2 pack of replacement polyester filters for the Z1 CPAP machine. One of these polyester filters - not a 2 pack, but just a single filter - is included with each Z1.
The frequency of replacement depends on the conditions of your environment, but a general rule of thumb is to replace this filter every month or two.
The filter end cap on the Z1 must be removed to replace the old filter with a new one. When installing the filter it's important to make sure all the air inlet holes in the end cap are covered by the new filter. When you've covered all the holes, you know you have the filter in the correct position.
This polyester filter 2 pack is an OEM replacement manufactured and distributed by HDM (Human Design Medical), the maker of the Z1 CPAP machine.