Or precautionary allergy labelling.

What are Precautionary allergy labels?

These are extra bits of information sometimes provided on labels, in addition to the actual ingredients.  Phrases like “May contain…”, “Made in a factory where…”.  These precautionary allergy labels are not legally required, but manufacturers are encouraged to use them to warn their customers of a risk of accidental contamination during the production process.  The Food Standards Agency  (FSA) (2006) encourages manufacturers to be as precise as possible eg which specific nut, or else peanut or “tree nuts” rather than just “nuts” but in reality different companies do different things eg Kelloggs does not differentiate between peanut and tree nuts when they put a warning label on about nuts.

Is there a real risk or not?

Contamination of chocolate is a particular problem, particularly with nuts, and with milk in the case of dark chocolate.  In some studies, half of all the chocolate tested was contaminated.

But most foods carrying such a label will not contain any of the allergen mentioned, indeed it is sometimes hard to imagine how it possibly could!  In an Irish study looking at foods labelled marked “may contain peanut/nut”, 5% had detectable peanut or nut, which is a significant proportion but actually the peanut or nut was present at such low levels they would be unlikely to cause a reaction in the majority of allergic people.

Aren’t they just a way of avoiding legal liability?

The FSA clearly state that these labels should only be used where there is a real and unavoidable risk.  And in any case, it’s not clear it changes the company’s legal responsibilities – if there is evidence that a manufacturer has been producing food in an unsafe manner, they would be liable regardless of whether there was a warning or not.

When is a trace not a trace?

The idea of threshold is important – how much of the allergen is actually present, and is it even enough to cause a reaction?  Not everyone reacts at the same threshold, and the differences between individuals can be a factor of ten or even a hundred.

In big study of peanut challenges in kids, those who got through to last stage were 13x more likely to have anaphylaxis (related to total amount of peanut consumed, presumably). Higher thresholds found in older kids, perhaps because they would have presented earlier if they had lower threshold? [PAI 2018 vol 29:754-761]

Australia and New Zealand ask manufacturers to look at actual levels of contamination before putting a warning on their products.

In the UK and Europe, the risk associated with the processes is what matters, rather than the actual levels of contamination. It’s not clear which is actually more useful.

So what should you do about traces?

Many people tend to ignore these warnings, particularly when it is something they have eaten many times before, or when it is a big brand name, and when the wording is ambiguous rather than direct.  Yet there is no good evidence that any of these things actually makes a difference to the real risk.

What is definitely true is that people have unexpected reactions and this can be after eating things marked with these warnings, but equally after eating things without these warnings.

What is also true is that allowing yourself to eat things marked with these warnings makes life much simpler!

Some patient organizations eg Anaphylaxis Campaign recommend avoiding anything marked with precautionary allergy labels, if only because this puts the control in your hands rather than leaving you at the mercy of the manufacturers.

Doctors often recommend avoiding anything with an allergy warning, because it “seems” safer and they don’t appreciate how difficult it is in day to day life.

So I think you have to make your own choice.  If you have a nut allergy, and it’s chocolate or something else that often does contain nuts eg a muesli bar, then to me the risk seems too much.  It it’s something that wouldn’t usually contain nuts, and you’ve had it before, and you’re at home with your family, then maybe the risk is acceptable to you.  But if you are away on holiday, and you’ve forgotten to bring your allergy medicines, and your asthma is playing up, and it’s late at night, then that’s probably the worst time to take a chance.    

[https://www.food.gov.uk/business-guidance/allergen-labelling-guidance-for-food-manufacturers]