>> As I sell food for human consumption, I certainly AM one to recycle brood combs on a strict 5-year schedule...

> The implication being, older brood combs contaminate honey? Any evidence for this? 

"Yes", and "None is needed". 
Adulteration under food safety regulations is evinced by the fact of mere "close contact", not by any detectable residue.

The mere "close contact" between old brood  combs contaminated with several years buildup of random environmental toxins and honey sold for human consumption is sufficient to render the honey "adulterated".  See 21 USC § 342 for the US law on this.  Clearly, "in the same beehive" is undeniably in "close contact with food". 

The concerns of the US government are very clear - except for "tolerances" established by the EPA and FDA, only substances that are Generally Recognized As Safe ("GRAS") are allowed in close contact or direct contact with food.  If food is exposed to any non-GRAS and non-Tolerance substance, it is considered "adulterated", and cannot be sold for human consumption.  It is crucial to understand that compliance with the regulations is not optional, and that it is the act of exposure that renders the food contaminated, even if there is "no detectable contamination" as a result.  The reason for this is that one cannot keep food safe on a "catch me if you can" basis, where every mouthful of food must be tested.  It is the food-handling facility that is inspected, and while no one yet has realized that a beehive is a food-handling facility, only the most dishonest beekeeper would try to deny it.

Despite hand-waving and avoidance of the evidence, there's a growing pile of evidence that bees bring back a wide range of environmental contaminants to the hive.  They do this so "well", Jerry Bromenshenk has published a significant number of papers and articles documenting the use of bees as "environmental monitors", and mentioned his work in Bee-L posts many times. 

The only way to avoid contamination of honey with these "random toxins" brought back by the bees is to replace the combs that get the most foot traffic. Try as I might, I cannot seem to convince my bees to simply wipe their little feet before entering the hive, and "travel stain" is a well-known phenomenon.  Wax is a magnet for "oil-based substances" and honey is a magnet for "water based substances", so to keep toxins that are inevitably absorbed by wax from coming into contact with honey, the regular removal and recycling of contaminated brood combs is a practice that is generally recognized as effective.  US beekeepers are, as a group, very lax in their food safety practices - it is a struggle to find a photo of a US beekeeper simply wearing a hairnet in his honey house, and that's something that would get any other type of food-processing plant shut down.

For some reason, European beekeepers "get it", and spent some of their time and effort working to keep a "clean hive" in a world that they recognize as inherently "dirty".  I do not know what the difference is, but I assume it has something to do with more humility and less arrogance in their underlying worldview.

Now, no one has ever enforced any of this, but there are lots of "little ethical issues" that are not practical to make the subject of enforcement, and beekeepers have flown under the food safety regulatory radar for over a century, as compared to, for example, meat and dairy producers.  

I've said it many times before - "Beekeeping Is Doing The Right Thing When No One Is Watching".
Each can decide for themselves if they are capable of doing the right thing without being bullied by regulations and enforcement.

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