I suppose doctors are accustomed to warning people about harmful things. This one from the doctors' group DEA is a bit different - but it's still a health warning (of sorts).
I like the comparison with smoking. There are obvious health impacts (both for us and those around us) and yet there is reluctance to change to a healthier option.
In the full poster version, the smoke carries the words: "lead, mercury, arsenic, dioxins, sulphur dioxide, NOx, fly ash". These are some of the toxic substances that result from coal mining and use - and lead to conditions including heart disease, lung cancer, asthma and stroke.
For more info on these and other health effects, check out the Coal and Health page on the DEA website. One interesting stat is that the health and social impacts are so severe that (if converted to financial terms) would triple the true cost of coal-based power.