But we're still confused about how what makes a unit. Mark Bennett at the Health Education Authority says that officially a unit equals 10ml of pure alcohol which translates as 3.5 per cent. The HEA say this used to be equivalent to a half-pint of standard strength beer or lager but that nowadays lots of lagers fall outside of this and have much higher percentages of alcohol. Similarly, a unit that equals a glass of wine is based on a small glass of wine that is 9 per cent alcohol. Pub glasses of wine are actually equivalent to 1.5 units in volume and most wine is more like 12 per cent alcohol.
Spirits are much easier - a standard pub measure of spirits equals one unit.
They are increasingly arbitrary. Last year the government asked manufacturers to list their unit strength on the product label. When we drink, alcohol passes from the stomach and intestines into the blood. The liver is reasonably happy when it's dealing with low levels of alcohol,' he says, 'but you really don't want to have any more than one or two units aboard at any one time. If you go over that amount regularly you will damage the liver.