You've almost certainly seen references to "parts per million" on various gluten-free products i.e., "This product contains less than 20 parts per million of gluten," or "Tested to contain less than 10 parts per million of gluten." But what does this mean in terms of how much gluten the products actually contain?
The term "parts per million" measures the percentage of one substance as part of another substance. For example, if a huge barrel of marbles contains 1,000 parts per million of sparkly red marbles, that means it contains 0.1% sparkly red marbles as a percentage of the whole barrel of marbles.
Most "gluten-free"-labeled commercial food products still contain some gluten, although some contain less than others. Foods that contain 1 part per million of gluten contain 0.0001% gluten as a percentage of the food, while foods that contain 20 parts per million of gluten contain 0.002% gluten.
That's not a lot of gluten, but it only takes a tiny bit to give you a reaction. (How little? Check out my article How Much Gluten Can Make Me Sick? for the answer.)
Obviously, the more of these foods you eat, the more gluten you're consuming. But if you choose foods with lower parts per million numbers, you'll be getting less gluten overall: One pound of something with 20 parts per million contains the same amount of gluten as two pounds of something with 10 parts per million of gluten.
Still with me? Lower is better when it comes to parts per million.
Foods that are certified gluten-free by an independent testing agency likely have less gluten in parts per million than other foods to get gluten-free certification, a food must have less than 10 parts per million gluten or less than 5 parts per million gluten, depending on the program.
According to the University of Maryland, people who eat a "standard" gluten-free diet with plenty of gluten-free grain products consume about half a kilogram (a little more than one pound) of gluten-free-labeled grain products a day (hey, those servings of gluten-free cereal, pizza and bread can add up!). Since those gluten-free-labeled grain products most likely contain between 10 and 20 parts per million of gluten each, someone who consumes that much "gluten-free"-labeled food per day actually is consuming a total of between 5 and 10 mg per day of gluten.
University of Maryland. How Much Gluten Is Safe For Me? Press release, Aug. 4, 2011.