The most common and most likely answer to this crossword clue is the 7 letter word MIASMAS
I've seen this crossword clue in The Times
M
I
A
S
M
A
S
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The straight part
We know that "Bad atmospheres" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case bad atmospheres = miasmas.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does MIASMAS mean?
MIASMAS noun- A noxious atmosphere or influence.
- A noxious atmosphere or emanation once thought to originate from swamps and waste, and to cause disease.