Which Substances in Food Should we Fear?

24. 9. 2020

“All things are poison and nothing is without poison. Solely the dose determines that a thing is not a poison,” says Miloslav Pouzar, a toxicologist, with reference to the famous quote of Paracelsus. He hastens to add, however, that the fear of toxins and poisons around us is often exaggerated. What poses a more serious hazard than substances like pesticide residues is our bad lifestyle. So how about hazardous substances and food?

Toxins are often mentioned, rather negatively, in relation to food. What, actually, are toxins?

A toxin is a natural substance produced within living cells or organisms. In addition to zootoxins, there are also mycotoxins and fungal toxins. Lay people often use the word toxin interchangeably with poison. However, the terms are not synonymous. Poison should be used to refer to substances with an acute effect which can, even in low doses and in short time, cause substantial damage to health or jeopardize human life. This is what people fear, though often in vain. Damage to health resulting from intoxication by poison is rather rare. When such incidents occur, they get extensive media coverage as was the case of the methanol scandal (editor’s note: the most serious case of mass poisoning by moonlit alcohol in Czech history, when 117 people got intoxicated and 47 died in 2012). This was an exception, which was shocking even for the regulatory authorities. It is hard to anticipate that people would be so stupid to take wood alcohol and mix it with ethanol. No system can prevent this; yet it provokes fear. There are some 120 000 deaths registered in the Czech Republic annually with only a few dozens of cases involving fatal intoxication.

Microbial toxins or mycotoxins have a potential to cause damage to health, or don’t they?

The most frequent causes of death in the Czech Republic include heart diseases, tumours and vascular diseases. People die of such diseases as a result of long-term damage to health. In other words, it takes some time to develop such diseases. So what is most damaging to human health is bad lifestyle and bad diet. If you take the big picture, hazardous substances present in food such as mycotoxins or pesticide residues have no actual effect on human health. Our environment is controlled to a great extent. Just think of the number of authorities that deal with food quality control; it is one of the commodities subject to the most stringent requirements when it comes to toxic substances. When you breathe outside, the air you inhale is subject to much less stringent control.

Is all food available in shops subject to control?

Quite logically, this would not be feasible; the checks are random. Some foods are checked more often since they are more risky in their essence. For example, mycotoxins produced by fungi that can be found in xylocarps, peanuts or corn. If you fail to comply with the technological requirements, even a little during food storage, the risk of contamination increases. This is not to say that nuts you buy in a supermarket are more hazardous than, say, carrots. First, you do not eat such a high quantity of nuts, and second, we know they tend to go mouldy, so they are subject to more frequent controls, and thus non-compliant samples are more often discovered. The best source in Czech I can recommend on the topic of food checks is the following portal: www.bezpecnostpotravin.cz.

If any substance present in food may be hazardous, how can we monitor their intake?

For any substances that may pose a risk, an acceptable daily intake (ADI) is calculated. The ADI is used to define limits regulating the presence of toxins in foods. The limits take into account what the usual intake of such food by an average consumer is. A problem may ocurr when you are not an average consumer. If you go vegan, for example, you eliminate some foods from your diet, and thus no longer comply with the average consumer basket. Thus, the risk that you may exceed the safe dose of a toxin in your restricted range of food increases. On the other hand, the limits are calculated to take such consumers into account, too. There should be no damage to your health, but you may be exposed to a higher risk than an average consumer.

Which toxins are your referring to?

Prototypical toxins found in food are pesticide residues; there is an ADI for each pesticide. Others include metals, substances that may leak into food from the packaging, or antibiotics, which are often discovered by the State Veterinary Administration in meat products. In fish, it may be malachite green, which is used to treat fish diseases; last year, levels exceeding the limits were discovered.

Does thermal processing help to eliminate such toxins from food? Let’s say in the case of meat.

