While it is easy to imagine a sharp rise in online orders for anti-pollution products such as masks and air purifiers on smoggy days, a boom in the sales of condoms and sports-wear might be a little unexpected.

According to search ratings for last week provided by Taobao.com, China's largest online shopping platform, searches for condoms were clearly correlated with those Chinese cities that had heavy smog.

In Beijing and some northern cities that were severely hit by smog, the rise in orders of condoms went beyond the sales in cities with cleaner air, as people have greater concerns of good child bearing and try to prevent getting pregnant on smoggy days, according to Taobao.