For the second time in 10 days, the amount of 2.5PM - the deadly tiny particles that get into the bloodstream and lungs - hit "emergency" levels, nearly 20 times the safe limit set by the World Health Organisation.
With the government facing new criticism over steps taken to counter the pollution, Prince Charles visited the Indian Meteorology Department as part of his two-day visit to India.
Charles, frequently outspoken on green issues, was briefed on the crisis by Sunita Narain, a leading environmental activist.
She did not reveal details of her royal talks, but in a commentary last week said: "Every breath we take is poison."
Narain has called for faster moves away from coal and other "dirty fuels" as energy sources, and said authorities are doing "too little too late".
India's Supreme Court joined criticism of the government on Wednesday saying authorities had made "little constructive efforts" to curb pollution.
The court has ordered a new clampdown on stubble burning - in theory already banned - on farms surrounding the capital. It was acting because pollution "affects the vast majority of Indians."
Research released this year said the toxic smog across northern India cuts short the lives of around one million people each year. The Delhi state government has taken measures including banning construction and only allowing cars on the road depending on whether the registration is an odd or even number.