A thick smog has been hanging over Beijing since Thursday. OK, so the capital is no stranger to pollution, but this is off the scale. In terms of the PM2.5 index (that's particulate matters of less than 2.5 micrometres in size), the World Health Organization considers a safe daily level to be 25 micrograms per cubic metre. When I asked my Aussie friend Colin what a bad level would be like in Sydney he said 40. In Beijing it's often into the 100s, but today it's around 700! We stayed indoors.