A high efficiency particulate air or HEPA (pronounced /ˈhɛpə/) filter is a type of high-efficiency air filter. HEPA filters, by definition, remove at least 99.97% of airborne particles 0.3 micrometers (µm) in diameter. Diffusion predominates below the 0.1 μm diameter particle size. Impaction and interception predominate above 0.4 μm. In between, near the 0.3 μm MPPS, diffusion and interception predominate. The filter resistance to flow is known as its pressure drop.