The prefilter is designed to protect your P3 HEPA main filter from contamination with large particles. It extends the life of your main filter and should be changed regularly. The filters come in a pack of 10 and are designed to be fitted easily on top of the main filter in the power unit. To keep you COVID confident we suggest you replace the prefilter every week. The life of the prefilter is dependent on the environment you are using your PAPR in and the quality of the air in that environment the unit needs to filter. An audible alarm tells you when air flow is compromised and this is usually an indication to replace the filter. You should always replace the prefilter when you replace the main filter to ensure optimum filtering efficiency.