The pre filter is designed as a barrier, to capture the larger dust and particulate, protecting the HEPA filter from premature blocking. The efficiency of the pre filter is directly related to the life of the HEPA in that the more particulates captured in it, the less can pass through and prematurely block the HEPA. Most pre filters have an efficiency grading of F5 - F7.

Donaldson BOFA's combined filters are a combination of a HEPA filter and a chemical/gas filter coupled together to create a more compact, cheaper and more effective solution. This joint filter method extends across most of the BOFA products creating a safer, cleaner system.