It is not as simple. There is a large number of toxins and each toxin shows different behaviour. Some may be eliminated by boiling. On the other hand, other toxins may actually be produced by thermal processing, if done incorrectly. For example, if you roast, grill or fry meat and it gets burned, carcinogenic polyaromatic hydrocarbons (PAH) are produced. These compounds are produced when you do barbecue with open fire or charcoal as the dripping oil is burning. You may also find them in grilled or smoked food.

Consumers are sometimes told that fish meat contains high levels of mercury, which is toxic. At the same time, they are told to eat fish for its high content of omega-3 fatty acids. Is fish good or bad for our health then?

You are right that seafood, i.e. not only fish, may contain heavy metals such as mercury that you have mentioned. What is important, though, is that the mere presence of mercury in food does not mean that its level is so high to pose a risk to our health. Sometimes, we focus on the toxic substances that we are able to discover. And since metals are the easiest of all substances to analyse, they get analysed most often. That is why they will also be discovered most often. In any food, you must considered the amount of the toxic substance. Metals get discovered quite often, but rarely exceed the limits. Therefore, it is more likely that you will cause damage to your health if you avoid fish, and thus get an insufficient intake of good nutrients from fish, than by the toxic effects of mercury. It is impossible to give across-the-board advice, and if such advice is given, it is unwise. You may have people who are more sensitive, as a result of a congenital defect or a condition, to specific substances than the rest of the population. Personally, I have confidence in the Czech food inspection authorities, but we must not forget that any substance may be toxic. The first sentence I tell may students at their first toxicology lecture is: All things are poison and nothing is without poison. Solely the dose determines that a thing is not a poison Just take the example of sugar, which is, in fact, also a poison.

So is it possible to overdose with sugar?

It is. But remember that the amount is as important as the frequency of the dose. It is almost impossible to have a fatal intake of sugar at once, but it is quite frequent in the Czech Republic to eat so much sugar every day that your life expectancy may be substantially reduced as a result of obesity-related diseases. After all, any substance may damage our health. Be it synthetic or natural. Take the example of mycotoxins which are natural yet highly toxic when present in food. Nearly all mycotoxins cause damage to the liver and kidney and compromise our immune system; some of them may be carcinogenic. Another example of natural toxic substances may be poisonous mushrooms. Even when mushroom picking is a favourite hobby of Czech people. People fear mostly synthetic substances, but it makes no sense to view toxicity as an opposition of the synthetic and the natural. When you make a ranking of substances according to their toxicity, there will be no synthetic substance among the top 10.

What substances rank highest?

Botulinum toxin is one of the most toxic natural substances. Botulinum toxin is a bacterial poison that is produced in anaerobic conditions under high temperatures in low-acidity foods such as sausages in PVC packaging. That is why it is also referred to as sausage poison.

I thought that Botulin is used in cosmetic to reduce wrinkles.

You are absolutely right; it is toxic only when exceeding certain concentrations. This brings us, once again, to the doses. In addition, the toxicity is also affected by the way of administration. Botulinum toxin is extremely toxic when digested in large amounts. If you do so, you may suffer from indigestion, and may even asphyxiate since your breathing muscles will stop working. If a small amount is injected into your skin, it is not such an issue. Botulinum toxin relaxes clenching of muscles, so it can be used to reduce wrinkles.

Food labels often read “Expiry date” which guarantees the time during which the food poses no health hazard. Tin and frozen food labels often read something different: “Best before...” What does it mean?

It denotes the time during which the manufacturer guarantees that the food poses no health hazard and maintains its sensory properties. In other words, on the best before date, the food should taste the same as on the date it was manufactured, and have the same nutrition properties. Many foods deteriorate in sensory terms. If we keep them stored for a long time, they will taste differently. For example, if you open a bottle of wine, it will not be very nice after two weeks, but will be no more damaging to your health than when you have opened it. In other words, it is not a matter of health and toxicology, but rather of quality. In general, food may pose no health hazard long after the best before date. That is why food banks can buy foods past its best before date since people in financial distress may not care that much about the best before date.

