UCUM is used widely in eHealth in the UK and internationally (HL7 and DICOM). It has been specifically developed to meet the requirements of machine processable representation of units in a form that is also convenient for human readers.
UCUM uses a limited characterset, and a simple formal syntax for expressing units in a way that is familiar easily understood by humans, and unambiguous for systems.
There is a formal sytax and defined set of terminal (atomic) units ensuring that the vast majority of units can be directly expressed. There is robust governance on the specification so the set can be extended as new requirements are identified, and a mechanism for indicating where locally defined units have been used where appropriate.