As particle filters become loaded with contaminants, the passage of particles becomes increasingly restricted, resulting in more intensive filtration. After some time of use, the filters become clogged, making it more difficult to breathe, as the user needs to exert more effort. The timing of this varies depending on the amount of particles in the inhaled air. Gas and vapor filters begin to allow contaminants to pass through when their absorbents become saturated. The useful life is not indicated due to changes in respiratory resistance. Instead, filters should be replaced according to local regulations; or if any irritation occurs due to the contaminant. If odor or taste is noticed and/or when the user coughs or feels discomfort, it means the filters are not being replaced frequently enough.