Beijing is no stranger to air pollution, but this time it was different. A lingering 4-day heavy smog with jaw-dropping pollution readings  hit the Chinese capital in mid-January, making news headlines and galvanizing an online community that has become increasingly vocal about environmental issues.
State media  called on citizens to take pre-cautions and avoid outdoor physical activities.
One key pollution indicator–PM2.5, otherwise known as particulates less than 2.5 micrometers in the air, reached a stunning high level on January 12. The highest reading on PM2.5 that day was beyond 700 micrograms per cubic meter, in what many Chinese are calling “exploding the index” (readings went off the charts.)
Of all the small particulates, experts believe PM2.5 poses the greatest risk to human health as they can penetrate into human lungs. Air pollution index over 500 is believed to be hazardous.In a rare move, the Beijing Meteorological Bureau issued  an orange warning on January 13, the highest haze warning the city has ever received.
The heavy pollution in Beijing has led to a surge in mask sales [zh] online. The masks were even sold out in some stores.
Beijing installed 35 PM2.5 monitoring stations  at the end of last year in a bid to better inform citizens on pollution levels. A lack of wind and explosive growth of automobiles over the years in the city have always been cited by state media  as reasons for pollution. International press blames  the problem on the government’s endless pursuit of economic growth and its ignorance over environmental degradation.
Netizens voiced their concerns over health on Sina Weibo:
但大街小巷人还是不少。地铁里也充斥着各种外地人的口音， 基本讨论也是北京污染严重，不应该来，北京越来越不好， 容易得病什么的。各位不如你们都回去建设自己家乡吧， 把首都还给我们做北京！
田源-Jason [zh]: Even though they know the air is heavily polluted, the streets and lanes, big and small have been packed with many people. Different dialects can be heard in the subway, the talks have basically been –the severity of air pollution, they shouldn’t have come, Beijing becoming worse, and (people) are becoming prone to diseases, etc. Hi, guys, perhaps you should go back to contribute to your hometown, leave the capital Beijing to us!
北京污染得好厉害啊～我说，是啊。你要不要回去嘛？她说不要。 她问我那些人为啥要待在那里呢？我 说，那里能赚很多钱，还有高楼大厦。她说赚钱后治病吗？ 我不知如何回复。她说they are real ill……有时候想想，成人有时候真不如个孩子！
黄金神棍 [zh]: My daughter turned on the iPad and saw the pollution index in the weather report. She said, Beijing is heavily polluted~ I said yes, are you gonna go back? She said no and asked me why people want to stay there? I said people make a lot of money there, there are also high rises. She said can sickness be healed after they make money? I didn’t know how to respond. She said they are really ill…. Sometimes, children are wiser than adults!
One Chinese pop singer [zh] gave a guttural yell:
Now is the Beijing toxic air time…. Wearing masks couldn’t prevent that odd smell! My head is dizzy! My eyes hurt! I have a hard time breathing.The air in Beijing what do you want????Do you want to turn yourself into solid????
One played mockery [zh] on the pollution:
It is said the PM index now in Beijing has reached 728, it has officially entered the Silent Hill model.
Another netizen took on a historic parallel [zh]:
Smoggy city Beijing. The British will feel a sense of history if they saw Beijing on Jan 12, 2013, it would be like travelling back to London 1945
Some were helpless [zh]:
We don’t need to see PM2.5 index, our noses could smell the severity of the pollution, making money is not easy, I continue undeterred even though there are dangers ahead, it’s not that I am brave, I don’t have a choice!