Some consumers, especially mothers of young children, are often obsessed with food additives. What are they?

Food additives are chemical substances added to food to preserve or enhance its taste, appearance, flavor, expiry, consistence or other qualities, no matter what their nutrition value is. They are, however, subject to stringent checks just like any other substances.

It must be stressed, however, that detrimental health effects have been proved for some food additives if taken excessively. Some food additives may have laxative effects, colouring agents may lead to the development of hyperactivity or ADHD, sulphites may cause throat swelling, mouth or skin itching etc. It is legitimate to be concerned about these.


Everything is undergoing developments, and toxicology is no exception in this respect. In fact, we always rely on incomplete information. When economists have some uncertainty, it is tolerated. When toxicologists have the same uncertainty, it is perceived to be an issue. In the past, a number of food additives have been proven to be harmful to health. This may have been proven, however, after many years of the use of such a substance. When such an effect is discovered, the substance gets regulated immediately and is no longer used. We know much more about new substances than the old ones.

Why is that?

The principle of long-term use applies to many substances, not only food. What it means is that some substances are viewed as harmless based on the fact that they have been used for a long time and no problem has arisen. If you study them in more detail, however, you find out that the evaluation of many substances is only based on experience, rather than extensive toxicological testing. Today, when you want to introduce a new additive, you have to comply with extremely strict requirements of the European Food Safety Authority, and the substance gets tested on all levels.

Do organic foods guarantee that we will not intake harmful substances?

As far as organic foods are concerned, you may be more confident about some types of toxins. If you feel you need to avoid pesticide residues, then it is more likely you will avoid them if you eat organic food.

Does it mean that no pesticides or other chemical substances are used produce such foods?

The use of chemical substances is subject to much stricter regulation.  But do not forget that if food is labelled as “organic” or “ecological”, it does not mean it poses no health hazard. That is something different. Organic food may involve another problem. In the 1990s, England saw a case of psoralen intoxication by organic celery. Psoralen is a toxic substance that is formed in celery when it is attacked by pests feeding on its flesh. Pesticides help us prevent the plant from being attacked and thus fight off psoralen production. What happened in organic farming in England was that celery was attacked by such pests and started to produce large amounts of such antibodies. It is just one example, which by no means captures the overall tendency, but we should realize that while we are trying to avoid synthetic toxins, we may suffer damage caused by the natural ones. It is, therefore, hard to tell whether organic food is better for our health; there is no evidence to support this. Organic foods are rather in line with our inner need to avoid synthetic substances and they may have better sensory properties; usually organic foods taste better. And since mental health is extremely important, they are, in fact, good for our health.

Do organic foods contain preservatives?

They do, since a number of natural substances qualify as preservatives. Some sellers tend to restrict the amount of preservatives in food, but food that does not contain preservatives must be eaten as soon as possible after it is produced. That is the case of farmers’ markets. There is no doubt that it is better to buy strawberries from the Czech Republic than from Chile. Our diet should be seasonal. But if we want to continue shopping in supermarkets, it is logical to accept some amount of preservatives since they actually protect us.

Should we pick fruit from trees by the road, or is it harmful because of the exhaust fumes?

Naturally, the risk of superficial contamination by metals and exhaust fumes is higher by the road. It is, however, not likely for the substances to penetrate inside the fruit, so you should be safe if you wash the fruit. Czech supervisory authorities have not focused on this specific aspect, but international studies show that washing reduces the contamination less for berries (raspberries, blackberries) than for other types of fruit (e.g. apples).

Is it possible to eat kernels from apricots or other fruits, just as children sometimes do?

