Organophosphate

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Organophosphates (OPs) are chemically esters of phosphoric acid. Many are used as insecticides (killing insects), anthelmintics (killing parasitic worms), and nerve agents (killing human beings). They also have other uses in chemical engineering and manufacture, but it is their use in agriculture that is particularly controversial today. Health risks from direct exposure are now well-known, particularly for agricultural workers, but there are also worries about safe levels in food and the effects of aerial spraying. They have been implicated in a variety of public health and environmental issues from ADHD to colony collapse disorder.

The good news is that OPs are often used to combat insects which act as disease vectors, such as mosquitos, as well as to combat agricultural pests. They became popular as a safer and less environmentally damaging alternative to organochlorine insecticides like DDT at a time when there were concerns about DDT poisoning the environment.

Organophosphate pesticides go by names such as parathion, malathion, methyl parathion, chlorpyrifos, diazinon, dichlorvos, phosmet, fenitrothion, tetrachlorvinphos, azamethiphos, azinphos-methyl, and naled. Other OPs include trichlorfon aka Metrifonate (formerly used to treat helminth infections), and echothiophate and isoflurophate (used in opthalmology to treat glaucoma).

How they work

Organophosphates work by destroying nerve function in insects, humans, and other things you want to kill. Specifically, they deactivate acetylcholinesterase (ACHE or AChE), which catalyses the breakdown of certain neurotransmitters and thereby terminates synaptic transmissions. Deactivation of ACHE causes an accumulation of acetylcholine (ACh), a chemical which carries stimulating signals across synapses between neurones. An accumulation of ACh means that instead of the normal rapid pattern of stimulation and destimulation of neurones that allows rapid messages to be sent, there is overstimulation and signalling between neurones is knocked out of balance.

In humans this results in symptoms including anxiety, headaches, convulsions, ataxia, lowered respiration and heart rate, general weakness, diarrhoeia, vomiting, visual disturbance, bronchoconstriction, bronchorrhea, convulsions, seizures, paralysis, and coma. The end result can be respiratory failure and death. They are also associated with psychological effects such as anxiety, memory impairment, irritability, and psychosis.

OPs can enter the body through ingestion, inhalation, or contact with skin or open wounds. Treatment is limited, using drugs such as atrophine, which blocks the action of ACh. However the efficacy is not great and even with treatment permanent brain damage can result. There is no vaccine.

Risk to humans

General

There is evidence that they are harmful to brain development in fetuses, babies, and young children, which poses a particular risk to children growing up in agricultural environments where OPs are used. Research on animals suggests it is plausible that OP exposure may cause respiratory problems in children. Tests on rats suggest parathion is particularly dangerous. Another study found that exposure to chlorpyrifos, formerly used for indoor pest control, resulted in lower IQs in children in New York City. However a 2013 study looking at levels of OP metabolites found no evidence of negative effects on neurobehaviour from low-level exposure (it also found that the highest levels of metabolites were in well-educated, white, married women, who presumably ate the most vegetables).

Some OPs may cause cancer. The International Agency for Research on Cancer (IARC) rates malathion and diazinon as "probably carcinogenic to humans" (Group 2A) and tetrachlorvinphos and parathion as "possibly carcinogenic to humans" (Group 2B - which is actually not a classification that requires a lot of hard evidence of risk). The US Environmental Protection Agency also says parathion may be a carcinogen.

Because of the risk of domestic poisoning, the EPA has called for their elimination in pesticides for domestic use, though there are no plans to end their use in agriculture.

There have been a number of cases of multiple deaths resulting from acute OP poisoning: 20 people in Sierra Leone died in 1986 after eating bread contaminated by OPs during transit; 25 children died in India in 2013 from eating free school meels apparently with dangerously high levels of OPs. Because they are toxic they are also sometimes used for suicide.

Occupational risks

Many people involved in agriculture, sheep dipping, and pest control work with OPs, and this has produced concerns about health risks. Symptoms of chronic occupational exposure include "difficulties in executive functions, psychomotor speed, verbal, memory, attention, processing speed, visual-spatial functioning, and coordination". Working with OPs is associated with an increased risk of Alzheimer's.

Davies et al describe an Irish farmer suffering from chronic exposure: "His mood became markedly unstable with brief swings into both tearful depression and extreme irritability. These periods generally lasted for hours only, although against a background from time to time of more protracted periods of low-grade depression of mood. He also experienced several episodes of impulsive suicidal thinking and on one occasion 'came to' in his yard holding a loaded shotgun to his mouth." He also suffered from memory problems, hypersensitivity to perfumes, deteriorating handwriting, and other difficulties with muscular activity; he later committed suicide.

