India's capital New Delhi and surrounding states saw a big spike in smog following Diwali, the year's biggest Hindu festival.
New Delhi's air quality index entered the severe zone Thursday night while revelers burst firecrackers to celebrate Diwali, known as the festival of lights.
The overall air quality index which includes several pollutants hit dangerous levels. Toxic PM10 particulates approached 1,000 micrograms per cubic meter in some areas. The World Health Organization recommends 50 as the normal level.
"As an eye surgeon, I think I will soon get many patients with complaints of eye irritation and itching. Such patients come to me every year. I get firecracker injury cases, too. I'm prepared for that," an ophthalmologist said.
Several states banned firecrackers and urged residents to stay away from fireworks in order to keep air pollution under control.
New Delhi's air quality generally worsens this time of the year. Pollution in northern India has surged due to stubble burning by farmers and dropping temperatures with the onset of winter.