The 1910 Halley's Comet panic
In May 1910 Earth was due to pass through the tail of Halley's Comet. Newspapers ran warnings of poison gas, and people bought gas masks and "anti-comet pills".
Newspapers named 18 May 1910 as the day Earth would meet the comet's tail. Wikipedia gives 19 May for the passage itself. On 18 May the Cincinnati Enquirer ran a Chicago story headed "Comet Hysteria", which said Chicago residents, women especially, had caught it and that the comet dominated talk on the streets and trains.
Spectroscopic analysis had found the toxic gas cyanogen in the tail, and the press speculated that life on Earth could be endangered. Scientists said the gas would present no danger. Wikipedia records that people still bought gas masks in a panic and that quack "anti-comet pills" were sold.
The astronomer Flammarion, as the Science History Institute reports, offered wilder ideas: hydrogen in the tail might unite with oxygen in the air and leave people choking, or carbonic acid might sear their lungs.
"On 19 May, Earth passed through the tail of the comet.[132][133] One of the substances discovered in the tail by spectroscopic analysis was the toxic gas cyanogen,[134] which led to press speculation that life on Earth could be endangered.[135][136] Despite reassurances from scientists that the gas would not present any danger,[136] public panic buying of gas masks was recorded, and quack "anti-comet pills" were sold.[137]"
- The Comet Panic of 1910, Revisited, Science History Institute, Distillations
- Haley's Comet and the End of the World, This Local Life