Year	mmol per litre
1980	4.368
1981	4.354
1982	4.34
1983	4.324
1984	4.313
1985	4.304
1986	4.296
1987	4.286
1988	4.276
1989	4.267
1990	4.257
1991	4.249
1992	4.245
1993	4.246
1994	4.251
1995	4.254
1996	4.26
1997	4.265
1998	4.27
1999	4.272
2000	4.267
2001	4.261
2002	4.253
2003	4.241
2004	4.228
2005	4.215
2006	4.201
2007	4.187
2008	4.173