Residue in food

The chief advantage of OPs over other pesticides is that they degrade rapidly in the environment, which means that soon after use they break down into harmless compounds, and thus won't hang around forever to kill your children and your children's children.

But although they degrade, they do not disappear completely, and small amounts are permitted as residue on crops. Levels from residue on food are far less than the amount a farmer will be exposed to during food production.

Research has linked OP exposure from pesticides to reduced IQs in children. Foods particularly at risk include green beans (snap beans), watermelon, and raw tomatoes, with toddlers particularly at risk - and even though some developed nations impose tight limits on OP usage, many foods are imported from countries with less strict regulations.

Organophosphates are prohibited by organic farming standards.

ADHD

OPs act on acetylcholinesterase in the nervous system, and acetylcholinesterase is closely related to processes implicated in ADHD.

A 2010 study showed that levels in children's urine of dialkyl phosphates, a breakdown product of OPs, correlated with ADHD. The study concluded: "These findings support the hypothesis that organophosphate exposure, at levels common among US children, may contribute to ADHD prevalence. Prospective studies are needed to establish whether this association is causal."

Nerve agents

Because of how they work, OPs can also be used as a chemical weapon, uses pioneered by the Germans and British before and during World War Two. Sarin, Tabun, Soman, and VX are examples of OP nerve agents (the first 3 are German, VX is Britain's contribution).

OP nerve agents were used by Saddam Hussein in the 1980s and in the Syrian civil war in the 2010s. Sarin was also used by Aum Shinrikyo in their 1995 Tokyo subway attack.

Environmental risk

Many OPs are highly toxic to birds, fish, and bees.

Although another group of pesticides, nicotinoids, is most strongly associated with colony collapse disorder (dead bees), some OP pesticides are also toxic to bees. Ironically, nicotinoids were introduced as replacements to OPs because they were believed to be less toxic to wildlife. But as yet there's no evidence to implicate OPs in the recent decline in bee populations.

Zika

This issue has reached new levels of controversy with the use of organophosphate spraying to combat the Zika virus, particularly when done in the USA (other countries with greater prevalence of mosquito-borne diseases face different cost-benefit trade-offs). For this purpose the American CDC in some cases recommends aerial spraying with temephos against larval mosquitos and chlorpyrifos, malathion, and naled against adult mosquitos, as part of a strategy that also involves physically removing breeding sites (stagnant water) and testing for insecticide resistance prior to use. Aedes aegypti, the mosquito which spreads Zika and many other diseases, is hard to combat because it can breed in almost any small pool or vessel of stagnant water, so targetting waterways from the ground is ineffective.

In Florida, spraying has involved a variety of pesticides including the organophosphate naled. Inevitably critics of organophosphates have been getting very angry, using studies of risks from sustained agricultural pesticide use to argue against spraying against mosquitos, despite the considerable differences in pesticide levels and length of exposure (remember, organophosphates break down quickly after use), and without any attempt to compare risks from OPs against risks from Zika. The EPA states that there is no danger from naled spraying even if it is used multiple times and a toddler eats some grass or soil afterwards. On the other hand, it's by no means clear that Zika is a significant public health hazard in the USA because Americans spend a lot of time indoors and public health officials tend to clamp down quickly on any outbreaks that do occur, so regardless of the danger of organophosphates, widespread spraying may not be necessary.

Conclusion

Any pesticide is likely to be somewhat toxic, because that's what they do. Pesticides are necessary when growing vegetables, even in organic farming where supposedly more wholesome organic pesticides are used. Eating vegetables is good for you and for the vast number of livestock who depend on fodder crops for most of their diet. Killing insects which spread diseases is also a good thing. So balance is required. And today, OPs are increasingly being taken off the market and replaced by nicotinoids, which appear less damaging to the environment although still not entirely benign: pesticide manufacturers and agronomists are always looking for better pesticides.

The evidence seems clear that exposure to high level of organophosphates is dangerous, particularly over a long period of time: this can occur in agriculture, when using them as domestic pesticides, or where poor regulation means large amounts are present in food. There is not sufficient evidence to say that there is a risk from exposure to the very small levels found in food grown in those countries that control the use of organophosphates and impose limits on residue, although the single study showing a link to ADHD needs further investigation.