New Delhi officials have declared the city’s smog a public health emergency, after the air quality index hit levels considered unsafe. Delhi has a history of toxic smog, which typically worsens in the winter months.The World Health Organization considers an Air Quality Index higher than 25 to be dangerous, but in some areas of Delhi people have measured rates of more than 900. The small particles, which the index measures, pose severe risks to the lungs.Authorities have closed schools in the city, suspended civil construction projects, and banned all trucks. Meanwhile, drivers are struggling to see through whiteout conditions on major roadways.