Māris kļaviņŠ, azamat azizov, JĀnis zaļoksnis environment, pollution, development: the case of uzbekistan

Figure 6.5. 
Eagles are 
especially susceptible in 
respect to presence of 
pollutants in their food
Concentration, mg/kg
25
2
0.5
0.04
0.000003
Figure 6.4. 
Bioconcentration of DDT 
in aquatic food chain 

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104 
ENVIRONMENT, POLLUTION, DEVELOPMENT:  THE CASE OF UZBEKISTAN
 6.2. 
 Toxicity assesment of substances
The  effect  of  any  substance  or  factor  on  living  organisms 
depends  on  the  nature  of  this  effect  (toxicity)  as  well  as  on  the 
quantities of substances that enter the body. Most substances are 
harmless  if  the  dose  affecting  the  organism  is  sufficiently  low, 
whereas even any well-known and widely used domestic substance 
in high doses may become hazardous to the human organism. Any 
effect on living organisms is dependent not only on the nature of the 
exposure, but also the amount of a substance that enters the body.
Any  substance  is  harmless  if  their  amount  enters  body  in 
mini mal amounts, but, but any in everyday life well-known and 
widely used substance in large doses becomes dangerous. The first 
this concept demonstrated Paracelsus (Fig. 6.6). His well-known 
statement  «All  substances  are  poisons;  there  is  none  that  is  not  a 
poison. The right dose differentiates a poison and a remedy» is a bsic 
for dose-response relationship. 
The amount of substance that a living organism takes in over a 
given period of time is called the dose. 
The assessment of the hazardousness of a substance and risk of 
its exposure is based on the correlation between the dose of the sub-
stan ce and the body’s response. The study of this correlation allows to 
evaluate the toxicity of a substance, for the toxic reaction usually de-
pends on both the dose of the substance under study and the con centra-
tion  of  the  substance  in  the  part  of  the  body  affected  first.  Usually 
exposure to a substance is toxic if there is an interaction bet ween the 
compound and a certain receptor in the body. The receptor might be an 
enzyme whose activity is inhibited. The binding to the receptor can be 
both reversible and irreversible; hence, the du ra tion of exposure to the 
toxic substance is crucial. Although the con centra tion of the substance 
in tissue is directly proportional to the dose of the compound, several 
factors may influence the amount of the substance that actually brings 
about the toxic effect. The ab sorpt ion of the substance, its distribution 
in the body, metabolic pro cesses and discharge mechanisms can affect 
its concentration when molecules reach the receptor.
In order to assess the body’s reaction to the amount of the sub-
stance taken in, other factors and conditions should also be taken into 
account. The key factors affecting the action of the substance are the 
number of doses, the nature of intake (continuous, in specific doses), 
the frequency of intake and the total time of action. Breakdown of 
the amount of the substance into separate doses, as a rule, reduces 
the toxicity of its action because the substance is transformed or 
eliminated from the body as a result of metabolism.
Curves in Figure 6.7. show reactions in the body depending on 
the quantity (dose) of the substance taken in.