The Air Quality Index (AQI) is an indicator of air quality, based on air pollutants that have adverse effects on human health and the environment. The pollutants are ozone, fine particulate matter, nitrogen dioxide, carbon monoxide, sulphur dioxide and total reduced sulphur compounds. Basically it is a number used by government agencies to communicate to the public how polluted the air currently is or how polluted it is going to become in the near future. As the AQI increases, an increasingly large percentage of the population is likely to experience adverse health effects. Different countries have their own air quality indices, corresponding to different national air quality standards.

India has launched its first national Air Quality Index (AQI) to provide real time information about pollution levels. It is based on the recommendation given by Indian Institute of Technology, Kanpur. This will put out real time data about the level of pollutants in the air and inform people about the possible impacts on their health. This index will initially cover ten cities – Delhi, Agra, Faridabad, Lucknow, Kanpur, Varanasi, Ahmedabad, Hyderabad, Bangaluru and Chennai. Later it will be extended to more than 60 cities with a population exceeding one million.

A study conducted by World Health Organisation (WHO) on 1,600 cities revealed that Delhi was the most polluted capital and thirteen of the dirtiest 20 cities worldwide were in India. Another study, conducted by economists and public policy experts from Energy Policy Institute at Chicago, Yale and Harvard University showed that India’s poor air quality reduces life span of the average citizen by 3.2 years.

The AQI may prove to be a major initiative for improving air quality in urban areas as it would enhance public awareness and involvement and create a competitive environment among cities to take steps for air pollution-mitigation.

According to Centre for Science and Environment (CSE), it an important step forward for building awareness and protecting public health and this is an appropriate time to push for aggressive and time –bound action in Delhi and other Indian cities to meet clean air standards and reduce the public health risk. Also with this step, India has joined the global league of countries like the US, China, Mexico and France that have implemented smog alert systems. These countries not only issue smog alerts, but also implement pollution emergency measures to bring down peak pollution levels.

The Index is based on five chief pollutants – particulate matter with a diameter less than 10 micrometers (PM10), particulate matter with a diameter of less than 2.5 micrometers (PM2.5), ozone (O3), nitrogen dioxide (NO2), and carbon monoxide (CO). A monitoring station will give the concentration of a particular pollutant and its average over a period of time – for CO and O3, the average is taken over eight hours, while for the other three, it is a 24-hour average. The unit of measurement is microgram (or milligram in the case of CO) per cubic meter. The AQI takes multiple data on pollution already available with the country’s Central Pollution Control Board (CPCB) and presents it as a colour coded scale with six levels.

There are six AQI categories, namely: Good, Satisfactory, Moderately polluted, Poor, Very poor and Severe. Dark green, the first level, indicates good quality air while maroon at the other end indicates severe pollution. For each category, the index identifies associated health impacts. For example when the scale touches maroon, the advisory reads: May cause respiratory impact even on healthy people, and serious health impacts on people with lung/heart disease.

So, AQI will be used to inform people about daily air quality and to provide advisories on health consequences. It will tell how clean or polluted the air is and what associated health effects might be a cause of concern. This can help people take precautions on “bad air” days.