Friday, April 18, 2014

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What is "AQI"?

The AQI is an index for reporting daily air quality. It tells you how clean or polluted your air is, and what associated health concerns you should be aware of. The AQI focuses on health effects that can happen within a few hours or days after breathing polluted air. EPA uses the AQI for five major air pollutants regulated by the Clean Air Act: ground-level ozone, particulate matter, carbon monoxide, sulfur dioxide, and nitrogen dioxide.

The graphic above represents the AQI for the Austin area in general. There are multiple monitoring sites for the Austin-San Marcos area. Index readings are available every hour for ozone, plus 8- and 24-hour readings for other pollutants.

You can 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 danger. For example, an AQI value of 50 represents good air quality and little potential to affect public health, while an AQI value over 300 represents hazardous air quality.

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. So, AQI values 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 get higher.

Ozone Color Code = Air Quality
Green = Good
Yellow = Moderate
Orange = Unhealthy for Sensitive Groups
Red = Unhealthy
Purple = Very Unhealthy

What is an Ozone Action Day?

An Ozone Action Day is issued when weather conditions are expected to become ideal for the development of ozone pollution. TCEQ meteorologists forecast an Ozone Action Day one day ahead of time. On these days, ozone concentrations are predicted to exceed the proposed Federal standard of 0.08 parts per million averaged over an 8-hour period. (Parts per million, or ppm, is used to express concentration.) Ozone levels above the standard are considered unhealthy to breathe, particularly among sensitive groups. Most ozone action days occur during the warmer months, from May through September. When the Clear Air Force expects ground-level ozone concentrations to reach the standard or above in Central Texas, an Ozone Action Day is issued.

What about the smoke from Mexico?

Particulate matter (PM 2.5) is another pollutant that shows up in our Central Texas air. It is matter that is 2.5 micrometers or smaller in size. 2.5 micrometers is approximately 1/30 the size of a human hair; so small that several thousand of them could fit on the period at the end of this sentence. A detailed PM explainer is provided online by the EPA. This shows up in its greatest concentrations during May when the Mexican smoke arrives. The Mexican smoke is a result of fires set annually in late winter in southern Mexico and northern Guatemala as part agricultural land clearing for spring planting. Wind trajectories bring it this way during May. ClientIP:,, UserAgent: CCBot/2.0 ( Profile: TWCSAMLSP