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Air Quality Index: Definition & Uses

Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

After completing this lesson, you will be able to explain what the air quality index is, what numbers mean on the US scale, and how the index is used. A short quiz will follow.

What is the Air Quality Index?

The air quality index (AQI) is a number used to report the quality of the air on any given day: it basically tells you how clean the air is. It measures particles and chemicals in the air that affect people's health (and ignores those that do not). The health effects from extreme pollution in places like China can be severe. These effects can range from slight irritations, to reduced endurance, to respiratory problems.

Smog in New York in 1988
Smog in New York in 1988

Different countries have different AQIs, so it is difficult to compare one location to another on a worldwide scale. Some countries are more safety-conscious than others. The United States uses a 500 point scale to report air quality. Any rating between 0 and 50 is considered good. A score between 51 and 100 indicates a moderate level of health concern. An AQI number between 101 and 150 is considered unhealthy for sensitive groups like the elderly or people with heart or breathing problems. Scores between 151 and 200 are described as unhealthy, while 201 to 300 is considered very unhealthy. And finally, air quality in the 301 to 500 range is deemed hazardous.

The US Air Quality Index
The US Air Quality Index

Air quality generally declines as the day goes on, because businesses and cars are releasing increasing amounts of pollution as they work. Air quality can vary by time of year, but this depends on the exact country or city. Winter can be worse in places where large amounts of fossil fuels are burned for heat, while the heat of summer can actually lead to greater ozone levels due to natural processes that occur in polluted air. Quality is also affected by stagnant air, wind speeds, and chemical reactions between pollutants in the air. If the air isn't moving much, for example, pollution won't dissipate as quickly.

The AQI does have limitations, however: it only records certain chemicals, and doesn't differentiate between them. For example, the exact mix of pollutants tends to be different in the summer versus the winter: carbon monoxide is common in the winter, whereas ozone is common in the summer. Since it is a general measure, the AQI can't tell you how dangerous the particular pollutants present on a day happen to be.

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