Metals Essential Zinc Copper Iron Chromium Phosphorus Selenium Cobalt All metals are toxic in excess Neutral or detrimental Tin Bismuth Boron Barium Antimony Arsenic Thallium Lead ID: 247542
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Slide1
Metal-Containing PesticidesSlide2
Metals
Essential*
Zinc
CopperIronChromiumPhosphorusSeleniumCobalt*All metals are toxic in excess.
Neutral or detrimental
Tin
Bismuth
Boron
Barium
Antimony
*Arsenic
*Thallium
*Lead
*Mercury
* Designates invariably harmful metalsSlide3
General Considerations
Elements do not degrade
Environmental accumulation is a significant risk of continuing use
As in soil of old orchards?Bioaccumulation may occur if metal is in a form that can be taken up by plantsMost metals are excluded by most plantsCadmium is taken up by some plantsEnvironmental presence in soil implies risk of transport into waterMovement from soil to air, air to soil also occursEnvironmental transformations may occur
Hg ---> MeHg
Toxicology
For most metal-containing pesticides, the toxicity is due to the metal
Example:
As-containing herbicide, fungicide will have similar toxicities despite very different structures
Exception:
Dithiocarbamate fungicidesSlide4
Barium
Barium carbonate: BaCO
3
Rodenticide20-25% active ingredient in baitsNeurotoxicantHuman poisoning?1 case of mass poisoningSubstituted for flour in pastrySlide5
Chromium
Sodium dichromate: Na
2
Cr2O7•2H2O Cotton defoliantWood preservativeToxicity:Bronchiogenic cancer suspectedMay require inhalationChromium is also an essential elementSlide6
Copper
Copper sulfate: CuSO
4
Also known as blue vitriolFungicide, algicideBordeaux mixture: CuSO4 and Ca(OH)2 Fungicide and seed treatmentVery alkalineToxicology
Acute:
Renal failure
Chronic:
Fatty degeneration of kidneys, liver
Pneumoconiosis from occupational exposure to Bordeaux mixture
Granulomas of lung, liver , kidneySlide7
Zinc
Zinc chloride: ZnCl
2
Wood preservativeLD50 : po, rat: 2,200 to 19,000 mg/kg Ip, mouse: 31 mg/kgZinc compounds tend to be emeticZinc phosphide: Zn3P2 RodenticideToxicity is due to production of phosphine (PH3
) by stomach acid
Phosphine causes pulmonary and cerebral edema
:Slide8
Cadmium
No known biological function
Cadmium chloride: CdCl
2 Turf fungicideToxicityAcuteSalivation, diarrhea, vomitingChronicPoor growth, anemia, kidney damageItai-itai from dietary exposure
May be aggravated by Ca and/or vitamin D deficiencies
High blood pressure?
Accumulates in kidneys
Human poisoning with CdCl
2
has not been reportedSlide9
Mercury
Toxicology
Major concern for
all forms is chronic toxicityElemental Hg:Little absorption through GI tractNot absorbed through skinVapors are primary dangerDyspnea, fatigue, gingivitis, loss of teeth, tremors, irritabilityDepression, despondency, lethargy, weakness, coma, deathInorganic Hg:Mercuric chloride: HgCl2 FungicideLD
50
37 mg/kg, po, ratSlide10
Organic Mercury Compounds
1915:
Phenylmercury salts
HgC6H6 1929: Alkylmercury saltsMethylmercury, HgCH3 LD50 ca 30 mg/kgNOAEL = 0.5 mg/kg/day for 150 days1960s-1970s - Hg-containing pesticides banned by most countriesSlide11
Organic Mercury Compounds
Toxicology
Absorbed through skin, GI tract
Crosses blood/brain barrierLittle excretion from CNSNeurological damage is 1st symptomCNSTremors, loss of peripheral vision, irritability progressing to dementiaSome improvement if exposure endsMost damage is permanent
Pregnant women excrete Hg to fetus
