Filters trap air particles as the wearer breathes in, protecting the wearer from particulates, but not gases or vapors. The most common type of filter is a “pancake filter,” which is round and flat. Filters receive the same letter and number combination that particulate respirators receive. N designates non-oil-based particles, while R means oil resistant, and R means oil-proof. The number is the filter efficiency rating. For example, N95 respirators protect against 95% of non-oil-based particles, while P100 filters protect against 99.9% of oil-based particles. Cartridges absorb hazardous vapors from the air. There are two types of cartridges: gas and vapor cartridges and combination cartridges. Gas and vapor cartridges are designed to protect against a specific gas or vapor. Combination cartridges protect against particulates as well as gases and vapors.