"The longest distance in the world," according to one joke making the rounds, "is not between life and death, but when I hold your hand at a crossing in Beijing and I can't see your face."

As with many of the country's more solemn situations, the choking smog blanketing Beijing since Monday has evoked wisecracks from social media users.

In the past few days, jokes on the topic have become one of the most discussed threads on China's largest micro-blogging site, Sina Weibo.