In recent years, Beijing officials have announced good or even excellent air quality about 80 percent of the time, notwithstanding the obvious and pervasive smog. But a monitor (with a Twitter feed) atop the United States embassy offers a different perspective: over 80 percent of days have unhealthy levels of pollution.

The Twitter feed, @BeijingAir, is widely viewed on mobile apps and reposted on Chinese microblogging sites like Sina Weibo. Many media reports have mentioned these hourly reports, which often assess the air to be hazardous. The biggest culprits are cars and coal and, despite various stopgap measures, air quality in China promises to be an ongoing health concern.