Final answer:
Obsolete fire extinguishers refer to units that are either out of date or no longer meet current safety standards, including those containing outdated materials like certain dry-cleaning solvents or foaming plastics.
Step-by-step explanation:
When referring to obsolete fire extinguishers, the term typically applies to fire extinguishers that are either no longer recommended for use, such as those containing certain dry-cleaning solvents, or those that have expired and cannot reliably provide fire suppression anymore. In a laboratory setting, it's crucial to know where specific safety equipment is located, including modern, functional fire extinguisher systems. Fire extinguishers containing foaming plastics or other older technologies may be considered obsolete if they no longer meet current safety standards or if they are no longer manufactured due to upgraded fire suppression technology.