Lecture
The toxicity of chemical compounds is due to the interaction of the organism, the toxic substance, and the environment. The toxicity of poisonous substances depends on the following factors:
- dose or concentration;
– physical and chemical properties;
- routes and rate of penetration of poisons into the body;
- age and sex;
- individual susceptibility to the poison, and so on.
Dose and concentration. One of the important factors determining
the toxicity of chemical compounds is their dose (concentration).
The therapeutic (curative) dose of a substance is the one that produces a certain therapeutic effect.
The toxic dose of a substance is the one that causes pathological
changes in the body that do not lead to a fatal outcome.
The lethal (fatal) dose of a substance is the one that leads to the death of the organism.
Doses of drugs and poisonous substances are expressed in mass (grams,
milligrams, micrograms), volume (milliliters, drops) units, and in
units of biological activity (IU – international unit).
The action of a substance that has entered the body depends not only on its dose,
but also on the time it remains in the body.
The resorption period lasts from the moment the poison enters the body
to the moment the maximum concentration in the blood is reached.
The elimination period begins from the moment the maximum
concentration of the substance in the blood is reached until it completely disappears from the blood.
Persons over 60 years of age are recommended somewhat lower doses
of drugs than younger persons, since in older
people the processes of metabolism and the rate of elimination from the body of drugs,
poisons, and their metabolites are somewhat slowed.
Children have a lower body mass than adults. Therefore, to achieve a therapeutic effect, children are prescribed relatively smaller doses
of drugs than adults. In addition, children are characterized by age-related features of sensitivity to drugs and poisons.
The toxicity of gaseous substances is characterized not by the dose, expressed
in units of mass or volume, but by concentration. An important parameter of the toxicity of gaseous substances is the MAC (maximum allowable concentration).
For the purpose of sanitary assessment of the air environment and the water of open
bodies of water (rivers, lakes), the MAC of toxic substances is determined in atmospheric
air, in the air of the working area, and in the water of reservoirs.
For gaseous toxic substances, the MAC is expressed in milligrams per
cubic meter (mg/m3), and for toxic substances in water – in milligrams per
liter (mg/L). Based on the results of determining the MAC, methods are developed for
purifying the air of enterprises and the water of reservoirs from contamination by toxic substances.
Physical and chemical properties of toxic substances. The toxicity
of chemical compounds is affected by their state of aggregation, solubility in water and fats,
dissociation into ions, and so on.
Gaseous substances and vapors of volatile liquids that enter the body through the respiratory tract exert a toxic effect considerably faster than liquids
or solids that get onto the skin or pass through the esophagus.
The toxicity of solid substances depends on particle size. Finely ground solid substances are more toxic than the same substances with larger particles. This is explained by the different solubility of small and large particles of the substances and, correspondingly, the unequal rate at which it enters the
blood. The toxicity of chemical compounds depends on their solubility in fats and
water. Fat-soluble substances easily penetrate into the body through the skin and from
blood into cells through membranes. The toxicity of water-soluble substances depends on their dissociation. Thus, barium chloride and barium nitrate dissociate well in water and are highly toxic, whereas barium sulfate does not dissolve in
water and has no toxic effect on the body.
Similar properties are characteristic of certain arsenic compounds. Arsenates that dissociate well in water are highly toxic. Poorly soluble in water
arsenic (III) oxide is less toxic than the arsenites and arsenates of alkali metals.
Water-soluble salts of heavy metals are also more toxic than their oxides.
The action of toxic substances depending on the routes and rate of their entry into the body. The toxicity of certain substances depends on the routes by which they enter the body. The same dose of a poison that has entered the body
by different routes can produce an unequal toxic effect.
For angina, patients are prescribed nitroglycerin in tablets or
in the form of an alcohol solution. Nitroglycerin is well absorbed by the mucous
membrane of the mouth (the sublingual area) and therefore acts quite quickly. The
same amount of nitroglycerin, taken internally, is absorbed more slowly and its action is delayed.
Chemical structure and action of toxic substances. It has been established that the action of many toxic substances depends on their chemical structure. However, the patterns of this dependence for some substances have not yet been established. It has been shown that
the toxicity of chemical substances is due to the presence in their molecules of certain functional groups or double bonds.
Many unsaturated compounds are more toxic than the corresponding
saturated substances of similar composition. Thus, allyl alcohol CH2=CH—CH2OH,
which belongs to unsaturated compounds, is more toxic than the corresponding
saturated propyl alcohol CH3—CH2—CH2OH of similar composition.
Toxic are substances in whose molecules the following groups
of atoms are present: =C=O, =C=C, S=, –N=O, – NO2, and so on.
The toxicity of certain organic substances is due to the introduction into
the composition of their molecules of atoms of chlorine, fluorine, arsenic, mercury, and so on. Certain
groups of atoms (–C=C–, –C6H5, –CH2–, –NH2 and others) contained in the molecules
of toxic substances enhance their toxicity.
The toxicity of chemical compounds depends on their position in
the corresponding homologous series. As molecular mass increases, the toxicity of the homologues increases. For example, propionic acid is more toxic than acetic acid, and butyric acid is more toxic than propionic acid.
Aliphatic alcohols exert a more pronounced toxic effect than their branched-chain isomers. Confirmation of this is the higher toxicity of propyl and butyl alcohols compared with their isomers
(isopropyl and isobutyl alcohols).
Pairs of cyclic hydrocarbons (cyclopropane, cyclobutane, cyclopentane, cyclohexane, and others) are more toxic than the corresponding pairs (by number of carbon atoms) of aliphatic hydrocarbons (propane, butane, pentane,
hexane, and so on).
As the number of carbon atoms in alcohol molecules increases, their toxicity increases.
Comments