The disruption of olfaction and potential effects to survival and reproductive success at environmentally-relevant concentrations metals, pesticides or surfactants have implications for fish and salmon recovery because these are commonly found in western United States streams. Conventional, acute and chronic toxicity testing do not explicitly address nervous system function and underestimate thresholds for toxicity in salmonids. Since these effects are not explicitly looked at during studies they oftentimes can go unnoticed. Olfactory toxicity occurring at environmentally relevant concentrations can induce reduction to food odor attraction and predator scent or alarm response pheromones can cause major problems with survivorship. Olfactory toxicity can also affect the ability of anadromous fish to find their natal stream causing them to stray to other streams.

Avoidance behavior exhibited by fish is species specific, Whitefish ("C. clupeaformis") showed a preference toward SLS at a concentration of 0.1 mg/L while rainbow trout ("Oncorhynchus mykiss") and common carp ("Cyprinus carpio") showed an avoidance response at a concentration of 0.01 ug/L. Past studies are difficult to compare due to differences in test and exposure conditions.

Some research has suggested that high levels of fluoride exposure may adversely affect neurodevelopment in children, but the evidence is of insufficient quality to allow any firm conclusions to be drawn.

Excess fluoride consumption has been studied as a factor in the following:

Estrogen birth control pills may increase the amount of copper in humans, but was not shown to increase absorption. Copper Intrauterine devices (IUDs) have been questioned anecdotally, with people claiming copper toxicity, but there is currently no scientific evidence to substantiate this claim. Estrogen increases the absorption of copper, making women more likely to carry excess copper even when no birth control is used.

The amount of estrogen (or copper) contained in these modern forms of contraception are generally considered safe, and the former restrictions for estrogen use (not to be used by women older than 40, 35 for smokers) were lifted in 1989.

There are conditions in which an individual's copper metabolism is compromised to such an extent that birth control may cause an issue with copper accumulation. They include toxicity or just increased copper from other sources, as well as the increased copper level of the individual's mother via the placenta before birth. The two hormones commonly used in birth control, estrogen and progestin, protect from each other's complications, so a combination method may work best. At least when existing imbalances have been treated.

In Northern Australia, where ciguatera is a common problem, two different folk science methods are widely believed to detect whether fish harbor significant ciguatoxin. The first method is that flies are supposed not to land on contaminated fish. The second is that cats will either refuse to eat or vomit/display symptoms after eating contaminated fish. A third, less common testing method involves putting a silver coin under the scales of the suspect fish. If the coin turns black, according to the theory, it is contaminated.

On Grand Cayman and other islands the locals will test barracuda by placing a piece of the fish on the ground and allowing ants to crawl on it. If the ants do not avoid the flesh and will eat it, then the fish is deemed safe.

In Dominican Republic, another common belief is that during months whose names do not include the letter "R" (May through August), it is not recommended to eat certain kinds of fish, because they are more likely to be infected by the ciguatera toxin.

The validity of many of these tests has been scientifically rejected.

The EPA lists no evidence for human cancer incidence connected with copper, and lists animal evidence linking copper to cancer as "inadequate". Two studies in mice have shown no increased incidence of cancer. One of these used regular injections of copper compounds, including cupric oxide. One study of two strains of mice fed copper compounds found a varying increased incidence of reticulum cell sarcoma in males of one strain, but not the other (there was a slightly increased incidence in females of both strains). These results have not been repeated.

Ethylene glycol poisoning is a relatively common occurrence worldwide. Human poisoning often occurs in isolated cases, but may also occur in epidemics. Many cases of poisoning are the result of using ethylene glycol as a cheap substitute for alcohol or intentional ingestions in suicide attempts. Less commonly it has been used as a means of homicide. Children or animals may be exposed by accidental ingestion; children and animals often consume large amounts due to ethylene glycol having a sweet taste. In the United States there were 5816 cases reported to poison centers in 2002. Additionally, ethylene glycol was the most common chemical responsible for deaths reported by US poison centers in 2003. In Australia there were 17 cases reported to the Victorian poison center and 30 cases reported to the New South Wales poison center in 2007. However, these numbers may underestimate actual numbers because not all cases attributable to ethylene glycol are reported to poison control centers. Most deaths from ethylene glycol are intentional suicides; deaths in children due to unintentional ingestion are extremely rare.