Congenital Minamata disease
No repair possible
Arylmercury compounds
Acrodynia or
“
pink disease
”Slide12
Thallium
Rodenticide
Home uses banned in US in 1975
ToxicologyCrosses skin, GI tractMechanism of action not well understoodInterferes with intracellular K?Thallium sulfate, Tl2SO4LD50 10.6 mg/kg, rat po
1/50 of LD
50
per day:
Depilation in 4 months
Death in 6 monthsSlide13
Arsenic
Insecticides
Paris Green
Copper acetoarsenite Cu(C2H3OO)2 •3Cu(AsO2)used against Colorado potato beetle, 1865Scheele’s Green
Copper arsenite, CuHAsO
NaAsO
2
still used in poison bait against grasshoppers
PbHAsO
4
used on apples against codling moths
Used against gypsy moth
38,000,000 kg in 1942
LD
50
in rats, po: 800 mg/kg
Herbicide
Cacodylic acid: (CH
3
)
2
AsO(OH)
LD
50
in rats, po: 830 mg/kg
Fungicide
Chromated copper arsenate
wood treatment
introduced 1940s
phased out 2003
12x2x6
”
board contains 27 g arsenic
5 Tbs ash contains enough As to kill a 1,100 lb cow
tastes saltySlide14
Human Toxicity of Arsenic
Acute toxicity
GI tract symptoms
Cumulative toxicity (rats)1.6 mg/kg/day is NOAEL of arsenites3.2 mg/kg/day is NOAEL of arsenates MechanismIncreases permeability of capillariesDecreases blood pressure ---> shock, death
Skin lesions
Also used therapeutically to clear skin
Polyneuropathy
Cancer
Skin
Following ingestion, not topical application
Lung
Following inhalation only?
Evidence is epidemiological
People living downwind from copper smeltersSlide15
Tin
Inorganic tin does not enter body
Organic tin compounds
RSnX3 RSn2X2 RSn3X
RSn
4
X
X =
Simple ion
Complex ion
R =
Alkyl
Aryl
Trifentin
acetate
Introduced 1954 as fungicide
Also used as
molluscicide
,
algicide
Bioaccumulates
in harbor ecosystems
Neurotoxic
Banned for environmental reasons
Trifentin acetateSlide16
Summary
All metals are toxic in excess
As, Hg, Cd, Pb
have no useful function in living organismsToxicity of metal-containing pesticides is usually that of the metalExceptions:Dithiocarbamate fungicidesZinc phosphideSlide17
Dithiocarbamate Fungicides
Ethylenebisdithiocarbamates
Maneb
NabamZinebDimethyldithiocarbamatesFerbam
ZiramSlide18
Ethylenebisdithiocarbamates
Zineb:
Acute toxicityLD50 porats: 1,850-8,900 mg/kgMice: 7,600-8,200 mg/kgRabbits: 4,450 mg/kgManebAcute toxicityLD50 po > 5,000 mg/kg in rats and mice
M
M = Zn, Zineb
= Mn, Maneb
= Na, Nabam
= Zn+Mn, MancozebSlide19
Ethylenebisdisdithiocarbamate
fungicides:
an alternative structural formSlide20
Dimethyldithiocarbamates
Ziram
LD
50 porats: 1400 mg/kgMice: 480 mg/kgGuinea pigs: 100-150 mg/kgSlide21
Common Features of Dithiocarbamates
Acute toxicity is low
Depress thyroid function
Common metabolite:EthylenethioureaInteract with alcohol consumptionBlocks oxidation of EtOH at acetaldehyde stepAcetaldehyde accumulatesDisulfiram (Antabuse) Used to prevent EtOH consumption by alcoholics
DisulfiramSlide22
ETHYLENETHIOUREASlide23
Developmental toxicity of
ethylenethiourea
Severe developmental toxicity in rats
Craniofacial malformationsLittle activity in miceSimilar pharmacokinetics in both speciesSlide24
Summary
Toxicity of most metal-containing pesticides depends on the metal
Toxicity of dithiocarbamates does
not depend on the identity of the metalDithiocarbamates areThyrotoxicSuspect carcinogensTheir metabolite, ethylenethiourea,Is a developmental toxic in rats but not in miceMany dithiocarbamates were voluntarily withdrawn by their manufacturers rather than carrying out currently required safety testing