30 PPI Filter Foam has larger cells than 45 or 60 PPI foam. This allows for more air to pass through.

45 PPI Filter Foam has larger cells than 60 PPI foam, which allows more air to pass through it than the 60 PPI foam. 45 PPI Filter Foam has smaller cells than the 30 PPI foam, this allows the 45 PPI foam to do a better job at filtering than the 30 PPI foam.