According to IQAir, a Swiss-based Air Quality Index (AQI) monitoring group, pollution levels in several parts of Delhi touched the 500 mark on Wednesday morning.
AQI measures the levels of particulate matter, also called PM 2.5, in the air. These tiny particles can enter the lungs and cause a host of diseases.
According to WHO guidelines, air with AQI values at or below 100 is considered to be satisfactory for breathing, while readings in the 400-500 range denote that pollution levels are "severe" in an area.
The satellite cities of Noida and Gurgaon also registered AQI levels touching the 500 mark.
Several northern states have been experiencing toxic air and poor visibility over the past few weeks. There have been reports of flights to and from Delhi being cancelled or delayed due to low visibility.
So thick is the smog that it is visible even from space. A few days ago, Nasa shared satellite images of a blanket of smog engulfing parts of northern India and neighbouring Pakistan.
The toxic air is also affecting people's health.
A survey by LocalCircles, an online community platform, in Delhi and nearby cities revealed that 81% of families reported at least one member suffering from health issues due to pollution in the last three weeks. Over a third of respondents said they had purchased cough syrup during that time.