Year	mmol per litre
1980	4.094
1981	4.097
1982	4.097
1983	4.097
1984	4.095
1985	4.09
1986	4.092
1987	4.091
1988	4.092
1989	4.089
1990	4.091
1991	4.091
1992	4.093
1993	4.086
1994	4.061
1995	4.052
1996	4.051
1997	4.047
1998	4.046
1999	4.047
2000	4.048
2001	4.051
2002	4.053
2003	4.06
2004	4.071
2005	4.08
2006	4.084
2007	4.089
2008	4.094