An iota of something; Does that sound like much to you? Measurement is about context. A substance can be toxic in minute amounts; however, information on what constitutes a toxic dose is the only manner in which one can compare exposure to potential danger. In the media, one often reads something like "20ppb lead found in drinking water at school". However, without the context of what 20ppb of lead means, and the relevant exposure data (acute vs. chronic) as well as target population (baby, child, adult, etc...), one would not be in a position to have an educated stance of the potential danger of a situation. What is a part-per-billion (ppb) anyway?

The 15c Swiss alchemist Paracelsus, who founded the discipline of toxicology, probably said it the best: "Sola dosis facit venenum". If you are not fluent in Latin, "Only the dose makes the poison" refers to the basic concept in toxicology that a substance considered toxic will be harmless in a small enough dose, and that ordinarily harmless substances can be deadly in high doses.

There is another concept in toxicity that involves dose over duration: chronic and acute. A chronic exposure to something tends to occur over a longer timeframe, and exposure to small amounts of a toxic substance over time can have significant health consequences. An acute exposure usually refers to a single exposure to a toxin.

Typical exposure limits for substances are often quoted in both acute and chronic exposure terms. For example, the Reference Exposure Level (REL) for mercury for chronic exposure is 0.004 ppb while the REL for acute exposure to mercury is 0.07 ppb. Typically these limits are set based on previous scientific studies involving animals and humans, and in the former case, usually extrapolates dose data based on a body weight (ie. mg substance per kg body weight). An additional safety factor is usually built in as well.

So what? The public is bombarded constantly with news, media, ads and advice to beware of exposure to this or that substance, generally with little regard for relevant dose information. However, comparing a reported toxic level of a material against its measured value is the only way that one can make an informed choice regarding exposure danger.

Once-rare disorders like autism, ADHD, learning disabilities, and obesity have become more common in the developed world. Many researchers suggest these disorders are due to widespread use of synthetic chemicals that mimic and disrupt the delicate function of hormones like estrogen and testosterone in the human body. Identifying the cause of these diseases or relating to exposure of a single substance is proving to be impossible. "It's not so easy to make the connections in humans because we live so long and are exposed to so many things across our lifetime that trying to link a single compound to a disease state is hard," says Andrea Gore, who researches neural endocrine disruptors.

Units of Size and Meaning

Understanding how substances are reported (ppm, ppb, etc…) is important as part of gaining a better understanding of the relative risks and consequences of a given amount of substance. For efficiency and clarity, Wikipedia has an excellent section on parts-per notation that I feel the need to share in abbreviated form here:

• One part per hundred is generally represented by the percent (%) symbol and denotes one part per 100 parts, one part in 10², and a value of 1 × 10^-2. This is equivalent to approximately one drop of water diluted into 5 milliliters (one spoonful), or about fifteen minutes out of one day.
• One part per million (ppm) denotes one part per 1,000,000 parts, one part in 10⁶, 1/1,000,000 * 100% = 0.0001% (or 1% = 10,000 ppm), and a value of 1 × 10^-6. This is equivalent to one drop of water diluted into 50 liters (roughly the fuel tank capacity of a compact car) or about 32 seconds out of a year.
• One part per billion (ppb) denotes one part per 1,000,000,000 parts, one part in 10⁹, 1/1,000,000,000 * 100% = 0.0000001% (or 1% = 10,000,000 ppb) and a value of 1 × 10^-9. This is equivalent to one drop of water diluted into 250 chemical drums (50 m3), or about three seconds out of a century.
• One part per trillion (ppt) denotes one part per 1,000,000,000,000 parts, one part in 10¹², and a value of 1 × 10^-12. This is equivalent to one drop of water diluted into 20 Olympic-size swimming pools (50,000 m3), or about three seconds out of every hundred thousand years.

These units can be helpful in expressing the content, of say, lead in drinking water. Federal limits set this at 15ppb. We can then quickly compare the results from a test sample with this limit to make a decision if that water is potable. If one were to only know the lead content in water, without any reference point of benchmark, 15 ppb may sound like a very small number in absolute terms, but the real knowledge value is to compare against a benchmark. The cautionary note here is that simply reporting numbers or basing decisions off your "intuitive" understanding of a result may not lead to a correct conclusion of the whole situation. As discussed earlier, limits would likely be different for acute and chronic exposure as well. Taking a sip of water with above-legal lead levels would be unlikely to be harmful, whereas drinking from the same source for a year would likely be harmful.

