Is the measure of the amount of alcohol in a person's blood stream. The legally permissible level of blood alcohol content for driving in Ireland is 80 mg /100ml or 0.08%. Blood alcohol content can be affected by a number of factors:

The same amount of alcohol consumed will result in different blood alcohol levels for different people. The effect on the individual varies considerably so it is impossible and incorrect to give absolute guidelines on the exact amount of alcohol that will put you over the limit. Alcohol is eliminated as soon as it enters the body at a rate of about one unit each hour, but your BAC can still rise after your last drink. Therefore trying to count units or estimate you BAC is very difficult and unreliable. By using a Safer Drive breathalyser a person can have a much more accurate indication on the effect the alcohol has had and whether they are above or below the legal driving limit.

Courtesy of www.saferdrive.ie