A brief chemistry lesson...

What I found confusing about this allergy is that you can't just check ingredient labels and expect to be safe.  This is because Balsam of Peru is not just one molecule, it is made up of hundreds of naturally occurring aromatic alcoholic compounds that are also found in many other natural foods.  So unfortunately, a lot of the time you just have to know where these chemicals are found.

Let's look at some molecules so we can visualize it all better.  The most frequent constituents of BOP are cinnamyl and benzoic esters, which make up between 45% - 70% of the overall composition.  More specifically, these esters are:

Benzyl benzoate

Cinnamyl cinnamate

Benzyl cinnamate

There are hundreds more aromatic compounds in B.O.P.; take a look at this page to get a broader understanding - http://www.cropwatch.org/Peru%20Balsam%20chemistry.pdf

The main problem we have is our reaction to various spices.  This is because what gives a spice its flavour is the numerous aromatic alcohols in its constitution.  All spices have these compounds in various forms and degrees.  As you can see from the names of the compounds above, cinnamon is most likely to be one of the worst for us.

Picking a spice at random, say cumin, we can look at the chemical composition and predict if it will affect is badly (http://books.google.co.uk/books?id=7KPUlXxOYZAC&pg=PA142&lpg=PA142&dq=cumin+benzyl+cinnamate&source=bl&ots=UPtek5hQEg&sig=pcWuVMET1jxIrbTX0TD_39p5VwM&hl=en&sa=X&ei=VkNyUvK-Mcel0AXkzIDoDw&ved=0CD0Q6AEwAg#v=onepage&q=cumin%20benzyl%20cinnamate&f=false ).  Straight away we can see that the essential oil of the cumin seed contains 4.47% benzyl cinnamate, and that's not taking into account all of the constituents of B.O.P.

The  bottom line- all spices are going to be a risk for us.  Not all of us will be allergic to all the constituents of B.O.P., but finding out which it is specifically will be a challenge!