Dear Dr. Roach: How much mercury is safe?
A.C.
The chemical element mercury exists in three forms: elemental mercury, which is the kind in thermometers; inorganic mercury, especially mercuric chloride; and organic mercury, including methyl and ethyl mercury. All of these have toxicities, but the types of toxicities are different for each, and the safe level depends on which type it is.
Elemental mercury is dangerous primarily when it is inhaled, which occurs mainly in occupational settings. Accidental swallowing of the small amount of mercury in a thermometer has very low toxicity, but I still don’t recommend it. Inorganic mercury is seldom a problem, as it is almost never used these days in the U.S. and Canada. Most products containing this form of mercury have been banned.
Methyl mercury is the major concern for consumers, as this is found in fish. It accumulates in the environment, and tends to be worst in larger fish, who eat the smaller fish and over their lifetime accumulate the methyl mercury, which was formerly used as a fungicide. The U.S. Environmental Protection Agency warns an average-size person not to exceed 70 mcg per day of methyl mercury: Below this dose, toxicity is unlikely. This translates to two to three servings of fish per week, but no more than one serving of highest-risk fish for methyl mercury per week. High-risk sources include most tuna, halibut and snapper.
Ethyl mercury is significantly less toxic than methyl mercury, as it is removed from the body more quickly. There is no EPA recommendation for ethyl mercury levels. Ethyl mercury has been used as a preservative in vaccines, but it has been removed from all childhood and most adult vaccines. It is still used in some flu vaccines — the ones in multidose vials: Single-use vials do not need a preservative. The amount of ethyl mercury in a flu shot is less than the more dangerous methyl mercury found in a can of tuna.
Dear Dr. Roach: I am curious on your opinion of the benefits of CBD. I am a 73-year-old female with spinal stenosis that is pressing on a nerve. I also have pretty severe insomnia. I take 1,000 mg of hemp oil for severe back pain. I noticed that the CBD has helped me sleep mostly without drugs. A surprise effect of the hemp oil is my resting blood pressure has dropped. It used to range from 140-125/90 at doctors’ offices. Now I measure 110-118/86. I was so shocked at this I made them take it in both arms. As for the back pain, it has reduced the degree of pain I experience. Not completely, but it’s tolerable. I would be interested in a medical point of view.
J.F.
Cannabidiol (CBD), one of the major pharmaceutical extracts from hemp (cannabis), has been recommended for many issues, but is still poorly studied. Medical cannabis, which contains the psychoactive THC in addition to the nonpsychoactive CBD, has been studied for chronic pain and has been shown to be better than placebo. THC alone has also been studied for pain, but I haven’t found good studies on CBD alone for pain.
I might argue that you don’t need a study, that you have tried it and it works. It is true that some or all of the benefit you experienced could be due to expecting to feel benefit (the placebo response), but if you are having no side-effects, I can’t argue with success.
I would note that chronic pain can cause sleep problems and raise blood pressure. Poor sleep itself can raise blood pressure. Thus, it’s not shocking that a treatment that improved pain also helped sleep and blood pressure.
Dr. Roach regrets that he is unable to answer individual letters, but will incorporate them in the column whenever possible. Readers can email questions to ToYourGoodHealth@med.cornell.edu
