Humidity and BGA Solder Sphere Sticking or Clumping

I have been applying solder spheres to BGA packages. During the process, the spheres are run over a stencil for placement. The spheres tend to stick to each other (and clump), even away from the stencil pattern, when it is humid in the work area. I'm not completely certain that humidity is the main factor, so I wanted to see if anyone else had experience with this clumping issue and had pinpointed humidity as the root cause. Thank you.

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