Possible Clues:

• Explosion somewhere under the bridge?
• 'Hellish dark, and smells of -!' (R. S. Surtees)
• Act of sternutation
• Sign of a cold or hay fever
• Reason to be blessed
• By the sound of it, one knows where this blow comes from
• Blow cold down your nose
• What a blow that it could be so cold
• An outbreak of germs one may even bless
• Sudden involuntary expulsion of air