NEW DELHI — A toxic cloud has descended on India’s capital, delaying flights and trains, causing coughs, headaches and even highway pileups, and prompting Indian officials on Wednesday to take the unprecedented step of closing 4,000 schools for nearly a week.

Delhi has notoriously noxious air but even by the standards of this city, this week’s pollution has been alarming, reaching levels nearly 30 times what the World Health Organization considers safe. On Tuesday, the government decided to close primary schools and on Wednesday the closings were extended to all public and most private schools.

For those of us living here, the air pollution saps our strength. Many people feel nauseated all day, like from a never-ending case of car sickness. The air tastes smoky and irritates the throat, and in some neighborhoods, it smells like paint.

Even if you have air filters in your house, as some of us do, a faint lingering chemical smell always seems to find its way in, through air-conditioner vents, open windows and cracks in the doors.