A covering of smog and grit so thick that it couldn’t even be measured on a traditional air quality index descended over Beijing Thursday, prompting heath concerns.

By 6 a.m. local time Thursday, the Twitter account @BeijingAir, which is operated by the U.S. Embassy, listed the air quality at “Beyond Index,” meaning it is off the charts.

02-28-2013 06:00; PM2.5; 525.0; 516; Beyond Index

— BeijingAir (@BeijingAir) February 27, 2013

The air quality remained at “Beyond Index” or “Very Unhealthy” or “Hazardous” for much of the rest of the day.

Here’s what it looked like in Tiananmen Square: Continue…