Why AQI Standards Vary Across the World – And Why Air Quality Index Matters More Than Ever

Air Quality Index (AQI) has become a daily reference point for millions of people, guiding decisions as simple as stepping outdoors or exercising.

However, a common question persists: why does AQI differ across countries, platforms, and standards? 

What Is AQI?

So, in simple terms, AQI is a numerical scale designed to translate complex air pollution measurements into an easily understandable format. Instead of presenting raw pollutant concentrations, AQI communicates health risk levels, categorizing air as Good, Moderate, Poor, Unhealthy, Severe, or Hazardous.

Most AQI systems track key pollutants:

• PM2.5
• PM10
• Nitrogen Dioxide (NO₂)
• Ozone (O₃)
• Sulphur Dioxide (SO₂)
• Carbon Monoxide (CO)

However, how these pollutants are weighted and interpreted differs globally. Then the highest sub-index among these becomes the AQI value for that location.

Major AQI Standards Explained

AQI-US (United States Environmental Protection Agency – USEPA)

Who defines it?

So, the AQI used in the US is defined by the United States Environmental Protection Agency (US EPA) and is widely adopted globally as a reference standard. Notably, AQI-US is health-first, short-term risk focused. Because of this, it reacts aggressively to pollution spikes.

PM2.5 Breakpoints for AQI-US 

AQI-IN (India National Air Quality Index)

So, India’s AQI framework was developed considering local exposure patterns and feasibility.

As a result, it uses less stringent thresholds for certain pollutants compared to US EPA. This means the same air can appear “Moderate” under Indian AQI but “Unhealthy” under US standards.

Who defines it?

India’s AQI is developed and regulated by the Central Pollution Control Board (CPCB) under the Ministry of Environment, Forest and Climate Change. AQI-IN is more context-aware, regulatory and communication focused. 

Pollutants Considered

AQI-IN includes more pollutants than AQI-US:

• PM2.5
• PM10
• NO₂
• SO₂
• CO
• O₃
• NH₃
• Pb

(At least 3 pollutants are required to compute AQI, with PM mandatory.)

PM2.5 Breakpoints for AQI-IN

Why AQI-IN Uses Higher Thresholds

• India faces chronic, long-term pollution, not occasional spikes
• Extremely strict limits would keep AQI in “Severe” for months
• AQI-IN is also a regulatory & policy communication tool, not only a health alert system
  

Same Air, Different AQI – As shown by AQI.in

PM2.5 = 87 µg/m³ for Mumbai, Maharashtra, India on 9th February, 2026 at 16:31 hours (Local Time)

• AQI-US: Unhealthy
• AQI-IN: Moderate

So, this does not mean the air changed, only the risk interpretation did.

Source : https://www.aqi.in/dashboard/india/maharashtra/mumbai 

Global Comparison of PM2.5 AQI Standards

A PM2.5 level considered “Moderate” in one country may be labeled “Unhealthy” in another.

The chart below compares PM2.5 concentration thresholds used by India (set by Central Pollution Control Board (CPCB), China (set by the Ministry of Ecology and Environment, MEE), European Union (EU) and USA (set by U.S. Environmental Protection Agency, US EPA)  for defining their air quality.

So, the same pollution level can be classified differently worldwide and how air quality is interpreted across the globe.

WHO Air Quality Guidelines

So, WHO does not operate an AQI scale but provides recommended pollutant limits based purely on health outcomes.

Also, these are the most conservative benchmarks, and many countries currently exceed them on a regular basis.

Why AQI Values Differ for the Same City

So, AQI variation is not an error, it reflects different public-health philosophies.

Key reasons include:

• Different pollutant breakpoints
• Health risk tolerance levels
• Pollutants prioritised in the index
• Communication goals (public alert vs regulatory reporting)

As a result, a city can report multiple AQI values depending on which standard is applied.

Why AQI Is So Important

1. Makes Invisible Risk Visible

Air pollution cannot be seen consistently, but AQI converts invisible exposure into actionable information.

2. Protects Vulnerable Groups

Children, elderly people, pregnant women, and those with asthma or heart disease depend on AQI alerts to reduce exposure.

3. Supports Public Health Decisions

AQI data informs school advisories, construction bans, traffic restrictions, and emergency health responses.

4. Enables Comparisons and Accountability

Tracking AQI over time reveals long-term exposure, not just extreme events which is crucial for policy evaluation.

The Bigger Picture

So, AQI is not just a number, it is a public-health communication tool. Understanding which standard is being used, and why, allows citizens, researchers, and policymakers to interpret pollution data responsibly.

 Air pollution has  become a year-round issue rather than a seasonal crisis. This means that AQI awareness is no longer optional, it is essential.

Visit AQI.in to check your local real-time AQI across both Indian and US standards leading to a much better understanding of the air surrounding you.