Apricot, apple or peach kernels contain amygdalin. If we crack the kernel with our teeth, the acidic gastric ulcers turn the amygdalin into cyanide, which is highly poisonous. The content of amygdalin depends on a number of factors and varies a lot, but its bitter taste helps us identify its presence in the kernels. There are some mentions of acute kernel poisoning in specialized literature, but not frequent ones. The poisoning occurs after eating 80 apricot kernels or 500 apple seeds at once. The amount of amygdalin which corresponds to the tolerable daily intake is contained in one or two kernels. On the other hand, amygdalin is marketed under as a B17 vitamin as an alternative cancer treatment. Clinical trials, however, failed to provide sufficient evidence for its therapeutic effects. Therefore, it is considered unlawful to market vitamin B17 in some countries.

How about moulds on food? Why is it that blue cheese is harmless, but mouldy jam should not be eaten?

Moulds are microscopic fungi that are similar in some respect to mushrooms. In other words, some are edible, others inedible and others poisonous, just like mushrooms. Edible moulds present on blue cheese do not produce any harmful secondary metabolites. This is unlike other moulds like Aspergillus flavus that produce aflatoxins, which are highly poisonous (editor’s note: this mould is often hosted by corn or peanuts, but may also be found in wet carpets, and cause pulmonary infections or attack brain or lung arteries, and thus cause myocardial infarction).

Does it mean that when mould appears on food, where it should not be, the food becomes poisonous?

You cannot put it like that since we do not know what mould it is. As a precautionary measure, it is advisable to dump mouldy food. Mycotoxins produced by some moulds migrate through the food and may get very deep, e.g. into the jam.

It is said that the oxalic acid may also be tricky and poisonous.

Indeed, it is found e.g. in spinach, mangel-wurzel or sorrel. However, these plants form an important part of healthy diet. To counter the toxic effects of the oxalic acid, it must be turned into calcium oxalate, which is insoluble. This happens when there is an interaction with calcium ions found e.g. in dairy products. That is why it is recommended to add milk to spinach.

Which foods contain aluminium?

Aluminium is naturally contained in spinach, jacket potatoes, plum juice or tea. It may also be contained in some food additives But whether it is toxic depends on the dose.

Is it harmful for our health?

Aluminium is considered to be toxic since it can cause damage to neurons when it comes into contact with them. We know it from the times of dialysis infancy. The water was not treated and the aluminium entered the blood flow of patients, who later died of dialysis dementia, whose symptoms are similar to those of Alzheimer’s disease. Under normal circumstances, we cannot absorb aluminium through our digestive tract, so when it is contained in food or water, it does not pose a serious risk. Therefore, I see no point in having fear of aluminium in food or food packaging and throwing away aluminium cutlery...

How about the inner walls of tins? Are not those covered with aluminium?

As far as I know, they are covered with tin compounds. After opening, some surfaces may undergo oxidation and release tin into the food. This may not be relevant in toxicological terms, but rather in sensory terms. Tins are designed to keep the food preserved; however, the content should be eaten as soon as the tin is opened.

Can fasting or a diet help us get rid of toxins?

Any short-term processes that do not result in long-term lifestyle changes have little impact on our health. In fact, most detoxication therapies do not have any short-term impact. Many popular detoxication therapies are based on rudimentary and simplified biological and toxicological assumptions, and try to offer one-fit-all solutions. The problem is that the range of substances that may get accumulated in our body for a variety of reasons (smoking, diet, environment, disease) is extremely wide. And just as there is no panacea for all diseases, there is no universal detoxication therapy; what is beneficial in some cases, is harmful in others.

Phthalates may also cause concerns in many people. One study conducted in the US suggests that phthalates pose a danger to reproductive system in unborn boys, and have an adverse role in premature deliveries. Where are phthalates present?

Phthalates are used to soften plastics, and thus may be found in some packaging materials that may get into contact with food, e.g. cap gaskets, but also some flooring materials, shower curtains, raincoats, healthcare aids or cosmetics. Where phthalates get monitored the most is in children toys since they interfere with the hormone system, which is more sensitive in children than in adults. That is where it is indeed necessary to monitor such substances.

What else can be found in households that poses a toxic risk?

