Year	mmol per litre
1980	4.434
1981	4.421
1982	4.411
1983	4.401
1984	4.394
1985	4.389
1986	4.385
1987	4.378
1988	4.374
1989	4.371
1990	4.366
1991	4.362
1992	4.361
1993	4.36
1994	4.361
1995	4.363
1996	4.369
1997	4.377
1998	4.385
1999	4.396
2000	4.41
2001	4.427
2002	4.444
2003	4.461
2004	4.478
2005	4.497
2006	4.516
2007	4.535
2008	4.555