My experience is that I react (diarrhea plus mood disturbance) to under 10 ppm, as I react to everything that has tested positive on my home testing kit which is sensitive to 10 ppm, plus I react to canned tomatoes which didn't come up as positive on my testing kit. And I've been ok with all the processed foods that are certified as under 5 ppm.

A lot of people on the celiac message boards are much less sensitive than I am. They don't have to be as careful to avoid symptoms, and yet they are still healed as confirmed by biopsy and blood test.

Based on these experiences, it seems that there are individual thresholds for gluten, with people being able to tolerate different amounts without symptoms, immune reactivity, or gut damage. However, I'm also convinced that it is impossible to actually encounter zero gluten. That stuff has so permeated our food supply and living spaces, I suspect it is in very trace amounts everywhere.