This document is part of our commitment to making the invisible visible, and the complex clear.