Pesticide Toxicity
Prof. C Kameswara Rao
Foundation for Biotechnology Awareness and Education,
Bangalore, India krao@vsnl.com, www.fbae.org, www.fbaeblog.org

'Pesticide' is a collective term for natural or synthetic substances, which inhibit or kill insects (insecticides), nematodes (nematicides), snails (molluscicides), bacteria (bactericides), fungi (fungicides) and weeds (herbicides).  Nevertheless, most often the term is used as synonymous with insecticides.   While there are several insecticides of biological origin, such as the products of the bacterium Bacillus thuringiensis, the neem tree and others, control of crop insect pests has been dominated by synthetic insecticides for over half a century. Some insecticides may also be bactericidal or fungicidal. 

There are more than 1100 officially recognized synthetic organic (that contain carbon) insecticides, which belong to a vast array of chemical groups, the more important being, a) the Organophosphates (OPs), with subgroups like organothiophosphates, derived from phosphoric acid, and b) Organochlorines (OCs), derived from chlorine.   Carbamates and Pyrethroids are some of the other groups.  Insecticides are referred by their common names, derived from the cumbersome scientific names.  The trade uses brand names, with the result the same insecticide may bear several trade names. 

OPs and carbamates constitute over 70 per cent of all synthetic insecticides. The OPs are also a component of herbicides and even nerve gasses such as sarin.    Disulfon, ethion, malathion, chlorpyrifos, and quanolfos are OPs, while DDT, lindane, methoxychlor, pentachlorphenol, camphechlor, endrine, endosulfon and imidacloprid are OCs.    Thiodicarb is a carbamate and fenvalerate is a pyrethroid ester. 

Chlorpyrifos, quinolfos, endosulfan, endrine, thiodicarb and fenvalerate are commonly used on the cotton crop.   Imidacloprid (trade name Gaucho) is routinely used as cotton seed dressing.

Insecticides are not crop pest specific.  They pose a serious risk to farm labour and farm animals, and most other animals in the environment that feed on insecticide sprayed crops, all of which suffer from insecticide poisoning, by dermal contact, inhalation or ingestion.   The lack of target specificity and an indiscriminate and excessive insecticide application has disturbed large components of biodiversity of agricultural lands.  

Most insecticides are not completely degraded and so leave residues in the food, feed, other agricultural produce, soil and water.   Insecticide (pesticide) residues are poisonous, when the food or feed from insecticide sprayed crops was consumed, without proper cleaning.  Insecticides and their residues also contaminate soil and water.   In the process the natural food chain gets contaminated.

The nature and intensity of insecticidal toxicity depends upon the chemical structure of the insecticide and the mode of its action and not on whether it is natural or synthetic.  The level of exposure and the concentration of the insecticide or its residue in the body are critical factors.

There are two states of insecticide poisoning, both of which are equally dangerous: the Acute State is when a single very high dose that may be lethal was administered and the Chronic State is one in which several sub-lethal doses were consumed over a long period of time, resulting in high concentrations in the body.  Some insecticides may accumulate in the body and cumulatively reach hazardous concentrations.  Chronic situations have indicated that about 60 per cent of the synthetic insecticides may cause cancer or Parkinson’s disease. 

Toxicity of insecticides manifests in several different ways.   OPs and carbamates are very potent neurotoxins that inhibit the enzyme choline esterase, leading to the loss of control of acetylcholine, the most important neurotransmitter.  The OPs are degraded rapidly on exposure to light, air, soil and water. 

The symptoms of OP poisoning are running nose, chest tightness, shortness of breath, sweating, nausea, vomiting, stomach cramps, muscle twitching, confusion, seizers, paralysis, and coma, which lead to death.  OPs can also cause the Mad Cow Disease. 

The symptoms of OC poisoning are tremors, headache, dermal irritation, respiratory problems, dizziness, nausea, and seizures.  The may also cause neurological and respiratory illnesses and are implicated in cancer.  The OCs are more persistent in nature, but are relatively less toxic than the OPs.

The wide use of synthetic insecticides and the common occurrence of illness and death due to exposure to them, have lead to very extensive and critical studies on antidotes and clinical treatment.   Studies on the safety of chemicals are conducted on rats since human subjects cannot be used. The critical concentrations are estimated as milligrams per kg of body weight, that would kill 50 per cent of the rats in the study group, a factor called LD50 (Lethal Dose Fifty) with an alternative measure LC50 (Lethal Concentration Fifty).  Chemicals with LD50 of less than five mg/kg are super toxic, while those that have values above 15,000 are practically non-toxic.  For insecticides the LD50 values range from 13 mg/kg (parathion) to 1375 mg/kg (malathion), which means that the former is extremely toxic and the latter is mildly toxic.  The LD50 values depend upon so many factors and so there is actually no standard.   Though such results cannot be directly extrapolated to humans, they are the only means of evaluating toxicity potential.   In general, children and small-bodied people (and animals) are at a greater risk.   

The farm workers and farm animals that are in direct contact with insecticides should be periodically tested for toxic concentrations, since it is not possible to estimate insecticide concentrations in every one at risk.   Farm animals should not be allowed to graze on crop stubble that was exposed to extensive pesticide application.  The local medical, veterinary and agricultural professionals are expected to be familiar with the safe use insecticides and treatment of people and animals that were affected, but more often than not, no such professional support is available.  The levels of awareness among consumers are poor and illness from toxicity of insecticides and their residues often goes unrecognized. 

July 13, 2007