Why does air quality stay good?

Air quality doesn't stay good. It fluctuates hour to hour and day to day depending on the weather, people burning trash, fires, forest fires, wind, rain, etc etc.

"Why is air quality important?
Local air quality affects how you live and breathe. Like the weather, it can change from day to day or even hour to hour. The U.S. Environmental Protection Agency (EPA) and your local air quality agency have been working to make information about outdoor air quality as easy to find and understand as weather forecasts. A key tool in this effort is the Air Quality Index, or AQI. EPA and local officials use the AQI to provide simple information about your local air quality, how unhealthy air may affect you, and how you can protect your health.

What is the AQI?
The AQI is an index for reporting daily air quality. It tells you how clean or unhealthy your air is, and what associated health effects might be a concern. The AQI focuses on health effects you may experience within a few hours or days after breathing unhealthy air. The AQI is calculated for four major air pollutants regulated by the Clean Air Act: ground level ozone, particle pollution, carbon monoxide, and sulfur dioxide. For each of these pollutants, EPA has established national air quality standards to protect public health. EPA is currently reviewing the national air quality standard for nitrogen dioxide. If the standard is revised, the AQI will be revised as well.

How does the AQI work?
Think of the AQI as 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. For example, an AQI value of 50 represents good air quality with little or no potential to affect public health, while an AQI value over 300 represents air quality so hazardous that everyone may experience serious effects.

An AQI value of 100 generally corresponds to the national air quality standard for the pollutant, which is the level EPA has set to protect public health. AQI values at or below 100 are generally thought of as satisfactory. When AQI values are above 100, air quality is considered to be unhealthy—at first for certain sensitive groups of people, then for everyone as AQI values increase.

What do the AQI values mean?
The purpose of the AQI is to help you understand what local air quality means to your health. To make it easier to understand, the AQI is divided into six levels of health concern:"