How did one Icelandic eruption poison the skies of Europe in 1783?

Laki, a 25 km fissure in Iceland, opened on 8 June 1783 and erupted for eight months. By volcano standards it was not huge: it is rated only 4 on the explosivity scale. What made it deadly was gas. Besides an estimated 14 cubic kilometres of lava, it poured out clouds of poisonous hydrofluoric acid and sulfur dioxide compounds, about 8 million tonnes of fluorine and 120 million tonnes of sulfur dioxide, six times Pinatubo's total.

In Iceland the gas contaminated the soil and the grass. About 80 percent of the sheep, half of the cattle and half of the horses died of fluorosis, which damages teeth and bones, and the vast majority of crops were destroyed. The famine that followed killed at least a fifth of the island's people. The minister Jón Steingrímsson became famous for a "fire mass" he delivered on 20 July 1783, as a branch of the lava flow halted not far from his church farm.

Then the wind took the cloud south-east. A rare high-pressure zone over Iceland sent the poisonous cloud over Prague by 17 June, Berlin by the 18th, Paris by the 20th and Great Britain by the 23rd. The fog was so thick that ships stayed in port, and the sun looked "blood coloured". Sulfur dioxide makes victims choke because it reacts with the moisture in the lungs and forms sulfurous acid, and an estimated 23,000 British people died of the poisoning.

Benjamin Franklin, who recorded his observations in America in a 1784 lecture, wrote that the fog made the sun's rays so faint that focused with a burning glass they would scarce kindle brown paper. He wrote that the cause was not yet ascertained, and wondered whether it was smoke from volcanoes in Iceland.