Ammonia fumes from soiled bedding can be an issue. Proper ventilation and removing soiled bedding will help eliminate the sneezing. Using DE can be a problem, pesticides, dander in bedding, feed dust, mold spores, pollen can also be problems.
If the sneezing persists; on a warm day thoroughly clean out the coop and nest boxes. Then spray activated oxine inside the coop. Let it dry and add fresh bedding. If you do this, start early in the morning.
Then using a spray bottle set on a fine mist, mix unactivated oxine with water and mist over the heads of your chickens 3 times a day for 10 days.
Here's a link regarding using oxine. Please read all of it. Revival AH sells it:
https://www.shagbarkbantams.com/the-many-uses-of-oxine-ah-animal-health/