Chemicals. At home, we store various caustic substances, acids, lyes, toilet detergents, oven cleaners and so on, and such substances may cause a variety of injuries. In elderly it is not common to mistake one substance for another and drink a corrosive substance, for example. That is not problem of the substance as such, but rather of the person that keeps toilet detergents next to cooking oil.

How about inhalation of detergents when cleaning the toilet or sink-holes?

Some detergents may release substances, such as chlorine, that may be harmful to our airways and cause damage to lungs. It is not advisable to inhale such substances and a lot of airing is necessary when using them.

The most common detergent is the dishwashing one. Shall we rinse the dishes after using the detergent or can we leave the detergent on the dishes?

Since detergents are surfactants, they involve some risk; the amount left on the dishes, however, does not pose any immediate health risk. When exposed to toxic substances, the human body has ways of countering the exposure. For example, the body starts producing enzymes that can decompose certain compounds. Or substances that help eliminate toxic substances in the body. Naturally, the capacity is not unlimited and when exceeded, some toxic substances may become harmful. As a general rule, when possible to avoid something, avoid it. You must ask the question whether to waste water on rinsing dishes, or increase the chemical burden experienced by your body. Personally, I would not be too obsessed about it, but I rinse the dishes anyway. But even if I did not, the effect of the detergent would be negligible compared to a shot of rum, smoking or a walk in the city.

Many people use candles or oil burners at home. Are these toxic?

It is a matter of the origin and quality of the product. You could buy low-quality candles that release toxic substances into the air, which was actually discovered by inspection authorities. In general, candles may be harmful for your health, but I cannot say whether it is the case of the candle on your table. It is not a property of the product class, but rather a case of a low-quality product. It is similar to tattooing. It is an area not covered by regulation; nobody knows who should be responsible for its regulation or monitoring. It is more likely to find harmful substances in candles than in food. From a precautionary point of view, I see oil burners more dangerous than any food.

And how about candle burning as such? The high-quality ones included.

When anything is burned at low temperature, be it candles or cigarettes, carbon monoxide and polyaromatic hydrocarbons are released, which should not be inhaled. Naturally, they are not released in amounts that pose an immediate risk, but they contribute to the overall burden of our body.

You have mentioned tattooing. Can it be harmful for our health?

When you get tattooed, you agree to a lifelong exposure to a (active) substance that you know nothing about. When performing risk analysis, toxicologists are very unhappy about high degrees of uncertainty. In the majority of cases of tattooing you do not know what is being applied onto your skin, but you know it will stay there for almost your entire life. I can keep repeating that small amounts of pesticide residues in food are not an issue, and people will get angry at me for downplaying the issue. And then they go and get tattooed. It is ironical since all pesticides get tested in a number of organisms, but we often have no knowledge about the colours used in tattooing.

Don’t do the tattoo shops have health certifications for the colours?

They do. Such certifications, however, cover the microbial contaminations, rather than the toxicity of substances found in the colours. In a way, it lacks reason that we can happily accept tattooing, of which we have little knowledge, but cannot accept food additives, which we know a lot about.

How about fluorides in toothpaste? Are they harmful?

Once again, anything that exceeds some reasonable level is toxic. If your intake of fluoride exceeds such a level, it will cause damage to your health. We know of cases of damage caused to tooth enamel; high concentration can result in the development of fluorosis. This disease is related to the creation of certain compounds in teeth that result in their decay. But if you use a toothpaste with fluorides to brush your teeth twice or three times a day, you need not worry. Drugs, cosmetics or food come together with instructions for use, which are good to follow. If you pour baking powder down your throat, carbon dioxide will be produced in your stomach, and you will be extremely sick, if nothing else. When you take this perspective, baking powder is also toxic. But you will never use it this way; you will add it when baking something, the carbon dioxide will be produced in the oven, and there is no reason to worry.

This interview is translated from Týdeník TÉMA

TEXT: Silvie Jurečková
Týdeník TÉMA