Here is the sentence:
Sneezing from the high pollen count, a clean handkerchief would come in
handy.
Good choice! You recognized that the modifier is dangling. Sneezing from the high pollen count, a participle phrase, has no target in the sentence to describe.
To fix the problem, you could revise the sentence like this: Because Gerald was sneezing from the high pollen count, a clean handkerchief would have come in handy. Adding Gerald to the sentence lets the reader know who is suffering without a way to wipe his nose!