Under the American system, only PM 2.5 values are taken to determine the level of pollution and the AQI number. Under the American system, values above 500 are considered beyond the AQI.
The US Embassy and Consulates' air quality monitors measure airborne fine particulate matter (commonly referred to as PM 2.5 because they are less than or equal to 2.5 microns in diameter) on the compounds of the Embassy and Consulates. The caveat is that data from a single monitoring station cannot be applied to an entire city.
The air quality data collected at the US Embassy and Consulates may differ from other monitors located in the same city.
Under the Indian computation, Central Pollution Control Board (CPCB) and Safar India give an index value for AQI which is a numerical scale used for reporting day-to-day air quality with regard to human health and the environment.
The CPCB launched AQI for effective communication of air quality status to people in terms which are easy to understand, and which transforms complex air quality data of various pollutants into a single number (index value), nomenclature and colour.
There are six AQI categories good, satisfactory, moderately polluted, poor, very poor, and severe.
So, there is a singe AQI number or index value which CPCB puts out or Safar India, another official data source, puts out.
AQ sub-index and health breakpoints are evolved for eight pollutants (PM10, PM2.5, NO2, SO2, CO, O3, NH3, and Pb) for which short-term (up to 24 hours) National Ambient Air Quality Standards are prescribed.
Apart from AQI, there is a count for major pollutants, PM 10 and PM 2.5 which are given separately by these monitoring and forecasting agencies.
In addition, there will be different AQI scores for different locations in a large metropolis like Delhi. For instance, at 5.15 p.m. on Friday, the AQI in Delhi at different locations varied from 450 in Vasant Vihar to a high of 643 in Siri Fort.
The AQI at different localities was mapped at Kalkaji (544), Okhla (553), Tughlakabad (588), Mundka (610), Punjabi Bagh (600) and Anand Vihar (602) all with varying readings.
With the public health emergency being declared, experts say the announcement could have come earlier.
Tanushree Ganguly, Programme Associate, Council on Energy, Environment and Water said, "Delhi has been experiencing severe air quality conditions for the past four days and PM 2.5 levels have now exceeded 300ug/m3. While declaring a public health emergency today is definitely a welcome move, the announcement could have come earlier".
She said the authorities must consider lowering the threshold concentration needed for declaring an emergency so that vulnerable sections of the population are not exposed to such high concentrations of PM2.5. "Also, at such high levels of ambient concentration, the indoor air quality also gets worse".
( With inputs from IANS )