Contamination comes from two directions. Dirt, water and debris can enter from outside the wheel-end. Wear particles and other material can also originate inside it. Effective seals address ingress, but once damaging particulate is circulating in the lubricant, sealing alone can't remove it.
Bearing manufacturers describe debris-contaminated lubrication as a significant contributor to premature bearing and gear failure, with lubrication and contamination -including wear particles, maintenance contamination, ineffective sealing and insufficient filtration - among the primary contributors to bearing damage.
