To put that into perspective, a dime weighs about 2,200 mg (2.2 g). Tricia Thompson, RD, an authority on celiac disease, explains that a one-ounce slice of regular white bread has approximately 3,515 mg of gluten, or 351 times the maximum daily amount that’s safe for celiacs. So basically, a crumb is too much.
However, this does not mean you can go cut a slice of bread into 350 little crumbs and then eat one, because even if you're otherwise gluten-free, odds are good that you're still getting at least some gluten every day anyway. In most parts of the world, regulations say that to be labeled gluten-free, a product can contain up to 20 parts per million (ppm) of gluten (equivalent to about 20 mg per kilo). So, even a slice of gluten-free bread can theoretically contain around 1/2 mg of gluten, if the manufacturer is close to the 20 ppm limit.
Most manufacturers probably don't go all the way to the limit, however. Nanna Mossberg, RD, a dietitian with Fria Brod AB, a large Scandinavian manufacturer of gluten-free bread products, told me that her company -- and other responsible companies -- "would never dare to have a product so close to 20 ppm... No, we have to be well below 20 ppm to dare to guarantee that our products do not contain more than maximum 20 ppm gluten, because there is always a variation that we have to take into account."
Sources:
Akobeng AK, Thomas AG. Systematic review: tolerable amount of gluten for people with coeliac disease. Alimentary Pharmacology & Therapeutics 2008;27:1044-1052.
Thompson, T. Gluten: Is It Okay to Have a Little Bit? [Accessed August 22, 2009] http://www.diet.com/dietblogs/read_blog.php?title=Gluten%3A+Is+It+OK+To+Have+A+Little+Bit%3F&blid=13481
