Toxins in Fish

How wonderful is fish? We’re told we should eat fish twice a week as it is packed with nutrition. These healthy nutrients are however easily obtained from other food sources, whereas fish may also contain large amounts of toxins. Mercury and dioxins enjoy the status of most researched toxin in fish.  Sadly – much of what we are told is healthy always seems to have some caviat or drawbacks.


The sea is not an unlimited cornucopia that we can continue to empty at will without serious repercussions.  It is stated that by 2048, the sea could be emptied of all living things.  It is not yet a sustainable food source.

All over the world, the oceans are seriously polluted due to plastic waste and chemical fertilizers. CO2 emissions caused by human activity and global warming are changing the chemical composition of the oceans, leading to increasing acidity of the water. As a result, the biodiversity in the oceans is under serious threat, the quality of the water is deteriorating and fish are poisoned.


Mercury is a global polluter, for which man is responsible for the lion’s share (67%). Fish and shellfish take up toxic substances from the water and through their food. It often takes a very long time before these substances are removed from their bodies, which means people are also exposed to these toxic substances when they eat seafood.

More than 90% of mercury in fish is the organic form: methyl mercury. Methyl mercury does not end up in the fish’s fat like other toxic substances, but is found in the entire body – muscles, skin and tissues.

The permitted level of methyl mercury consumption in the Netherlands is 200 µg per person per week, a standard advised by the World Health Organisation (WHO). The National Institute for Public Health and the Environment (RIVM) however advises a much lower amount: 39.2 µg per person per week. These differences arose because WHO has a lower standard of safety and therefore allow greater methyl mercury consumption because they have judged fish to be extremely healthy.

1 kilogram of fish may contain 0.5 – 1 mg of mercury. This level is lower in the United States where 1 kilogram of fish may only contain 0.3 mg/kg. Some examples of popular fish species to eat and their methyl mercury rates:

Mackerel can contain 0.66 mg/kg. One portion of mackerel of 150 grams means a methyl mercury content of 99 µg.

Tuna has an average of 0.675 mg/kg of methyl mercury. One portion of tuna of 150 grams means a methyl mercury content of 101 µg

Herring has 0.04 mg/kg. One portion of herring of 150 grams means a methyl mercury content of 6 µg

Therefore only herring is safe to eat according to The National Institute for Public Health and the Environment (RIVM) standards. Just one portion of mackerel or tuna exceeds The National Institute for Public Health and the Environment (RIVM) standards and the WHO standards are exceeded with two portions of tuna a week. This naturally merely covers methyl mercury. We still need to consider the risks of dioxins, fire retardants and other toxic substances that are found in fish. Nutritional advice organisations separate these various substances out so as not to scare the consumer but it would be a lot fairer to total the poisons up.

There are also indications that mercury cancels out the positive effects of omega 3 fats.

Dioxins are very toxic, carcinogenic substances, largely released into the environment by polluting industries. Dioxin concentrates in fish fat, so the fattier the fish, the more dioxins.

Even extremely low levels of dioxin are carcinogenic and cause other harmful effects such as skin complaints, neurological damage and immunity issues.

Based on animal experiments, the Scientific Committee on Food (SCF) has determined the maximum level at 14 picograms of dioxin and dioxin-like PCBs per kilogram of body weight per week (= 2 picograms TEQ/kg lg/day). This is the standard employed in the Netherlands. The matter was discussed a few years ago when they deliberated halving the standard to 1 pg/kg lg/day. This is a much safer standard, but the measure was not adopted.

Fresh fish is allowed contain 4 pg per gram of weight. The exception is eel which may contain 12 pg/g. This means that if you weigh 70 kg, you may eat a maximum of 82 grams of eel a week, and then nothing else that may contain dioxins. Or to put it another way: eel contains a great deal of poison but preventing the fall of the eel industry seems more important that a population’s health.

Fish has 2.4 – 214.3 pg I-TEQ per gram of fat (an average of 21.2 pg I-TEQ/g fat).This large spread is due to a large difference in the dioxin contained in fatty and non-fatty fish (fatty fish contains a lot more dioxin). This means that if a 60 kilogram woman eats a portion of fatty fish with the maximum level of dioxin (200 grams, 15% fat), she eats more than 100 times the allowable daily limit.

16% of the dioxins consumed by the Dutch enter their bodies through fish consumption. This is based on current Dutch eating habits. A Dutch national eats an average of one portion of fish every 15 days. The Health Council of the Netherlands and the Netherlands Nutrition Centre however advise eating fish twice a week. This means that fish consumption in the Netherlands should be four times higher than it is now.  The British Government is also encourage families to eat fish twice weekly via their NHS website. Besides the fact that this is irresponsible in light of fish stocks worldwide, it would also drastically increase dioxin consumption.

If fish consumption were to increase fourfold, total dioxin consumption as well the percentage that comes from fish, would increase dramatically. Total intake would rise by 48% and the percentage that comes from fish would be 43.2% instead of 16%.

The average dioxin intake and PCBs is 1.2 – 3 pg/kg lg/day. A 48% increase would push these numbers to 1.8 to 4.4 pg/kg lg/day. The permitted standard is 2 pg/kg lg/day.

If fish is consumed twice a week, the population will sit just under or over the standard.


Note: Eat fish or shellfish in moderation, and certainly no more than once or twice weekly.

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