A hermetic seal is a seal which, for practical purposes, is consiered airtight. For example, tin cans are hermetically sealed. The term is often used to describe electronic parts that are designed and intended to secure against the entry of microorganisms and to maintain the safety and quality of their contents. Applications include semiconductor electronics, thermostats, optical devices, and switches. The food, chemical, and pharmaceutical industries all have applications for the use of such "airtight" packaging, such as glass, metals, and high barrier plastics (with effective heat seals). High-end coffins, too, are often made to be "hermetically sealed" and must be of metal or of other material with metal lining, and constructed so that when closed and fastened the coffin is completely airtight. In some nuclear reactor designs, the reactor is housed in a hermetically sealed reactor vessel.