Year	mmol per litre
1980	4.371
1981	4.363
1982	4.356
1983	4.349
1984	4.344
1985	4.336
1986	4.327
1987	4.318
1988	4.311
1989	4.308
1990	4.3
1991	4.293
1992	4.287
1993	4.278
1994	4.268
1995	4.256
1996	4.239
1997	4.225
1998	4.212
1999	4.202
2000	4.191
2001	4.18
2002	4.17
2003	4.162
2004	4.155
2005	4.147
2006	4.142
2007	4.137
2008	4.132