In an effort to prevent poisoning, often a bittering agent called denatonium benzoate, known by the trade name Bitrex, is added to ethylene glycol preparations as an adversant to prevent accidental or intentional ingestion. The bittering agent is thought to stop ingestion as part of the human defense against ingestion of harmful substances is rejection of bitter tasting substances. In the United States, eight states (Oregon, California, New Mexico, Virginia, Arizona, Maine, Tennessee, Washington) have made the addition of bittering agents to antifreeze compulsory. Three follow up studies targeting limited populations or suicidal persons to assess the efficacy of bittering agents in preventing toxicity or death have, however, shown limited benefit of bittering ethylene glycol preparations in these two populations. Specifically, Mullins finds that bittering of antifreeze does not reduce reported cases of poisoning of preschoolers in the US state of Oregon. Similarly, White found that adding bittering agents did not decrease the frequency or severity of antifreeze poisonings in children under the age of 5. Additionally, another study by White found that suicidal persons are not deterred by the bittered taste of antifreeze in their attempts to kill themselves. These studies did not focus on poisoning of domestic pets or livestock, for example, or inadvertent exposure to bittered antifreeze among a large population (of non-preschool age children).

Poisoning of a raccoon was diagnosed in 2002 in Prince Edward Island, Canada. An online veterinary manual provides information on lethal doses of ethylene glycol for chicken, cattle, as well as cats and dogs, adding that younger animals may be more susceptible.

Some of the toxic effects of mercury are partially or wholly reversible, either through specific therapy or through natural elimination of the metal after exposure has been discontinued. Autopsy findings point to a half-life of inorganic mercury in human brains of 27.4 years. Heavy or prolonged exposure can do irreversible damage, in particular in fetuses, infants, and young children. Young's syndrome is believed to be a long-term consequence of early childhood mercury poisoning.

Mercuric chloride may cause cancer as it has caused increases in several types of tumors in rats and mice, while methyl mercury has caused kidney tumors in male rats. The EPA has classified mercuric chloride and methyl mercury as possible human carcinogens (ATSDR, EPA)

Heavy metals "can bind to vital cellular components, such as structural proteins, enzymes, and nucleic acids, and interfere with their functioning". Symptoms and effects can vary according to the metal or metal compound, and the dose involved. Broadly, long-term exposure to toxic heavy metals can have carcinogenic, central and peripheral nervous system and circulatory effects. For humans, typical presentations associated with exposure to any of the "classical" toxic heavy metals, or chromium (another toxic heavy metal) or arsenic (a metalloid), are shown in the table.

Once kidney failure has developed in dogs and cats, the outcome is poor.

Various Caribbean folk and ritualistic treatments originated in Cuba and nearby islands. The most common old-time remedy involves bed rest subsequent to a guanabana juice enema. Other folk treatments range from directly porting and bleeding the gastrointestinal tract to "cleansing" the diseased with a dove during a Santería ritual. In Puerto Rico, natives drink a tea made from mangrove buttons, purportedly high in B vitamins, to flush the toxic symptoms from the system. There has never been a funded study of these treatments.

An account of ciguatera poisoning from a linguistics researcher living on Malakula island, Vanuatu, indicates the local treatment: "We had to go with what local people told us: avoid salt and any seafood. Eat sugary foods. And they gave us a tea made from the roots of ferns growing on tree trunks. I don't know if any of that helped, but after a few weeks, the symptoms faded away."

Senescent leaves of "Heliotropium foertherianum" (Boraginaceae), also known as octopus bush, a plant used in many Pacific islands as a traditional medicine to treat ciguatera fish poisoning, contain rosmarinic acid and derivatives, which are known for their antiviral, antibacterial, antioxidant and anti-inflammatory properties. Rosmarinic acid may remove the ciguatoxins from their sites of action, as well as being an anti-inflammatory.

It is difficult to differentiate the effects of low level metal poisoning from the environment with other kinds of environmental harms, including nonmetal pollution. Generally, increased exposure to heavy metals in the environment increases risk of developing cancer.

Without a diagnosis of metal toxicity and outside of evidence-based medicine, but perhaps because of worry about metal toxicity, some people seek chelation therapy to treat autism, cardiovascular disease, Alzheimer's disease, or any sort of neurodegeneration. Chelation therapy does not improve outcomes for those diseases.

Copper toxicity, also called copperiedus, refers to the consequences of an excess of copper in the body. Copperiedus can occur from eating acid foods cooked in uncoated copper cookware, or from exposure to excess copper in drinking water, as a side-effect of estrogen birth control pills, or other environmental sources. It can also result from the genetic condition Wilson's disease.

A toxic heavy metal is any relatively dense metal or metalloid that is noted for its potential toxicity, especially in environmental contexts. The term has particular application to cadmium, mercury, lead and arsenic, all of which appear in the World Health Organisation's list of 10 chemicals of major public concern. Other examples include manganese, chromium, cobalt, nickel, copper, zinc, selenium, silver, antimony and thallium.

