Selective toxicity can be defined as the property of a toxic agent to harm or eradicate one species without affecting the other even if the two species are in a very close association. Selective toxicity of a chemical agent is evaluated by using selectivity co-efficient which is the ratio between LD50 of the two species concerned:
Chemicals having selectivity co-efficient considerably higher than unity are said to be highly specific or selective in their action. As this value approaches unity the selective action of the toxic agent diminishes, both the species involved are harmed equally.
As a matter of fact, selective action of a toxic agent on different species or organisms forms the very basis of Economic Toxicology. It is the property of a chemical agent to harm one species while inflicting little or nominal damage to other species which is used to protect the desired species against the harmful action of undesired ones.
The selective action of chemical agent may be due to following reasons:
- Due to differences in the process of absorption, distribution and accumulation the toxic agent may attain toxic concentrations in some species while in others it remains well below the toxic level. Thus, only those species are harmed which can absorb more toxic agent.
- The low toxicity of fungicide Captan to plants can be explained by the fungicide’s lower capacity to penetrate the plant cells while in cells of the fungus it readily enters and kills them. Simply, a large surface area as compared to unit weight may at times result in greater absorption of the toxic chemical in some species. Insects for example, have a greater surface area per unit weight as compared to plants or mammals. This causes a number of insects to absorb much more insecticide than plants or mammals do. The effectively of radio-active iodine in the treatment of hyperthyroidism is due to its selective uptake by tissues of the thyroid gland.
- Rapid detoxification and excretion of chemical agent in one species while a slower rate or total absence of these processes in another may result in attainment of toxic levels in the latter. The relatively low toxicity of Organophosphate insecticide Malathion in mammals is due to its decomposition into harmless compounds while in insects this process is much slower and results into a large concentration of the insecticide which becomes toxic to the insect.
- Selective toxicity of many chemicals is due to the fact that they are capable of disturbing certain specific structure or bio-chemical processes which may not be present or are not very significant in the desired species. For example, chemicals acting on Central Nervous System (like most of the insecticides) affect only animals while causing no damage to plants as a nervous system is absent in plants.
Organophosphate insecticides do not harm plants because they act on the process of synaptic transmission of nerve impulses. Herbicides like Carbamate derivatives have little action on animal life as they selectivity inhibit the process of photosynthesis which does not occur in animals. Antibiotics like Penicellin, Cephalosporine etc. inhabit cell wall formation in bacteria due to which they are killed while no adverse effects occur in man in whose body these bacteria live as there are no cells similar to bacteria. Like-wise Bacteria synthesise Folic acid from p-amino-l benzoic acid, glutamic acid and pterdine.
Sulphonamide drugs which resemble p-arainol benzoic acid in both charge and dimensions antagonise the incorporation of p-amino-l benzoic acid and therefore inhabit folic-acid synthesis. It is due to lack of folic acid anil bacteria disappear. The chemical pathway is absent in humans which have to absorb folic I acid from intestines – from the food which we eat. Sulphonamide drugs therefore do not harm humans.
Concept of risk and safety of toxic substances
Risk of toxic substances
Toxic materials can cause serious health effects in an exposed individual. The degree of hazard associated with any toxic material is related to the exact material you are exposed to, concentration of the material, the route into the body and the amount absorbed by the body (the dose). Individual susceptibility of the user also plays a role.
The health effects may occur immediately or the effects may be delayed. Health effects that occur immediately after a single exposure are called acute effects. In other cases, health effects will not occur until some point after the exposure. This is called a chronic effect. A chronic effect may occur hours, days, months or even years after exposure. Generally, acute effects are caused by a single, relatively high exposure. Chronic effects tend to occur over a longer period of time and involve lower exposures (e.g., exposure to a smaller amount over time). Some toxic materials can have both acute and chronic health effects.
Safety of toxic substances
Whenever possible, it is always best to avoid using a toxic material either by eliminating its use (by changing the method or process for example) or by substituting the toxic material with a less hazardous material. Unfortunately, it is not always possible to find a non-toxic substitute that still does the job effectively and safely.
When considering substitution, the first step is to obtain the Material Safety Data Sheets (MSDSs) for all possible substitute materials. Find out about all of the hazards (health, fire, chemical reactivity) of these materials before making any changes. Caution must be exercised so as to avoid introducing a potentially more hazardous situation. Choose the least hazardous materials that can do the job effectively and safely. Learn how to work safely with them, too.
To prevent exposure to a toxic material, control measures are used. Ventilation is a very common control measure for toxic materials. Well-designed and well-maintained ventilation systems remove toxic vapours, fumes, mists or airborne dusts from the workplace before workers are exposed. Removing the contaminated air reduces the hazard of toxic materials.
For the storage of toxic materials, ensure that the storage area is clearly identified with warning signs, is clear of obstructions, and is accessible only to trained and authorized personnel. Before storing toxic materials, inspect all incoming containers to ensure that the containers are undamaged and are properly labelled. Do not accept delivery of defective containers. Also, be sure to store toxic materials in the type of containers recommended by the manufacturer or supplier.
Safe handling and work procedures are crucial for workplaces where individuals use toxic materials. It is vital that people working with hazardous materials such as toxics are properly trained regarding the potential hazards. Remember, if, at any time an individual is unsure or has questions about working with a toxic material, they should always talk with the supervisor.
Waste toxic material must be disposed of properly. Careless disposal of any hazardous waste presents a potential hazard to many individuals who may not be trained or equipped to deal with unexpected hazardous materials (e.g. caretaking staff, garbage collectors, plumbers, water treatment plant workers, firefighters, etc.). Careless disposal can also cause significant damage to the environment.
Control measures such as ventilation, enclosure and work practices are examples of the preferred methods of protecting workers. If these measures are not feasible or unable to provide appropriate worker protection, then personal protective equipment may be required. Some toxic materials can be harmful when in contact with your skin. In these instances, it may be necessary to wear protective equipment such as gloves, aprons, boots, hoods or other clothing, depending on the risk of skin contact. Choose clothing made of materials that resist permeation, penetration or damage by the chemical.
Eye protection is important when working with toxic materials. Selection of the most appropriate type depends on factors such as how the material is used, physical characteristics (e.g. fine powder, liquid, vapour, etc.) and potential health effects (e.g. eye irritant, skin irritant, toxicity through skin absorption, etc.).