I was in the last foul ‘pea-soup’ fog in London, in 1963. It was thick dark yellowish toxic brown. It took life.
Several British monarchs grappled with the effects of burning coal in London, beginning with King Edward I, 14th century, right through to several governments under Queen Elizabeth II, 20th/21st century.
After almost 700 years of frequent, deadly, poisoned air, the answer became apparent.
Guess what it was: if your answer is correct, you win a buy-one-get-one-free consultation. Contact for details.
Here’s a clue: when they stopped burning a certain substance, pretty soon people could breathe again.