# Air Quality Index (AQI) Calculation: Review and Formulas

## Introduction

The Air Quality Index (AQI) is a vital tool used to assess and communicate the quality of air in a particular region. It provides information about the potential health risks associated with air pollution, aiding policymakers, health officials, and the public in making informed decisions to protect their health and the environment. This article will delve into the details of AQI calculation, exploring its components, methodologies, and the formulas used for determination. Additionally, we will present real-world examples to illustrate how AQI is derived in practice.

## 1. What is the Air Quality Index (AQI)?

The AQI is a numerical scale that quantifies the level of air pollution in a specific area at any given time. It takes various air pollutants into account, including particulate matter (PM2.5 and PM10), ground-level ozone (O3), sulfur dioxide (SO2), nitrogen dioxide (NO2), and carbon monoxide (CO). The AQI typically ranges from 0 to 500, where lower values represent better air quality, and higher values indicate increased levels of pollution.

## 2. Components of the Air Quality Index

Before delving into the formulas used for AQI calculation, let's understand the key components involved:

• Pollutant Concentration: The concentration of individual pollutants is the primary data needed to calculate the AQI. These concentrations are measured in micrograms per cubic meter (µg/m³) for PM2.5, PM10, SO2, NO2, and in parts per million (ppm) for CO. Ozone (O3) is measured in parts per billion (ppb).
• AQI Categories: To make AQI more accessible to the general public, it is divided into various categories, each representing a specific level of health concern. Commonly, there are six AQI categories: Good, Moderate, Unhealthy for Sensitive Groups, Unhealthy, Very Unhealthy, and Hazardous.

## 3. AQI Calculation Formulas

The AQI calculation involves determining the individual AQI for each pollutant and then selecting the highest value among them to represent the overall AQI for a specific area. Here are the formulas for calculating the AQI for each pollutant:

### 3.1 Particulate Matter (PM2.5 and PM10)

AQI_PM25 = [(IHI - ILO) / (BPHI - BPLO)] x (Cp - BPLO) + ILO

Where:

• AQI_PM25 is the AQI for PM2.5
• IHI is the AQI value corresponding to the upper boundary of the AQI category
• ILO is the AQI value corresponding to the lower boundary of the AQI category
• BPHI is the concentration value corresponding to the upper boundary of the AQI category
• BPLO is the concentration value corresponding to the lower boundary of the AQI category
• Cp is the actual concentration of PM2.5 in µg/m³

### 3.2 Ozone (O3)

AQI_O3 = [(IHI - ILO) / (BPHI - BPLO)] x (Cp - BPLO) + ILO

Where:

• AQI_O3 is the AQI for Ozone
• IHI is the AQI value corresponding to the upper boundary of the AQI category
• ILO is the AQI value corresponding to the lower boundary of the AQI category
• BPHI is the concentration value corresponding to the upper boundary of the AQI category
• BPLO is the concentration value corresponding to the lower boundary of the AQI category
• Cp is the actual concentration of Ozone in ppb

## 4. AQI Calculation Example

Let's consider an example to demonstrate how to calculate AQI:

### Example Scenario:

• PM2.5 concentration (Cp) = 30 µg/m³
• Ozone concentration (Cp) = 70 ppb
• AQI category boundaries and corresponding AQI values can be found in AQI reference tables.

### Calculation for PM2.5:

IHI = 50, ILO = 0
BPHI = 35, BPLO = 0
AQI_PM25 = [(50 - 0) / (35 - 0)] x (30 - 0) + 0
AQI_PM25 ≈ 42.86

### Calculation for Ozone:

IHI = 100, ILO = 51
BPHI = 86, BPLO = 0
AQI_O3 = [(100 - 51) / (86 - 0)] x (70 - 0) + 51
AQI_O3 ≈ 86.63

In this example, the AQI for PM2.5 is 42.86, and the AQI for Ozone is 86.63. Since the Ozone AQI is higher, the overall AQI for this scenario is 86.63, categorized as "Moderate."

## 5. Interpretation of AQI

The final AQI value, as obtained using the formulas above, falls into one of the predefined AQI categories. Each category corresponds to a level of health concern and is associated with specific health recommendations. Here's a general interpretation:

• Good (0-50): Air quality is considered satisfactory, and air pollution poses little or no risk.
• Moderate (51-100): Air quality is acceptable; however, there may be a moderate health concern for a very small number of sensitive individuals.
• Unhealthy for Sensitive Groups (101-150): Members of sensitive groups may experience health effects, but the general public is unlikely to be affected.
• Unhealthy (151-200): Everyone may begin to experience health effects; members of sensitive groups may experience more serious health effects.
• Very Unhealthy (201-300): Health alert: everyone may experience more serious health effects.
• Hazardous (301-500): Health warnings of emergency conditions; the entire population is likely to be affected.

## Conclusion

The Air Quality Index is a powerful tool that empowers individuals and communities to stay informed about the quality of the air they breathe. By understanding the AQI calculation, its components, and the associated health risks, we can take necessary precautions to safeguard our health and the environment. Regular monitoring, timely information dissemination, and collective efforts to reduce pollution can lead to better air quality and a healthier future for generations to come.