Year	mmol per litre
1980	4.402
1981	4.395
1982	4.388
1983	4.381
1984	4.375
1985	4.368
1986	4.362
1987	4.353
1988	4.344
1989	4.337
1990	4.328
1991	4.321
1992	4.316
1993	4.313
1994	4.311
1995	4.312
1996	4.314
1997	4.316
1998	4.318
1999	4.323
2000	4.325
2001	4.328
2002	4.331
2003	4.337
2004	4.342
2005	4.348
2006	4.356
2007	4.365
2008	4.376