Dr. Roach: Many factors influence calcium absorption
Dear Dr. Roach: What is the body's rate of absorption for calcium?
Dear A.: The textbook answer is 20 percent to 40 percent, but the answer is very complicated, as it depends on total body and intracellular calcium, vitamin D levels, presence of phosphates in food and other factors. The short answer is that it's usually exactly what it needs to be when things work right. If the body is calcium deficient, then absorption is maximal, but still most calcium in food is not absorbed.
Dear Dr. Roach: I'd like to comment on the letters about mercury in (and out) of thermometers. The advice given by the Poison Control Center is insufficient. Elemental mercury would sink to the bottom of the cup, however, it does have a significant vapor pressure and can produce inhalable mercury. When rinsed down the drain, the mercury could be in the drain trap and emit mercury vapor. This is why all sources of "escaped" mercury, including spent or broken CFL light bulbs, should be recovered and recycled.
Dear R.B.: Thank you for writing. Because elemental mercury can vaporize, it is important to clean up properly. Small mercury spills can be cleaned according to instructions from the department of health (such as health.ny.gov/environmental/chemicals/hsees/mercury/brochures/cleanup.htm). Larger spills require professional help. Mercury in a drainpipe trap (where my reader's mercury may have ended up) should be removed by a plumber and disposed of properly.
Email questions to ToYourGoodHealth@med.cornell.edu.