Heavy metals are found naturally in the earth. They become concentrated as a result of human caused activities and can enter plant, animal, and human tissues via inhalation, diet, and manual handling. Then, they can bind to and interfere with the functioning of vital cellular components. The toxic effects of arsenic, mercury, and lead were known to the ancients, but methodical studies of the toxicity of some heavy metals appear to date from only 1868. In humans, heavy metal poisoning is generally treated by the administration of chelating agents. Some elements otherwise regarded as toxic heavy metals are essential, in small quantities, for human health.

In the brain, domoic acid especially damages the hippocampus and amygdaloid nucleus. It damages the neurons by activating AMPA and kainate receptors, causing an influx of calcium. Although calcium flowing into cells is a normal event, the uncontrolled increase of calcium causes the cell to degenerate. See reviews by Ramsdell (2007) and Pulido (2008).

Gastrointestinal symptoms can appear 24 hours after ingestion of affected molluscs. They may include vomiting, nausea, diarrhea, abdominal cramps and haemorrhagic gastritis. In more severe cases, neurological symptoms can take several hours or up to three days to develop. These include headache, dizziness, disorientation, vision disturbances, loss of short-term memory, motor weakness, seizures, profuse respiratory secretions, hiccups, unstable blood pressure, cardiac arrhythmia and coma.

People poisoned with very high doses of the toxin or displaying risk factors such as old age and renal failure can die. Death has occurred in 4 of 107 confirmed cases. In a few cases, permanent sequelae included short-term memory loss and peripheral polyneuropathy.

There is no known antidote available for domoic acid, so if symptoms fit the description, it is advised to go quickly to a hospital. Cooking or freezing affected fish or shellfish tissue does not lessen the toxicity.

New research has found that domoic acid is a heat-resistant and very stable toxin which can damage kidneys at concentrations that are 100 times lower than what causes neurological effects.

Methylmercury is the major source of organic mercury for all individuals. Due to bioaccumulation it works its way up through the food web and thus biomagnifies, resulting in high concentrations among populations of some species. Top predatory fish, such as tuna or swordfish, are usually of greater concern than smaller species. The US FDA and the EPA advise women of child-bearing age, nursing mothers, and young children to completely avoid swordfish, shark, king mackerel and tilefish from the Gulf of Mexico, and to limit consumption of albacore ("white") tuna to no more than per week, and of all other fish and shellfish to no more than per week. A 2006 review of the risks and benefits of fish consumption found, for adults, the benefits of one to two servings of fish per week outweigh the risks, even (except for a few fish species) for women of childbearing age, and that avoidance of fish consumption could result in significant excess coronary heart disease deaths and suboptimal neural development in children.

The period between exposure to methylmercury and the appearance of symptoms in adult poisoning cases is long. The longest recorded latent period is five months after a single exposure, in the Dartmouth case (see History); other latent periods in the range of weeks to months have also been reported. No explanation for this long latent period is known. When the first symptom appears, typically paresthesia (a tingling or numbness in the skin), it is followed rapidly by more severe effects, sometimes ending in coma and death. The toxic damage appears to be determined by the peak value of mercury, not the length of the exposure.

Methylmercury exposure during rodent gestation, a developmental period that approximately models human neural development during the first two trimesters of gestation, has long-lasting behavioral consequences that appear in adulthood and, in some cases, may not appear until aging. Prefrontal cortex or dopamine neurotransmission could be especially sensitive to even subtle gestational methylmercury exposure and suggests that public health assessments of methylmercury based on intellectual performance may underestimate the impact of methylmercury in public health.

Ethylmercury is a breakdown product of the antibacteriological agent ethylmercurithiosalicylate, which has been used as a topical antiseptic and a vaccine preservative (further discussed under Thiomersal below). Its characteristics have not been studied as extensively as those of methylmercury. It is cleared from the blood much more rapidly, with a half-life of seven to 10 days, and it is metabolized much more quickly than methylmercury. It is presumed not to have methylmercury's ability to cross the blood–brain barrier via a transporter, but instead relies on simple diffusion to enter the brain. Other exposure sources of organic mercury include phenylmercuric acetate and phenylmercuric nitrate. These compounds were used in indoor latex paints for their antimildew properties, but were removed in 1990 because of cases of toxicity.

On June 22, 2006, a California brown pelican, possibly under the influence of domoic acid, flew through the windshield of a car on the Pacific Coast Highway. The phycotoxin is found in the local coastal waters.

Since March 2007, marine mammal and seabird strandings and deaths off the Southern California coast have increased markedly. These incidents have been linked to the recent and dramatic increase of a naturally occurring toxin produced by algae. Most of the animals found dead tested positive for domoic acid.

According to the Channel Islands Marine and Wildlife Institute (CIMWI), "It is generally accepted that the incidence of problems associated with toxic algae is increasing. Possible reasons to explain this increase include natural mechanisms of species dispersal (currents and tides) to a host of human-related phenomena such as nutrient enrichment (agricultural run-off), climate shifts or transport of algae species via ship ballast water."