Performance vs. Survival

Related to the intake of nutrients and food is a concept that applies more specifically to athletes. There is a difference between performance and survival. The US RDA charts for vitamins and minerals list the "recommended daily allowance" for basic health and survival. These are minimum target levels one should attempt to attain from a normal diet or with supplements. There are also performance goals for nutrition (as previously written about here) that are usually greater than the survival goals, and satisfy the unique nutritional needs of a given athlete. These nutritional goals would be different between different individuals and different sports, optimized to individual performance. There is also a "Tolerable Upper Limit" (TUL) that reflects the ceiling for consumption of a given nutrient. (A good reference is here). There is some individual variability for each nutrient, and in some cases the performance intake may even exceed the TUL. Athletes should be cautious about over-consumption of certain vitamins such as A, B3, B6, C, and minerals such as fluoride and zinc, as these have TULs that are lower than some companies would have you believe.

Natural vs. synthetic

There is an ongoing push in the media and in our culture in general towards "natural" products. At the heart of this push is the repulsion of "synthetic" products, usually on the simple premise that "natural" is good and "synthetic" is bad. It is worth pointing out that one of the most toxic substances in the world, the neurotoxin Botulinum is totally natural. As are toxic minerals like mercury and arsenic, and their nasty organic cousins, methylmercury and trimethylarsine. No safe threshold for lead exposure has been discovered—that is, there is no known sufficiently small amount of lead that will not cause harm to the body. The next time somebody tells you "such-and-such is okay because it's natural", a red flag should go up: Don't take that statement at face value: each substance should be evaluated on a scientific basis.

Macro vs. Micro, and the Food you Eat.

Our typical omnivorous diet provides important building blocks for a healthy life: Carbohydrates, protein and fat. We consume each of these categories of products in large amounts- one can call them "macro" amounts. Other macronutrients in our diet, which should be consumed in as small an amount as possible with a nutritionally adequate diet, are dietary cholesterol, trans fatty acids, saturated fats, and added sugar. There are also some macro nutrients, like sodium and potassium, which have a "recommended daily allowance (RDA) in the US of and 1.5g and 4.7g respectively. Finally, there are dozens of "micronutrients" that include vitamins and minerals, that are needed for optimal health. While some supplement companies may have you believe that the average person is chronically low in one or more micronutrients, an individual with an average Western diet is usually over-consuming all nutrients. For individuals on restricted diets or athletes with higher dietary needs, deficiencies can become significant. For example, vegan and vegetarian athletes face a special issue with lack of vitamin B12 and L-carnitine, both found commonly in high doses in meat products, small amounts in milk products, and little else. With an RDA of 1.5ug/day, vitamin B12 is easily found and absorbed via nutritional supplements. L-Carnitine does not have a recognized RDA, and it can be synthesized within the body. A performance goal intake of about 60–180 milligrams of carnitine per day is recommended by the NIH. To relate back to the concept of dose and context, a reminder is in order that the long list of minerals and vitamins on the daily supplement container is only as useful as comparator information (such as %RDA), and in the context of toxicity, the reported relevant upper limit or toxicity level.

A Model Diet

With this discussion of micro and marco-nutrients comes some advice on diet. The Dietary Guidelines for Americans describes a healthy diet as one that:

• emphasizes a variety of fruits, vegetables, whole grains, and fat-free or low-fat milk and milk products;
• includes lean meats, poultry, fish, beans, eggs, and nuts;
• is low in saturated fats, trans fats, cholesterol, salt (sodium), and added sugars; and
• stays within your daily calorie needs.

An excellent read is "The Omnivore's Dilemma: A Natural History of Four Meals" by Michael Pollan published in 2006. To summarize in a sentence: "Eat food, not too much, mostly plants"

So next time somebody warns you about the dangers of such-and-such contaminant or toxin, you should ask yourself (and them) at least two questions: What is the dose to which I'm being exposed, and what is the dose that constitutes a toxic level. Without those two pieces of information, you are driving in the dark without headlights.

[Jonathan Toker is a Canadian elite-level runner and triathlete. He received a Ph.D. in organic chemistry from The Scripps Research Institute in 2001, and raced in the professional ranks for 5 years. Dr. Toker also worked as a scientist in the biotech industry for 5 years prior to launching his unique SaltStick Electrolyte Capsule and Dispenser lineup.]