Understanding the Air Quality Index (AQI)

Pollution levels are measured every day in over 900 counties throughout the US, and these measurements are used to calculate each area’s Air Quality Index (AQI), as determined by the US Environmental Protection Agency (EPA). The EPA measures the level of five major air pollutants so you can determine if going outdoors under these conditions may be a danger to your health.
The five pollutants measured by the AQI are nitrogen dioxide, sulfur dioxide, carbon monoxide, ground-level ozone and particle pollution (which is also referred to as particulate matter). Of these, airborne particulate matter and ground-level ozone are the most dangerous to your health.
Particulate matter is composed of very tiny particles mixed with liquid droplets, which may include sulfates, nitrates, metals, organic chemicals, soil and dust particles. They are emitted from such things as industrial plants, power plants, automobiles and forest fires. The problem with these is primarily their size. Because they are so tiny, they are not trapped by the nose and throat as larger particles normally are so they can be expelled. Instead, they enter the lungs, causing serious health problems.
Ground-level ozone is created when nitrogen oxides and volatile organic compounds (VOCs) undergo a chemical reaction in the presence of sunlight. The source of ground-level ozone is due to many of the same things listed above, and major health problems can occur when the ozone level is high.
The health effects from breathing this polluted air may appear within either hours or days of breathing it in. Depending on how high the index is on any particular day, it can affect anyone, even those who are generally healthy, especially if they are working or exercising outdoors. However, some people are more prone to the toxic effects of polluted air, including the elderly, children, people with asthma or other lung diseases, those with diabetes, cardiovascular disease, and those who have previously suffered a stroke or heart attack.

The levels of the Air Quality Index are divided into six categories as follows:
0-50 (Good) – No problems with air quality for anyone
51-100 (Moderate) – Those unusually sensitive should limit prolonged outdoor exertion
101-150 (Unhealthy for sensitive people) – Children, those with respiratory disease and people exercising should limit prolonged outdoor exertion
151-200 (Unhealthy) – Children, those with respiratory disease and people exercising should avoid prolonged outdoor exertion; everyone else should limit prolonged outdoor exertion
201-300 (Very unhealthy) – Children, those with respiratory disease and people exercising should avoid any outdoor exertion; everyone else should limit any outdoor exertion
301-500 (Hazardous) – Everyone should avoid all outdoor physical activity.

Keeping the above scale in mind can help you determine what is a safe time to enjoy the outdoors without worrying about the risk to your health.

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