Beijing air pollution levels soared off the charts on Thursday morning for the second time this week, as a wave of heavy smog engulfed the city.
The haze will be compounded by a sandstorm  brewing in northern China, which has already affected parts of Inner Mongolia, Gansu, Shanxi and Hebei.
“A sandstorm in Mongolia and Inner Mongolia is blowing west to east and will gradually affect our city, raising the concentration of fine respirable particulates. Overall pollution levels are now serious. Friends from the public are advised to stay indoors,” the Beijing Municipal Environmental Monitoring Centre said on its website  at 12pm.
The US embassy air quality monitor for PM2.5 – airborne particles small enough to enter the lungs and blood – reached levels “beyond index” and hit 502 micrograms per cubic metre at about 6am. The index goes up to only 500.
As of 9.45am on Thursday, PM2.5 levels were still at a hazardous 469 at a 24-hour exposure at this level. The World Health Organisation recommends PM2.5 levels be kept below 25.
The Ministry of Environmental Protection in Beijing recorded a PM2.5 level of 416, a level considered “severely polluted”.
02-28-2013 10:00; PM2.5; 510.0; 506; Beyond Index
— BeijingAir (@BeijingAir) February 28, 2013 
Several “side-by-side” photo comparisons of Beijing smog surfaced on social media on Wednesday including this one  on Twitter from Bill Bishop, author of China newsletter Sinocism, and this one  on Google Plus from William Farris.
Beijing authorities issued a yellow haze warning on Wednesday.
Frustrated Beijing citizens were not particularly shocked to hear how bad the pollution readings were after experiencing nearly two months of chronic bad air.
"Just another day in Beijing," a disgruntled netizen tweeted on Thursday.
On Sina Weibo, a few users questioned the sudden spike in PM2.5 levels on Wednesday. From just a moderate level of 75 in the afternoon, it shot up to the 200-300 level by evening.
"Thick air, can't see, no wind...this is what I woke up to today," one microblogger said.
Beijing Capital International Airport gave no warning of heavy smog, but its website listed  14 flights as cancelled. Ten of these were international flights.
About four of the city's highways were partially closed on Thursday morning, according to Beijing news site Qianlong . Visibility was at a relatively safe three kilometres.
Other mainland cities experiencing high air pollution readings included Baoding and Shijiazhuang in neighbouring Hebei province, as well as Huhhot in Inner Mongolia. All had PM2.5 readings of 500.