Understanding Smog and the Air Quality Index (AQI)
Smog is a type of intense air pollution. The word "smog" was coined in the early 20th century as a portmanteau of the words smoke and fog to refer to smoky fog, its opacity, and its odor. This kind of visible air pollution is composed of nitrogen oxides, sulfur oxides, ozone, smoke, and other particulates. Man-made smog is derived from coal combustion emissions, vehicular emissions, industrial emissions, forest and agricultural fires, and photochemical reactions of these emissions.
To quantify the level of smog and other air pollutants, governments and environmental agencies around the world use an Air Quality Index (AQI). The AQI is a yardstick that runs from 0 to 500. The higher the AQI value, the greater the level of air pollution and the greater the health concern. This calculator serves as a conceptual tool to help you understand how different pollutants contribute to the overall AQI and what those levels mean for your health. It focuses on two key components of smog: Ground-Level Ozone (O₃) and Particulate Matter (PM₂.₅).
How to Use the Smog Calculator (AQI Estimator)
This calculator provides an estimated AQI based on the concentrations of major pollutants. Here's how to interpret and use it:
- Enter Ozone (O₃) Concentration: Input the concentration of ground-level ozone in parts per billion (ppb). This is a primary component of photochemical smog.
- Enter PM₂.₅ Concentration: Input the concentration of fine particulate matter (particles with a diameter of 2.5 micrometers or less) in micrograms per cubic meter (µg/m³). This is a major cause of haze and health issues.
- View the Estimated AQI: The calculator will determine the individual AQI for each pollutant and display the highest value as the overall AQI.
- See the Health Implications: Based on the final AQI value, the calculator will show the corresponding health category (e.g., Good, Moderate, Unhealthy) and a brief description of the associated health advice.
The AQI Calculation Formula
The AQI is calculated using a piecewise linear function that relates a pollutant's concentration to a standardized index value. The formula is:
I = [ (I_high - I_low) / (C_high - C_low) ] * (C - C_low) + I_lowWhere:
Iis the Air Quality Index.Cis the pollutant concentration.C_lowandC_highare the concentration breakpoints that are less than and greater than C.I_lowandI_highare the AQI breakpoints corresponding to C_low and C_high.
The calculator applies this formula to both Ozone and PM₂.₅ based on the EPA's established breakpoint tables. The final reported AQI is the higher of the two individual pollutant AQIs.
Practical Example
Imagine a city's environmental agency reports the following pollutant levels for a summer afternoon:
- Ozone Concentration: 80 ppb
- PM₂.₅ Concentration: 25 µg/m³
The calculator would perform the following steps:
- Calculate Ozone AQI: For an 80 ppb 8-hour ozone concentration, the EPA breakpoints are 55-70 ppb (AQI 101-150). The calculation puts it in the "Unhealthy for Sensitive Groups" category, with an AQI of approximately 114.
- Calculate PM₂.₅ AQI: For a 25 µg/m³ 24-hour PM₂.₅ concentration, the breakpoints are 12.1-35.4 µg/m³ (AQI 51-100). The calculation puts it in the "Moderate" category, with an AQI of approximately 78.
- Determine Final AQI: The final AQI is the higher of the two values, so the reported AQI would be 114.
The calculator would then display an AQI of 114, categorized as "Unhealthy for Sensitive Groups," advising that children, older adults, and people with lung disease should reduce prolonged or heavy exertion outdoors. This demonstrates how even if one pollutant is at a moderate level, another can drive the overall air quality into a more dangerous category. Understanding your personal impact through tools like the CO₂ Breathing Emission Calculator can provide a wider context on air composition.