Longevity of a storage medium is measured by how long the data remains readable, assuming compatible devices exist that can read it: that is, how long the disc can be stored until data is lost. Numerous factors affect longevity: composition and quality of the media (recording and substrate layers), humidity and light storage conditions, the quality of the initial recording (which is sometimes a matter of mutual compatibility of media and recorder), etc. According to NIST, "temperature of 64.4 °F (18 °C) and 40% RH [Relative Humidity would be considered suitable for long-term storage. A lower temperature and RH is recommended for extended-term storage."