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IAM often approached by people looking for definitive answers about plant toxicity. It seems like an area that needs urgent clarity, given that you see well-being influencers using potentially deadly, exotic flowers as decorations on smoothie bowls or online diet gurus claiming that everyday fruit and vegetables are toxic and should be eliminated from the diet. Surprisingly, however, determining whether a plant is 鈥渢oxic鈥� or not is actually quite tricky.

The first thing you need to know about toxicity is that it isn鈥檛 binary, but a sliding scale determined largely by dosage. Take alcohol, for example. A single drop of vodka in a 1-litre jug of water is extremely unlikely to have any measurable biological effect on your body. However, swap that jug for a litre of pure vodka and this obviously becomes a very different story, with a continuum of risk between the two extremes.

Now, in plants, the combinations and concentrations of their constituent substances can vary enormously depending on a complex range of factors, including genetics, soil chemistry, sunlight levels, pest damage, harvest stage and even how a fruit or vegetable has been stored and cooked. This can mean that 鈥� much like those two jugs 鈥� two virtually identical looking peppers sitting on the same shelf can contain a 100,000 fold difference in their levels of capsaicin, the highly irritant compound responsible for the spicy flavour of chillies.

Indeed, the mildest peppers and the chillies so fiery that they are used to make pepper spray are the exact same species, just slightly different genetic selections within that.

Even on the same plant, apples at the top of the tree, for example, can contain nearly . That is because apples generate these compounds partially as a sunscreen to help shield their delicate tissues from the damage associated with ultraviolet light. You can often actually see this phenomenon in action on the same fruit, because these compounds are also pigments. The redder side of any apple is the side that was exposed to higher levels of UV light and is richer in potentially beneficial compounds.

But why am I talking about beneficial compounds in an article on toxicity? Well, counter-intuitively, many nutrients are also toxins, depending on the dose. Vitamin A, for example, is essential to human health but a chronic excess of it can, in rare cases, cause .

鈥淎pples at the top of the tree can contain twice as many antioxidants as those closer to the ground鈥�

As if the huge range of factors that can determine the levels of these compounds wasn鈥檛 tricky enough, the exact level at which they become problematic to health is similarly hard to determine. This is because the precise dose at which a compound starts to cause damage can only really be determined by running a clinical trial where you intentionally give a large group of people varying levels of known toxic substances and record the concentration at which negative effects start to become apparent.

I know we all probably know people we might like to enrol in such trials, but ethics boards exist for a reason. That is why many safety or toxicity levels are based on estimates. The problem here is that estimates can leave an awful lot of flexibility when it comes to interpretation, particularly in the context of cultural pressure.

For example, selling apricot kernels, which contain a defence compound that is converted to cyanide by the body, is banned due to their .

The same seeds are, however, legal in many countries, but with government advice to greatly limit consumption. In Ireland, as little as one small kernel is , while the European Food Safety Authority recommends up to .

At the same time, all over the world, Italian Amaretti cookies and liqueurs made with apricot seeds as one of their key ingredients are sold without the . To me, it is all fascinatingly confusing and sometimes highly contradictory.

This nuanced, often quite fuzzy, picture of toxicity means that it is indeed technically true to say that all sorts of everyday foods contain toxins, leaving the term open to misinterpretation, particularly if you would like to deploy it to fit a cultural narrative.

Likewise, one might argue, depending on your perspective, that it can mean potentially dangerous foods can be widely sold. That is a real headache for those seeking simple solutions in our beautifully complex world.