Small white looking dust when refurbishing

Hi guys,

Wonder if any of you have had the same issue as below.

So while refurbishing iPhone screens 9/10 times I get 1 or 2 very small white looking dust when putting the 3 in 1 frames on.

I use a mini dust free room and a humidifier but almost always get this issue. I assume it’s dust anyways. If anyone has had this issue what can be done to prevent this?

I have attached some pictures as well. Thanks in advance.