Year	mmol per litre
1980	4.208
1981	4.201
1982	4.194
1983	4.188
1984	4.187
1985	4.189
1986	4.193
1987	4.199
1988	4.208
1989	4.214
1990	4.218
1991	4.226
1992	4.235
1993	4.245
1994	4.259
1995	4.275
1996	4.289
1997	4.306
1998	4.321
1999	4.336
2000	4.351
2001	4.367
2002	4.385
2003	4.404
2004	4.422
2005	4.44
2006	4.46
2007	4.479
2008	4.499