Bisphenol A (BPA), a chemical that can leach out of polycarbonate bottles, is clearly the "toxin du jour."
Environmental organizations label it as a clear threat to our health and some politicians have even begun to clamor to ban any substance that can release BPA.
The current media focus on BPA was stimulated by a couple of studies that measured the amount of this chemical that leached into water stored in polycarbonate bottles. Such studies are motivated by one of the well-established chemical characteristics of BPA, namely that it has hormone-like effects. And since hormones can be physiologically active at very small doses, the potential effects of BPA certainly merit investigation, especially given that some hormone-driven cancers appear to be increasing.
Of course, before we attempt to evaluate the meaning of the amount of BPA leaching out of bottles, we have to have some idea of the dose at which the chemical presents a hazard. Although there is no universal agreement on what is a safe intake, there is a consensus among regulatory agencies that rodents treated with five milligrams of BPA per kilogram of body weight do not experience adverse effects. This is referred to as the "no observed adverse effect level," or NOAEL. Building in a safety factor of 100, these agencies have proposed a Tolerable Daily Intake (TDI) for humans of 0.05 milligrams per kg.
Let's get back to the bottles, and consider worst-case scenarios. We'll focus on baby bottles, because if BPA presents a risk, it is expected to be most significant during the developmental stage. In the most recent study, the maximum amount of BPA that leached into water from a polycarbonate bottle was eight nanograms per milliliter. Let's assume a baby were to drink a liter of this water. One milligram is a million nanograms, so the total intake would be 0.008 milligrams. If the baby weighs 5 kg (11 pounds), we have an intake of 0.0016 mg per kg of body weight. This is about one-thirtieth the TDI and one three-thousandth the "no observable adverse effect level," in test animals.
That's theory. What about measuring how much BPA we are actually exposed to? That's been done. The Centers for Disease Control in the U.S. sampled urine from more than 2,000 people age 6 to 85 and found an average of about 2.7 nanograms per mL. Since bisphenol A does not accumulate in the body, the urinary output can be used to estimate the amount taken in through food and water. This calculates to 50 nanograms per kg of body weight. And how does that compare with the TDI? It is 1,000 times less. And 100,000 times less than the dose that causes no effect in test animals. Some researchers argue that the calculation of oral intake based on urinary output is flawed and that if one goes by studies in animals the value should be at least 100 times greater. Even if we accept this argument, we are still looking at an intake that is a thousand times less than the dose that causes no effect in test animals. So there seems to be a significant safety factor here, even if one argues about the exact value of the NOAEL.
Since the human is not a giant rat, the possibility exists that we are more sensitive to hormone disrupting chemicals than rodents. But if that is the case, we have a lot more to worry about than just BPA. Remember the joke about the drunk who was walking back and forth below a street lamp? What did you lose, he was asked? My keys, came the reply. Did you drop them here? No, he answered, but this is the only place where there is light!
Right now, the light is being cast on bisphenol A, while numerous hormone-like substances lurk in the darkness.
Take lavender-scented soaps and lotions, for example. These have been linked with breast growth in young boys. It turns out lavender oil activates estrogen regulating genes in human breast cells. Alfalfa sprout extracts display increased breast cancer cell proliferation above levels seen with estradiol, an estrogen. Soybeans contain natural estrogenic compounds, and so does milk. Milk represents a far greater estrogenic exposure than we experience from BPA. Our average daily intake of estrogens through milk is about 370 nanograms which is roughly what what would be found in 50 mL of water from a polycarbonate bottle. Nobody suggests banning milk even though it contains a good dose of estrogenic compounds. Neither should they.
That's not all. Nonylphenol, an ingredient in numerous detergents, is estrogenic. It ends up in sewage, along with natural estrogens and birth control pill remnants excreted by women. Sewage treatment plants do not remove these substances and they can end up in surface water as well as in ground water when sewage sludge is spread on fields as fertilizer.
We are awash in a sea of both natural and synthetic hormone disrupting substances and it is unrealistic to accuse a specific one of being the devil. This does not mean we should be cavalier about hormone-like substances in the environment.
Even though there is no evidence that at the levels encountered, BPA is a risk to humans, we can't rule out the possibility that babies may not excrete BPA as efficiently as adults, or that the chemical might have a synergistic effect when combined with other endocrine disrupting substances. Baby bottles made of glass or other plastics are available, and it seems a good idea to search for viable alternatives to the epoxy lining in canned foods. But panic over drinking from polycarbonate bottles is unwarranted, and talk of banning polycarbonate plastics is naive.
Joe Schwarcz
Prof. Joe Schwarcz is Director of McGill University’s Office for Science and Society (OSS). It is dedicated to disseminating up-to-date information in the areas of food, food issues, medications, cosmetics and health topics in general. Dr Schwarcz teaches a variety of courses with emphasis on health issues and on the application of chemistry to everyday life. Professor Schwarcz has received numerous awards for teaching chemistry and for interpreting science for the public. Among these are the Royal Society of Canada’s McNeil Award and the American Chemical Society’s prestigious Grady-Stack Award. Dr. Schwarcz is the only non-American ever to be honored with this prize. His latest award is the Royal Canadian Institute’s Sandford Fleming Medal.
He was the chief consultant on the Reader’s Digest best sellers “Foods That Harm, Foods That Heal” and “The Healing Power of Vitamins, Minerals and Herbs” and contributed the chemistry chapter to the best-selling “Mental Floss.” His books “Radar, Hula Hoops and Playful Pigs,” “The Genie in the Bottle,” “That’s The Way The Cookie Crumbles,” “Dr. Joe and What You Didn’t Know,” “The Fly in the Ointment” and “Let Them Eat Flax” have been best sellers. The books have been translated into five languages and are sold around the world. His next work, “An Apple A Day,” has just been released (Feb. 2008).
