Sorry, came upon this while reading something else.
Elemental Fluorine...?
Color me rather skeptical and highly surprised.
If true, nasty horrible stuff--and a terrible way to go.
The Laki Fissure event was a major calamity, and while sulfur fumes are more commonly associated with volcanic events, Iceland seems especially prone to fluorine fumes. The death toll was very high amoung livestock, and about 10,000 Icelanders died mostly of starvation effects. Benjamin Franklin who was in France at the time remarked on the weird weather and blue haze which affected Europe’s climate at the time. He commented that it was probably something volcanic in Iceland. Subsequent European crop failures are thought to have worsened conditions in France helping to promote the French Revolution.
With regard to historical records, I read somewhere (probably in Simon Winchesters book “Krakatoa”) that efforts to determine earlier ages of major eruptions there were using the absence of records as a sign that something catastrophic happened at certain dates. Regarding 540AD, I believe that this was the period referred to by Cassiodorus in his writings. Check Google. There was also famine in China at the same time. I suspect it was more likely volcanic in cause than from a major boloid strike. I think a boloid event of that size would have left physical evidence which I don’t think has been found.