Year	mmol per litre
1980	4.54
1981	4.532
1982	4.522
1983	4.511
1984	4.498
1985	4.486
1986	4.473
1987	4.462
1988	4.453
1989	4.443
1990	4.434
1991	4.427
1992	4.419
1993	4.412
1994	4.4
1995	4.385
1996	4.372
1997	4.364
1998	4.355
1999	4.347
2000	4.34
2001	4.335
2002	4.331
2003	4.33
2004	4.33
2005	4.33
2006	4.331
2007	4.334
2008	4.338