What is Ambient Air Quality?

(by Yuvika Ramsaran , WKC Group)

The Earth’s atmosphere is divided into five major layers: namely, the tropospherestratospheremesospherethermosphere, and the exosphere. The troposphere is the atmospheric layer closest to the Earth’s outer surface contains bulk of the atmosphere’s mass, is the layer in which all weather occurs, and contains the air that we breath.

Ambient air is atmospheric air in its natural uncontaminated state, and typically consists of 78% nitrogen, 21% oxygen and other trace gases including carbon dioxide, helium, methane, argon, and hydrogen.

The United States Environmental Protection Agency (US EPA) identified six criteria pollutants contained within ambient air that are of concern with regards to impacts to human health and the environment. The six criteria pollutants are:

  • Sulphur Dioxide (SO₂)

  • Nitrogen Dioxide (NO)

  • Particulate Matter (PM₂.5 and PM₁₀)

  • Ground Level Ozone (O)

  • Lead (Pb)

  • Carbon Monoxide (CO)

Why is Ambient Air Quality Monitored?

The purpose of ambient air quality monitoring is to quantify and assess the existing concentrations of selected air quality pollutants against applicable local and international standards or limits. This is usually undertaken by regulatory bodies or as part of an Environmental and Social Impact Assessment (ESIA) process prior to the development of a Project.

How is Ambient Air Quality Measured?

Ambient air quality is measured using the Air Quality Index (AQI) which works like a thermometer that provides readings from 0 to 500 degrees. The only difference between an AQI and a thermometer is that the AQI provides information about the changes in the amount of pollution in the air. An AQI that reads under 50 (low AQI level) means that the air quality is good, and a person can spend long periods of time outdoors and air pollution will pose very little to no risk to their health. As the AQI number increases, the risk to human health also increases.

The AQI is divided into six categories. Each category/colour represents a different level of health concern. Colour coding the level of health concerns ensures efficient comprehension in determining whether the air quality in a particular surrounding is reaching unhealthy levels.

Daily AQI Colour

Levels of Concern

Value of Index

Description of Air Quality



0 to 50

Air quality is satisfactory, and air pollution poses little or no risk.



51 to 100

Air quality is acceptable. However, there may be a risk for some people, particularly those who are unusually sensitive to air pollution.


Unhealthy for Sensitive Groups

101 to 200

Members of sensitive groups may experience health effects. The general public is less likely to be affected.



151 to 200

Some members of the general public may experience health effects; members of sensitive groups may experience more serious health effects.28


Very Unhealthy

201 to 300

Health alert: The risk of health effects is increased for everyone.



301 and higher

Health warning of emergency conditions: everyone is more likely to be effected.

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