CLASSIFICATION OF TOXICANTS

CLASSIFICATION OF TOXICANTS

            Toxic agents are classified in a number of ways depending on the interests and needs of the classifier. Dioscorides classified substances using general char­acteristics i.e., whether they are toxic or therapeutic. An early scheme by Orfila classified substances as being of animal, vegetable or mineral origin. No single classification is applicable for the entire spectrum of toxic agents and combina­tions of classification systems based on other factors may be needed to provide the best rating system. Nevertheless, classification systems that take into ac­count both the chemical and biological properties of the agent, and the exposure characteristics are most likely to be useful for toxicology in general. Many clas­sification schemes for toxic agents are available which are based on following points:

Based on the source of toxicants:

a.                   Plant toxicants, e.g., morphine, curare, strychnine.

b.                  Animal toxicants, e.g. toxins (zootoxins), venoms.

c.                   Mineral toxicants, e.g. copper, lead, selenium, iron.

d.                  Synthetic toxicants, e.g. organophosphates, carbamates, aluminium phosphide.

Based on the Physical state of toxicants:

a.                   Gaseous toxicants, e.g. hydrocyanic acid (HCN), sulphur dioxide, carbon monoxide, phosphine.

b.                  Liquid toxicants, e.g. sulphuric acid, carbon disulphide, nicotine.

c.                   Solid toxicants, e.g. strychnine, opium, atropine.

d.                  Dust toxicants, e.g. asbestos dust, silicon dust, metallic dusts.

Based on the physical characteristics:

            Inflammable / Non-inflammable,

            Explosive / Non-explosive

Based on the physical effects:

            Irritant / Non-irritant

            Corrosive / Non-corrosive

Based on the Target organ/system: It is difficult to classify a toxicant on the basis of its target organ or system as it may affect other systems also. How­ever, action on the primary site has been taken as a basis of classifica­tion.

a.                   Hepatotoxins, e.g. carbon tetrachloride, aflatoxins, phenol.

b.                  Neurotoxins, e.g. organophosphorus insecticides, pyrethroids, anaesthetics, nicotine.

c.                   Nephrotoxins, e.g. heavy metals (lead, arsenic, cadmium), oxa­lates.

d.                  Pulmonotoxicants, e.g. alpha-naphthylthiourea (ANTU), hydrogen sulphide, ammonia gas.  

e.                   Haematotoxins, e.g. warfarin, cyanide, phenothiazine, snake venom (pit viper venom).  

f.                    Dermatotoxicants, e.g. coal tar compounds (petroleum oils), heavy metals (arsenic, mercury), p-tertiarybutyl phenol.

Based on the Chemical nature/structure of toxicants:

a. Inorganic toxicants: These include metals, metalloids, non-metals and their salts and derivatives, acids and alkalies.

i. Metals, e.g., lead, copper, mercury, antimony.

ii. Non-metals, e.g. phosphorus, sulphur, chlorine, nitrate/nitrite.

iii. Acids and alkalies, e.g. hydrochloric acid, sulphuric acid, potassium hydroxide.

b. Organic toxicants: These include all carbon compounds other than the oxides of carbon, the carbonates, and the metallic car­bides and cyanides.

1.                  Hydrocarbons, e.g. cyclopropane, benzene, paraffin, naph­thalene.

2.                  Halogen derivatives of hydrocarbons, e.g. chloroform, BHC, DOT, carbon tetrachloride.

3.                  Alcohols and phenols, e.g. methyl alcohol, ethyl alcohol, phenol, cresol, pentachlorophenol.

4.                  Ethers, e.g. diethyl ether, divinyl ether.

5.                  Aldehydes and ketones, e.g. formaldehyde, paraldehyde.

6.                  Organic acids, e.g. formic acid, phenoxy acetic acid, salicylic acid.

7.                  Esters, e.g. organophosphorus insecticides, succinylcholine.

8.                  Amines, e.g. adrenaline, ephedrine, amphetamine.

9.                  Amides, e.g. phenacetin, sulphonamides.

10.              Glycosides, e.g. digitoxin, cyanogenetic glycosides.

11.              Alkaloids, e.g. nicotine, atropine, strychnine.

12.              Proteins, e.g. ricin, crotin, abrin.

Based on the Analytical behaviour of toxicants: Toxicants can be classified into separate characteristic groups according to the analytical procedures involved. In the Stas-Otto scheme, toxicants have been divided into the following groups.

a. Volatile toxicants, e.g. hydrocyanic acid, alcohols, acetone, phenol, chloral hydrate.

b. Extractive toxicants

i. Toxicants extractable by ether from acid solution, e.g. organic acids,                       nitro compounds.

ii. Toxicants extractable by ether from alkaline solution e.g. alkaloids

iii. Metals and metalloids, e.g.  Copper, mercury, zinc, silver, antimony.

Based on the Toxic effects: It is difficult to classify toxicants on the basis of toxic effects as a single compound may have number of effects.

a. Carcinogens, e.g. thiouracil, vinyl chloride, nickel.

b. Mutagens, e.g. ethyl methane sulphonate, UV light, nitrogen mus­tards, nitroso       compounds.

c. Teratogens, e.g. phenylmercuric acetate, triazines, thalidomide.

d. Clastogens, e.g. UV light, caffeine.

Based on the principal uses of toxicants:

a.Insecticides, e.g. organophosphorus insecticides, carbamates, pyrethroids.

b.Fungicides, e.g. captan, folpet, pentachlorophenol.

c.Herbicides, e.g. triazine, paraquat, 2,4-D.

d. Rodenticides, e.g. warfarin, fluoroacetate, red squill.

e. Food additives:

i.                     Preservatives, e.g. ascorbic acid, sodium bisulphite.

ii.                   Antioxidants, e.g. ascorbic acid.

iii.                 Emulsifying agents, e.g. cholic acid, desoxycholic acid.

iv.                  Colouring agents, e.g. amaranth, tartrazine.

v.                   Anticaking agents, e.g. aluminium-calcium silicate.

Based on the Toxicity/Poisoning potential:

	Group	Lethal dose
a.	Extremely toxic	< 1 mg/kg
b.	Highly toxic	1-50 mg/kg
c.	Moderately toxic	50-500 mg/kg
d.	Slightly toxic	0.5-5 g/kg
e.	Practically non-toxic	5-15 g/kg
f	Relatively harmless	> 15 g/kg.

Based on the Mechanism of action:

a.  Anticholinesterase agents/cholinesterase inhibitors, e.g. organophosphorus         insecticides, carbamates.

b. Sulphydryl (-SH) inhibitors, e.g. mercury, arsenic.

c. Protoplasmic toxicants, e.g. heavy metals.

d. Corrosive toxicants, e.g. caustic alkalies, acids, heavy metals, ir­ritant gases.

e. Methaemoglobin producers, e.g. nitrite

f.  Inhibitors of mixed function oxidases (MFO), e.g. pipronyl butoxide.

g. Inhibitors of Kreb's cycle, e.g. fluoroacetate.

h. Uncoupler of oxidative phosphorylation, e.g. dinitrophenols, chlorophenol            fungicides.

Based on the environmental and human health consideration:

            Air pollutants, water pollutants, radiation hazards, occupational hazards, etc.