Possible Clues:

• Common reaction to dust
• "Gesundheit!" prompter
• Sound preceding "Bless you"
• "Gesundheit" prompter
• Sound prompting a 'bless you!'
• Reflexive outburst
• Outburst usually acknowledged by others
• What 'Gesundheit!' is a response to
• Allergy season outburst
• Sound preceding 'Gesundheit!'