DEAR DR. ROACH: I love fish, but I am worried about mercury. I read that one could get too much mercury by eating too much fish. I can find no reference to mercury on the packaging. I also like to finish a package before going on to something else. By eating fish three or four times a week, am I getting too much mercury at one time?

— G.W.H.

ANSWER: Some fish have much more mercury than others, and I would advise finding fish you like with low mercury levels. Shrimp, canned light tuna, salmon, pollock and catfish are all low in mercury. You should avoid shark, swordfish, king mackerel and tilefish, as these have quite high levels. Many types of frozen tuna, especially bigeye, also are high in mercury and should be eaten sparingly.

The actual detailed data of mercury content in fish are available for those who want at tinyurl.com/FDA-fish-mercury.

If you eat a lot of fish in one week, cut back in the next week or two. Your body can get rid of the type of mercury in fish over time.

DEAR DR. ROACH: In a recent column, you commented on a reader’s obstructive sleep apnea and a CPAP question. You wrote, “The person eventually will wake up … due to lack of oxygen …”

When I worked in heavy industry, I was told in safety training programs that it isn’t the lack of oxygen in a person’s body that causes the gasping reflex, but rather the buildup of carbon dioxide.

If a worker in a plant entered an enclosed area that had been purged with nitrogen, breathing would seem normal because the body would exhale carbon dioxide. However, within seconds, the brain would experience oxygen deficiency and the person would pass out without any warning symptoms. This would lead to a quick death.

Was my safety training in error, or was your column an oversimplification to make your answer more easily understood by readers?

— G.C.

ANSWER: Your safety training was mostly right. In the case of breathing an inert gas like nitrogen, oxygen levels in the brain will drop precipitously, and a person can be rendered unconscious within a minute and perhaps even within seconds. There may not be time for symptoms, which is why safety training is so critical for workers. This situation doesn’t happen in normal environments, and the body’s defense systems cannot deal with it.

In the case of sleep apnea, there are both high carbon dioxide and low oxygen “alarm systems” in the brain. People with severe sleep apnea develop resistance to the high CO2, so low blood oxygen levels are of great importance in sleep apnea. Giving supplemental oxygen alone does improve the low oxygen of sleep apnea; however, it may make the duration of apnea (the period of time not breathing) longer, confirming that both low blood oxygen and high carbon dioxide are important. Addressing the obstruction with CPAP reduces apnea, prevents low oxygen and reduces high CO2, so it’s a better choice for most sleep apnea patients than oxygen supplementation alone.

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Dr. Roach regrets that he is unable to answer individual letters, but will incorporate them in the column whenever possible. Readers may email questions to ToYourGoodHealth@med.cornell.edu or send mail to 628 Virginia Dr., Orlando, FL 32803.