The E1 filter is a type of replaceable gas filter for respirators and masks, which provides protection against specific chemical substances. The letter "E" in the name indicates that the filter is effective against acidic gases and vapors, such as sulfur dioxide or hydrogen chloride. The number "1" specifies the filter capacity class, which in this case is low. This means that the filter is designed for work environments with a low concentration of these harmful substances, specifically up to a volume concentration of 0.1%.