Cumulation
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Cumulation: accumulation of a drug or chemical in the body. If a chemical enters the body continuously, its amount in the body increases until the elimination will reach the same rate as the intake; in other words the same amount of chemical is eliminated per unit of time as is entering the body. This is called the steady state. If elimination is very fast, this steady state level is reached quickly, but if elimination is very slow (in other words Half-life is very long), a long time is needed to reach steady state. As a thumb rule, the body burden in a steady state is the daily dose multiplied by 1.5 times half-life (in days), e.g. in the case of PCDD/Fs, about 5000 daily doses. A bathtub with a leaking bottom plug can illustrate this. If the leak is large, pouring water from a tap to the bathtub at a constant rate will raise the water level rapidly to such a relatively low height that as much pours out through the leak as is coming in from the tap. But if the leak is little, water level will rise longer and to a higher level, until the pressure will increase the output of water through the small leak to match the rate of the incoming water. Dioxins and PCBs leak out of the body very slowly, and therefore they keep cumulating even for decades until the elimination rate will finally be as great as the intake rate. The half-time of cumulation (the time during which 50% of steady state level will be reached) is the same as the half-life of elimination of the chemical. The half-life of TCDD is 7 to 8 years. This means that at a constant intake rate the body burden (the total amount of chemical in the body) will reach 50% of steady state in 7-8 years, 75 % in about 15 years and reach the steady state only in 40 to 50 years.
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