call ducklings sneezing?

[kikibon01]

I have four call ducklings about two weeks old, totally completely healthy. Two of them have been sneezing very very often though- it started maybe.. a day or two ago? they have access to clean water that they can dip their entire bills in, and do so often. they are still under a heat lamp, and have wood chips covering the floor. could it just be dust or something? or is it something way more serious- like a respiratory problem? I looked at the ducks thoroughly, none of them have runny noses, or any other symptoms. thanks in advance!