According to IQAir, a Swiss-based Air Quality Index (AQI) monitoring group, pollution levels in several parts of Delhi touched the 500 mark. 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. Delhi was the most polluted capital city in the world in 2023, according to IQAir. India was also ranked as the world's third-most polluted country after neighbours Bangladesh and Pakistan, IQAir said.