Compression Wear Ratings: What Does mmHg Mean?
If you've ever tried to buy or even looked into buying compression clothing or supports, you've probably come across "mmHg". This usually follows a series of numbers and looks a lot like a computer error message, but is actually how the strength of compression is measured.
Millimetres of Mercury
Compression clothing is usually measured in millimetres of Mercury, hence the mmHg - Hg is the chemical symbol for mercury and mm means millimetres. This came from the extra pressure generated by a 1mm high column of mercury, although this method of measuring pressure isn't used anymore because of the different variables that can affect the mercury it is still applied in compression wear.
1 millimetre of mercury is most easily understood as 1/760th of an atmosphere of pressure.
Why Is mmHg Used?
While in science the Pascal is the pressure measurement of choice (1 mmHg is 133.322367415 pascals, if you were curious), millimetres of mercury is still the most widely used metric for pressure in medicine. Both blood pressure and intracranial pressure (the pressure inside the skull) are measured in mmHg, for example.
Because of its use in medicine it became the standardised unit for measuring the levels of compression in compression stockings.
While the millimetre of mercury is the standardised unit for measuring the strength of compression clothing, there isn't a universal classification for levels of compression. The two most common classifications are Afnor and RAL.
The Afnor standard comes from the AFNOR organisation (Association Française de Normalisation), the French body for standardisation and normalisation. The Afnor classes are:
Class 1: 10 - 15 mmHg (Very Light Compression for Fitting)
Class 2: 15 - 20 mmHg (Light Compression for Active Use)
Class 3: 20 - 36 mmHg (Moderate Compression for Active Use & Recovery)
Class 4: 36+ mmHg (Strong compression Medical Grade)
The RAL standard comes from Germany, and is based on an attempt by European Union to create an EU-wide standard classification system for compression. The RAL classes are:
Class 1: 18 - 21 mmHg (Light compression)
Class 2: 23 - 32 mmHg (Moderate compression)
Class 3: 34 - 46 mmHg (Strong compression)
Class 4: 49+ mmHg (Very strong compression)
The UK also has its own BSI standard from the British Standards Institute. The BSI classes are: