Air pollution has reached alarmingly high levels in Delhi over the past few decades due to vehicular and industrial emissions. An estimated 2000 metric tones of air pollutants are emitted into the atmosphere every day in Delhi. Vehicular emissions contribute to about 72 per cent of total pollutants emitted (CPCB, 2001-02), followed by 29 per cent from industries and thermal power plants, and 8 per cent from the domestic sector (NEERI, 1991). In Delhi, the contribution of vehicular pollution has increased only in the past 2-3 decades as the share, which was a paltry 23 per cent in 1971, rose to 43 per cent in 1981 and became 63 per cent in 1991 (WWF, 1995).

CSE's Pollution Monitoring Laboratory monitors ambient air quality at Tughlakabad Institutional Area for particulate matter, sulfur dioxide, nitrogen dioxide. This area is considered a 'sensitive area' because of the proximity of Batra Hospital, a leading area hospital and several academic institutions within a 50 m radius of the monitoring site.

Analysis of the ambient air quality data monitored over the last four months reveals that most of the pollutants exceed the National Ambient Air Quality Standards (NAAQS) set by the Central Pollution Control Board (CPCB). For example, the Respirable Suspended Particulate Matter (RSPM) concentration of 303.03 mg/m³ recorded at the Tughlakabad monitoring site is 4 times the stipulated 24-hour standards of 75 mg/m³ recommended for sensitive areas, as set by CPCB in NAAQS. Suspended Particulate Matter (SPM) concentration of 582.33 mg/m³ recorded on the same day is more than five times the maximum permissible limits (100mg/m³) recommended for sensitive areas (Analysis of Tughlakabad Air Quality).

Total suspended particulates are associated with decline in lung function, increased respiratory symptoms and mortality from cardio-respiratory causes (Vincent and Tan, 1997). As Indians have short life span, the total impact on life-years lost due to air pollution is greater than in developed countries. If total suspended particulates were reduced by 100 mg/m³ (the current average is 378), there would be 1,385 fewer deaths in Delhi every year, representing a total of more than 50,000 life-years.

It is necessary to use a rating system, such as the Air Quality Index (AQI) which provides information about the air quality of that area. Daily data generated for ambient air at the Tughlakabad monitoring site will be used to develop an AQI which will be posted on our website every day to enable the public to take appropriate precautions to safeguard themselves their families and and communities against exposure to air pollution levels.

The Air Quality Index (AQI) is an index, or rating scale, for reporting daily the ambient air pollution recorded in the monitoring sites. The two objectives of AQI are: a) to inform and warn the public about the risk of exposure to daily pollution levels; and b) to enforce regulatory measures for immediate local impact. The AQI informs the public about the quality of air and associated health effects resulting from inhaling polluted air. The higher the AQI value, greater is the level of air pollution. Data obtained from monitoring of ambient air at Tughlakabad monitoring site by the Laboratory is used to calculate the AQI. Four major pollutants that have adverse effects on human health, including Suspended Particulate Matter (PM100), Respirable Suspended Particulate Matter (PM10), Nitrogen Dioxide and Sulfur Dioxide, have been considered for reporting AQI.

The maximum permissible limits set for sensitive areas by NAAQS is used to develop an Air Quality Index for Tughlakabad. Tughlakabad is considered as a sensitive area as Batra Hospital, a major area hospital, and several educational institutions are located nearby. An AQI of 100 is equivalent to the NAAQS for 24 hours for sensitive areas.

For each pollutant, an intermediate value of 50 is considered equivalent to one half the value of NAAQS (upper breakpoint of the "good" category). An AQI greater than 100 is considered to be above NAAQS and is given index values of 200, 300 and 300+ to represent poor, very poor and critical air quality respectively.

At the end of the day, the concentration of each pollutant is converted into AQI for the pollutant using the above-mentioned formula. The pollutant with the highest AQI at a given day becomes the AQI reading for that day. It can be used to describe the impact of the pollutants on human health and the environment. The pollutant with the highest AQI number becomes the "overall" AQI for a particular location.

The higher the AQI value, the greater is the level of air pollution and the greater is the danger to health. If AQI for suspended Particulate Matter is 305 and is highest out of the five pollutants, it is reported as the overall AQI or quality of air for a particular location. It would be reported as an "AQI of 305; reason: SPM." AQI indicates the maximum safe levels for a pollutant. Above this level the pollutant has undesirable impact on people and environment. If the AQI falls below 50, the air quality is considered "good". An AQI reading between 51-100 indicates "moderate" air quality and an AQI reading from 101-200 indicates "poor" air quality. A reading over 201 indicates "very poor" air quality and an AQI of above 301 is regarded as "critical" to human health.

"Good"—If the AQI value is between 0-50, air quality is considered good with no or minimal health effects.

"Moderate"—If the AQI value is between 51-100, air quality is acceptable. However, for some, it may be a moderate health concern with possible respiratory or cardiac effects for most sensitive individuals.

"Unhealthy"—If the AQI value is between 101-200, air quality is unhealthy. There is an increasing likelihood of respiratory and cardiovascular symptoms or illnesses in members of sensitive groups.

"Very Unhealthy"— If the AQI value is between 201-300, air quality is very unhealthy. It might trigger a health alert, resulting in aggravation of heart or lung disease. Other effects include increased risk of death in children, elderly or person with severe cardiopulmonary (heart and lung) disease, increased risk of respiratory effects in general pollution.

"Critical"— AQI values over 300 triggers health warnings of emergency conditions. The entire population is more likely to be affected with serious aggravation of heart or lung disease, increased risk of premature death and serious risk of cardio respiratory in general population.

A specific colour is assigned to each AQI category to make it easier for people to understand the significance of air pollution levels in their communities. For example, the colour green means that the quality of air is "good". The colour purple means that conditions may be "critical " for everyone. The colours can help to determine whether air pollutants are reaching unhealthy levels the area.