The day after Diwali festivities Delhi woke up to watery eyes, irritated throats, and a blanket of smog thanks to a severe dip in air quality, with parts of the city reporting 'hazardous' to 'very hazardous' air.

After a relatively clean October, the air quality in the capital saw a decline in the last couple of days, slipping from 'severe' to 'hazardous' a day after Diwali.

The blanket ban on the sale and purchase of firecrackers, including green crackers, seemed to do little to deter people in many parts of Delhi and surrounding areas.

Earlier this week, the Ministry of Earth Sciences' System of Air Quality and Weather Forecasting And Research (SAFAR), had warned, that even if we saw half the firecracker emission that we had last year, the air quality would deteriorate to 'severe' on the night of Diwali.

According to SAFAR, stubble share and the calm wind conditions also contributed to the air quality degrading.

It isn't just Delhi, other parts of the country have also witnessed a fall in air quality in the last 24 hours. Delhi's neighbours Ghaziabad (UP) and Faridabad (Haryana) recording AQIs upwards of 500 on Friday morning.

Here is a look at PM2.5 level in other major cities in the country.

According to the System of Air Quality and Weather Forecasting And Research, Air Quality index is calculated by turning complex data of various pollutants into one index value, nomenclature, and colour for better understanding.

Typically, while calculating the AQI of a place, 8 pollutants are taken into account. These include,

• PM2.5

• PM10

• O3 (Ground level Ozone)

• SO2 (Sulphur dioxide)

• CO (Carbon Monoxide)

• NO2 (Nitrogen dioxide)

• NH3 (Ammonia)

• Pb (Lead)

As per the CPCB, the AQI is calculated using the average concentration value of each pollutant over a standard time interval of 24 hours (8 hours in case of CO and O3), and should include data for at least 3 of these 8 pollutants.

What Do The Numbers Mean?

According to the Central Pollution Control Board's National Air Quality Index, pollution levels range from good to severe.

• 0 - 50: Good. Air quality is considered satisfactory, and air pollution poses little or no risk.

• 51-100: Satisfactory. Air quality is acceptable; may be a moderate health concern for sensitive people.

• 101-200: Moderate. Sensitive groups (with lung, heart diseases) may experience health issues.

• 201-300: Poor. Everyone may begin to experience health issues like respiratory problems.

• 301-400: Very Poor. Health warnings of emergency conditions. The entire population is likely to be affected.

• 401-500: Severe. Health alert: everyone may experience more serious health issues.