Are we heading for another 'killer smog'?

Is London poisoning itself all over again?

Just under a week ago we were told that the pollution was about to get so bad in London that any strenuous activity might physically harm us. This might have been less surprising seventy years ago when vast industrial chimneys were spewing out soot into the sky. But how does this happen in 2022? And what does a pollution level of ‘ten’ even mean? What causes it? What is it exactly that we’re breathing in? And is anyone doing anything to stop it getting worse?