More and more often we can hear that fish are healthy, since they are full of unsaturated fatty acid, vitamins and microelements. We are literally inundated by dieticians, alarming that we eat too little fish and that it is still – although the situation is improving – too small part of our menu, since the fish should be eaten at least twice a week.

We performed a quality measurement of two sorts of herrings bought in a well-known German supermarket. We send to our laboratory Sultan-style herring and herring marinated in oil.

There is said, especially in the Internet, that fish contain an enormous amount of mercury, so it poisons us. Is there any truth in the web-myth?


When you hear about mercury in fish, you are likely to ask – where does it come from? The answer is quite easy: mercury lands in water due to industrial pollution. Herring is being filled with mercury in two ways: by food it eats and by gills.

The amount of mercury in fish body depends on some factors, for instance, on the age of given fish (mercury tends to accumulate in body – the older fish is, the more mercury it contains), on condition of water where the fish lives and on predacity of the fish, because predators are more exposed to mercury in body, since they feed themselves with small fish.

The bad news concerning mercury is that traditional methods of preparing the fish – baking, grilling – do not lead to rid fish of mercury, since it accumulates firstly in muscles.

What are the symptoms of mercury poisoning like? Generally, our central nervous system is damaged. We cannot speak properly, our mouth and tongue are becoming numb, we have problems with hearing and moving.

Therefore especially pregnant woman should be careful about fish. Mercury penetrates into blood and accumulates in foetus’ tissue (and brain) by placenta. That can cause curbing mental development and damaging health in other ways.



We tested herrings marinated in oil and Sultan-style herrings with raisins and have found mercury and uranium in both sorts of fish. However, the amount of elements does not exceed norms – there is no need to worry.

More uranium and mercury have been found in Sultan-style herrings. This tendency can be observed on the chart:


Sultan-style herring – red; marinated herring – green

What is interesting is the presence of uranium. Uranium’s influence on body is still questionable. It accumulates in kidneys and skeleton. There is still no evidence of the speed of its excretion from body, it can however cause damage to kidneys (depending on its water solubility). Like mercury, it is probably able to reach placenta, which can affect development of foetus.


So, shall we stop eating fish? Obviously not! The most important is to choose sorts which are fished young and are no predators – this way we can eat fish containing less mercury than the other ones.

Heavy metals can be found everywhere in the environment, so a small amount of them is also in our food. However, we should remember that one mole is about 6×10^23, i.e. 600 000 000 000 000 000 000 000. Depending on methods of analysis and type of the tested material we can go below 10^-12, that is 600 000 000 000 particles” Radosław Łapczyński of Sigmatik says. “For comparison, 1 g of sugar is about 1 800 000 000 000 000 000 000 particles. Thus, the most important in observing pollution is to understand the scale and to remember that the problem is not the presence of something in food (there is everything, including plutonium and dioxins) but the amount of the pollution.

Although to exceed the norm of mercury in body we should have eaten an enormous number of herrings, it is a good idea to be reasonable. While we should not avoid fish, we also should not eat them every day. We need to remember that mercury does not cause sudden poisoning or affect our well-being. It is an element which slowly accumulates in our body.

Unfortunately, we still suffer a lack of certain studies on influence of fish-rich (and, accordingly, mercury-rich) diet on our health.

Even more mysterious is the uranium, with its presence in, for example, the given above herrings. Is it dangerous? Although its concentration in fish does not exceed the norms, at the moment there is no detailed test on influence of eating fish with uranium on human health in long-terms.