"Killer clouds" are a curious weather-related phenomenon that have been fortunately rare occurrences throughout history. They are caused when atmospheric conditions trap hazardous emissions near the Earth's surface, and they can indeed kill. The worst recorded occurrence of a "killer cloud" happened in the summer of 1783 in Europe, claiming an estimated 23,000 lives in Great Britain and many hundreds if not thousands more on the continent.
The killer cloud that engulfed Britain and Europe was caused by the eruption of the Laki Volcano on Iceland. On June 8, 1783 the volcano began to erupt from a long fissure. It was not the spectacularly explosive eruption of the kind most familiar to history, like those of Mt. St. Helens in the United States or Mt. Pinatubo in the Philippines, but it was unusually deadly in a couple unique ways. The first was in its chemical makeup, which had a high concentration of sulfur dioxide and fluorine gases. The second was in its longevity; it erupted continuously for five months, and did not stop erupting completely until February of 1784. In that extended period, Laki ejected an astonishing volume of material into the atmosphere: an estimated 120 million tons of sulfur dioxide, eight million tons of fluorine, and 14 cubic kilometers of lava.
The people of Iceland suffered the worst of the volcano's effects, not just from lava flows and falling ash and rock, but from the poisonous effects of the fluorine. Up to 80 percent of the island's livestock died as a result of eating contaminated grass, and as a consequence, an estimated 9,000 people died of starvation over the next two years. The prevailing winds and an uncommon high pressure area over Iceland drove the ash and gas cloud from Laki, which was estimated to have reached 15 kilometers into the sky, toward the southeast and the rest of Europe.
All of Europe from Prague to Britain was affected by the cloud, which took the form of a thick, dry fog. Boats were forced to remain in port, unable to navigate. The sunlight was dimmed. Anyone venturing outside was affected by the cloud, which contained sulfur dioxide and its by-product, sulfuric acid. Eyes, noses, throats, and lungs were made raw by the toxic fog, and many people, especially those who worked outdoors, died. The cloud also caused violent thunderstorms; it reflected sunlight and heated the atmosphere above it, creating unstable and turbulent air masses. By autumn, the cloud had dissipated, but the effects from the volcano in far-away Iceland would continue to be felt for the next two years. The aerosol cloud of sulfuric acid in the atmosphere lowered temperatures worldwide, making the following winter one of the coldest on record. Researchers have also discovered that the patterns of the Indian and African monsoons were disrupted, causing a drought that reduced the flow of the Nile River.
Even though the killer cloud of 1783 is not well known to modern people, the likelihood is that it will happen again. Iceland is one of the most volcanically active places on Earth, with at least 18 active volcanoes. There are other parts of the world, such as Indonesia, that are home to volcanoes that could produce killer clouds. And thanks to industrialization and the rapid growth in areas like China and India, mankind may not have to wait for a volcano to produce a killer cloud; for several years, a huge plume of pollution known as the "Brown Hole" has formed over South Asia. If it continues to grow, and the right atmospheric conditions affect it, it could become a killer cloud every bit as dangerous as that caused by a volcano. Doing something about it before it's too late should be a priority; after all, Mother Earth has already demonstrated she can throw enough nightmares at us without our finding new ways to create them ourselves.
Thordarson, Thorvaldur and Self, Stephen (2003) Atmospheric and environmental effects of the 1783-1784 Laki eruption: a review and reassessment. Journal of Geophysical Research, 108 (D1). 4011. ISSN 0148-0227