Systems and methods for polishing a substrate with reduced contamination are described. Moist air is directed to one or more surfaces in proximity to the polishing surface and exposed to airborne slurry particles generated during polishing. By maintaining the atmosphere in the vicinity of the exposed surfaces at an elevated relative humidity level, airborne slurry particles adhering to the exposed surfaces remain in suspension and, therefore, may be easily cleaned, e.g., during a high pressure rinse cycle. This feature reduces the likelihood that slurry particles will accumulate on exposed surfaces of the polishing apparatus and flake off while a substrate is being polished, reducing the likelihood of substrate defects caused by such slurry contamination.