More than half a century after SI units became standard, pockets of resistance to their adoption still persist — at least in medicine (see also Nature 537, 279; 2016).

Many specialists in the radiological disciplines, including myself, still think in terms of the old units — not for diagnostic dosing (becquerels are well established) but for radiation treatment, measured in curies.

And measuring blood pressure in millimetres of mercury (mm Hg) dates back to 1896, when the Italian physician Scipione Riva-Rocci introduced his mercury manometer. This is still common practice worldwide, perhaps because the SI unit of pressure, the pascal, yields readings that are much less user-friendly: 18.665/11.999 kilopascals for systolic/diastolic blood pressure rather than 140/90 mm Hg, for instance.