Visitors cause sneezing

A bit of a weird one but here goes:

Every time we have visitors, my wife will start sneezing. No issues when we visit people in their houses, no problems at work or if we meet up with someone at a restaurant.

Any suggestions? We’ve ruled out perfume, detergents and things like that since the sneezing would then also happen when we go somewhere. But it only happens at our place and it doesn’t matter who visits.

Thanks in advance!