Air is purified by the main P3 HEPA filter that operates at 99.97% efficiency. However, nothing lasts forever. To keep you COVID confident we suggest you replace the filter every month. The life of the filter 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. The PAPR also has pre-filters that protect the main filter from large particles being trapped within it. We suggest you replace the pre-filter when you replace the main filter to ensure optimum filtration.