A classification used to distinguish between the different rates at which inhaled radionuclides are transferred from the respiratory tract to the blood. • ICRP 71 classifies materials into three lung absorption types: - Type F (fast) are materials that are readily absorbed into the blood; - Type M (moderate) are materials that have intermediate rates of absorption into the blood; - Type S (slow) are materials that are relatively insoluble and are only slowly absorbed into the blood. The lung absorption types supersede the inhalation classes D (days), M (months) and Y (years) recommended by ICRP 30 previously (often referred to informally as 'lung classes'). There is an approximate correspondence between lung absorption type F and inhalation class D, between lung absorption type M and inhalation class M and between lung absorption type S and inhalation class Y. See also: Gut Transfer Factor, A Similar Concept For Ingested Radionuclides In The Gastrointestinal Tract. Main Safety Function: See: Safety Function