Year	mmol per litre
1980	4.519
1981	4.504
1982	4.478
1983	4.448
1984	4.413
1985	4.381
1986	4.351
1987	4.329
1988	4.308
1989	4.284
1990	4.263
1991	4.245
1992	4.234
1993	4.228
1994	4.223
1995	4.219
1996	4.218
1997	4.215
1998	4.212
1999	4.213
2000	4.2
2001	4.184
2002	4.173
2003	4.162
2004	4.156
2005	4.138
2006	4.123
2007	4.112
2008	4.101