Year	mmol per litre
1980	4.544
1981	4.532
1982	4.521
1983	4.508
1984	4.492
1985	4.473
1986	4.45
1987	4.431
1988	4.412
1989	4.395
1990	4.374
1991	4.357
1992	4.347
1993	4.338
1994	4.33
1995	4.329
1996	4.33
1997	4.334
1998	4.34
1999	4.346
2000	4.356
2001	4.362
2002	4.366
2003	4.363
2004	4.36
2005	4.364
2006	4.369
2007	4.376
2008	4.384