PESTICIDE TOXICITY C Kameswara Rao Foundation for Biotechnology Awareness and Education '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 use 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 |