Unlike many types of food poisoning, scombroid form is not brought about by ingestion of a pathogen. Histidine is an amino acid that exists naturally in many types of food (including fish) and at temperatures above 16 °C/60 °F it is converted to the biogenic amine histamine via the enzyme histidine decarboxylase produced by symbiotic bacteria such as "Morganella morganii" (this is one reason why fish should be stored in the freezer). Histamine is not destroyed by normal cooking temperatures, so even properly cooked fish can still result in poisoning. Histamine is the main natural chemical responsible for true allergic reactions, so the symptoms produced are almost identical to a food allergy.

Disease cures are almost always more expensive and less effective than simple prevention measures. Often precautions involve maintaining a stable aquarium that is adjusted for the specific species of fish that are kept and not over-crowding a tank or over-feeding the fish. Common preventive strategies include avoiding the introduction of infected fish, invertebrates or plants by quarantining new additions before adding them to an established tank, and discarding water from external sources rather than mixing it with clean water. Similarly, foods for herbivorous fish such as lettuce or cucumbers should be washed before being placed in the tank. Containers that do not have water filters or pumps to circulate water can also increase stress to fish. Other stresses on fish and tanks can include certain chemicals, soaps and detergents, and impacts to tank walls causing shock waves that can damage fish.

Ornamental fish kept in aquariums are susceptible to numerous diseases. Due to their generally small size and the low cost of replacing diseased or dead fish, the cost of testing and treating diseases is often seen as more trouble than the value of the fish.

Due to the artificially limited volume of water and high concentration of fish in most aquarium tanks, communicable diseases often affect most or all fish in a tank. An improper nitrogen cycle, inappropriate aquarium plants and potentially harmful freshwater invertebrates can directly harm or add to the stresses on ornamental fish in a tank. Despite this, many diseases in captive fish can be avoided or prevented through proper water conditions and a well-adjusted ecosystem within the tank.

Zinc has been used therapeutically at a dose of 150 mg/day for months and in some cases for years, and in one case at a dose of up to 2000 mg/day zinc for months. A decrease in copper levels and hematological changes have been reported; however, those changes were completely reversed with the cessation of zinc intake.

However, zinc has been used as zinc gluconate and zinc acetate lozenges for treating the common cold and therefore the safety of usage at about 100 mg/day level is a relevant question. Thus, given that doses of over 150 mg/day for months to years has caused no permanent harm in many cases, a one-week usage of about 100 mg/day of zinc in the form of lozenges would not be expected to cause serious or irreversible adverse health issues in most persons.

Unlike iron, the elimination of zinc is concentration-dependent.

Scombroid food poisoning is a foodborne illness that results from eating spoiled (decayed) fish. Along with ciguatera, it is listed as a common type of seafood poisoning. The toxin believed to be responsible is histamine, formed as the flesh of the fish begins to decay. As histamine is also the natural agent involved in allergic reactions, scombroid food poisoning often gets misidentified as a food allergy.

The syndrome is named after Scombridae family of fish, which includes mackerels, tunas and bonitos, because early descriptions of the illness noted an association with those species; however, the Centers for Disease Control and Prevention (CDC) have identified other, nonscombroid vectors, such as mahi-mahi and amberjack. Scombroid syndrome can result from inappropriate handling of fish during storage or processing. Cooking the food does not prevent illness, because histamine is not destroyed at normal cooking temperatures.

Supplemental zinc can prevent iron absorption, leading to iron deficiency and possible peripheral neuropathy, with loss of sensation in extremities. Zinc and iron should be taken at different times of the day.

Ichthyoallyeinotoxism, or hallucinogenic fish inebriation, comes from eating certain species of fish found in several parts of the tropics, the effects of which are reputed to be similar in some aspects to LSD. Experiences may include vivid auditory and visual hallucinations. This has given rise to the collective common name "dream fish" for ichthyoallyeinotoxic fish.

The species most commonly claimed to be capable of producing this kind of toxicity include several species from the "Kyphosus" genus, including "Kyphosus fuscus", "K. cinerascens" and "K. vaigiensis". It is unclear whether the toxins are produced by the fish themselves or by marine algae in their diet, but a dietary origin may be more likely.

"Sarpa salpa", a species of bream, can induce LSD-like hallucinations if it is eaten. These widely distributed coastal fish are called "the fish that make dreams" in Arabic. In 2006, two men who ate fish, apparently the "Sarpa salpa" caught in the Mediterranean were affected by ichthyoallyeinotoxism and experienced hallucinations lasting for several days.

Other hallucinogenic fish are "Siganus spinus", called "the fish that inebriates" in Reunion Island, and "Mulloides flavolineatus" (formerly "Mulloidichthys samoensis"), called "the chief of ghosts" in Hawaii.