2,4,6-Trichlorophenol (or 2,4,6-TCP) is a chlorinated phenol that has been used as a fungicide, herbicide, insecticide, antiseptic, defoliant, and glue preservative. It is a yellow solid with a strong, sweet odour. It decomposes on heating to produce toxic and corrosive fumes including hydrogen chloride and chlorine. The technical grade of this substance may contain polychlorinated dibenzodioxins (PCDDs), polychlorinated dibenzofurans (PCDFs), and other contaminants. It is an environmental pollutant that has been found in fresh water lakes such as the Great Lakes. (4)
2,4,6-Trichlorophenol is a cholinesterase or acetylcholinesterase (AChE) inhibitor. A cholinesterase inhibitor (or 'anticholinesterase') suppresses the action of acetylcholinesterase. Because of its essential function, chemicals that interfere with the action of acetylcholinesterase are potent neurotoxins, causing excessive salivation and eye-watering in low doses, followed by muscle spasms and ultimately death. Nerve gases and many substances used in insecticides have been shown to act by binding a serine in the active site of acetylcholine esterase, inhibiting the enzyme completely. Acetylcholine esterase breaks down the neurotransmitter acetylcholine, which is released at nerve and muscle junctions, in order to allow the muscle or organ to relax. The result of acetylcholine esterase inhibition is that acetylcholine builds up and continues to act so that any nerve impulses are continually transmitted and muscle contractions do not stop. Among the most common acetylcholinesterase inhibitors are phosphorus-based compounds, which are designed to bind to the active site of the enzyme. The structural requirements are a phosphorus atom bearing two lipophilic groups, a leaving group (such as a halide or thiocyanate), and a terminal oxygen.
Metabolism
Human studies indicate that sulfation and glucuronidation are the main metabolic pathways of 2,4,6-TCP, which is excreted in the urine as conjugated metabolites. In general, 2,4,6-TCP undergoes biotic isomerization to other trichlorophenol isomers and conjugation with glucuronic acid. Glucuronic acid accounts for approximately 80% of the conjugates detected in urine. Metabolites generated following incubation of 2,4,6-TCP are 2,6-dichloro-1,4-hydroquinone and two isomers of hydroxypentachlorodiphenyl ether. The 2,6-dichloro-1,4-semiquinone free radical was also identified. (5)
2,4,6-Trichlorophenol has been used as a fungicide, herbicide, insecticide, antiseptic, defoliant, and glue preservative. Skin contact while treating wood with the tetrachlorophenols is the most likely route of exposure. Exposure may also occur from drinking water that has been disinfected with chlorine and breathing air contaminated by 2,4,6-trichlorophenol. (5)
Acute exposure to cholinesterase inhibitors can cause a cholinergic crisis characterized by severe nausea/vomiting, salivation, sweating, bradycardia, hypotension, collapse, and convulsions. Increasing muscle weakness is a possibility and may result in death if respiratory muscles are involved. Accumulation of ACh at motor nerves causes overstimulation of nicotinic expression at the neuromuscular junction. When this occurs symptoms such as muscle weakness, fatigue, muscle cramps, fasciculation, and paralysis can be seen. When there is an accumulation of ACh at autonomic ganglia this causes overstimulation of nicotinic expression in the sympathetic system. Symptoms associated with this are hypertension, and hypoglycemia. Overstimulation of nicotinic acetylcholine receptors in the central nervous system, due to accumulation of ACh, results in anxiety, headache, convulsions, ataxia, depression of respiration and circulation, tremor, general weakness, and potentially coma. When there is expression of muscarinic overstimulation due to excess acetylcholine at muscarinic acetylcholine receptors symptoms of visual disturbances, tightness in chest, wheezing due to bronchoconstriction, increased bronchial secretions, increased salivation, lacrimation, sweating, peristalsis, and urination can occur. Certain reproductive effects in fertility, growth, and development for males and females have been linked specifically to organophosphate pesticide exposure. Most of the research on reproductive effects has been conducted on farmers working with pesticides and insecticdes in rural areas. In females menstrual cycle disturbances, longer pregnancies, spontaneous abortions, stillbirths, and some developmental effects in offspring have been linked to organophosphate pesticide exposure. Prenatal exposure has been linked to impaired fetal growth and development. Neurotoxic effects have also been linked to poisoning with OP pesticides causing four neurotoxic effects in humans: cholinergic syndrome, intermediate syndrome, organophosphate-induced delayed polyneuropathy (OPIDP), and chronic organophosphate-induced neuropsychiatric disorder (COPIND). These syndromes result after acute and chronic exposure to OP pesticides.
Symptoms
Inhalation of 2,4,6-trichlorophenol may cause coughing and sore throat. Eye or skin contact causes redness and pain at the site of contact. Convulsions, diarrhoea, dizziness, deadache, shortness of breath, vomiting, weakness, and ataxia may occur after ingestion. (6)
Treatment
If the compound has been ingested, rapid gastric lavage should be performed using 5% sodium bicarbonate. For skin contact, the skin should be washed with soap and water. If the compound has entered the eyes, they should be washed with large quantities of isotonic saline or water. In serious cases, atropine and/or pralidoxime should be administered. Anti-cholinergic drugs work to counteract the effects of excess acetylcholine and reactivate AChE. Atropine can be used as an antidote in conjunction with pralidoxime or other pyridinium oximes (such as trimedoxime or obidoxime), though the use of '-oximes' has been found to be of no benefit, or possibly harmful, in at least two meta-analyses. Atropine is a muscarinic antagonist, and thus blocks the action of acetylcholine peripherally.
