The Sort of Sad Death of the Mercury Thermometer

No element gets people telling crazy stories like mercury does. People have told me tales about pharmacists waxing floors with mercury, mothers rubbing it into babies’ skin to kill germs, and 10-year-olds coating dimes in it to make them shine, then blithely carrying them around in their pockets.

But most people who have encountered mercury have done so after breaking a mercury thermometer. And many of us who saw the liquid balls of mercury scatter across a floor or countertop considered the element the most beautiful on the periodic table.

Those days have passed. Since 2001, 20 states have banned mercury “fever thermometers” for medical use, and regulations tighten every year. Many pharmacies now carry only sterile digital replacements or the less accurate ones with red glop in the bulb.

Mercury thermometers have held out in labs and industry: the National Institute of Standards and Technology says that temperature is the second-most measured quantity on earth (after time), and most chemical process in most industries require scientists to monitor temperatures closely. But as of today the federal government has more or less killed the mercury thermometer in the United States—NIST has announced it will no longer calibrate mercury thermometers. This means companies and labs will have a harder time ensuring the thermometers’ accuracy, all but forcing a switch to other instruments. (NIST will continue to calibrate other thermometers.)

There’s no secret reason NIST (and partners like the Environmental Protection Agency and United Nations) have pushed scientists away from mercury. It’s a neurotoxin—exposure can cause tremors, partial blindness, deafness, memory loss, and many, many other problems—and, if mercury does spill, it’s very hard to clean up. But toxicologists have known these facts for ages—what took so long? Inertia, partly. People grew up using mercury thermometers, and damn it, they want to keep using them. But mercury thermometers also have real advantages over the alternatives. They’re often cheaper and more accurate, especially at higher temperatures. And unlike one common alternative, electronic thermometers, they’re self-contained (no wires sticking out) and require no power. Crucially, it’s also obvious when a mercury thermometer isn’t working properly—the mercury is probably dribbling out. Digital thermometers can go on the fritz in many subtle ways and require regular checking to make sure their sensitive parts still take readings accurately.

For the most part, mercury thermometers require only an initial calibration, and this was the role NIST, until today, played. Every glass thermometer has subtle variations in the size and shape of the bulb at the bottom and the capillary tube inside, as well as variations in the width of gradations on the side. The compounded effect of these uncertainties is that each thermometer reads temperature slightly differently.

NIST, as a federal government agency, served as a clearinghouse where everyone could get thermometers matched to a precise and uniform standard under controlled conditions. NIST started calibrating mercury thermometers (usually in ice baths) in 1901, and the thermometers became ingrained in our scientific culture. Many federal standards—like the temperature at which vaccines and blood in blood banks are stored—once required (or still require) taking measurements with mercury thermometers. NIST itself had 8,000 mercury thermometers lying around in drawers throughout its offices, all of which it had to discard. (NIST recycled the thermometers, and there’s a good chance the mercury ended up in compact fluorescent light bulbs, which contain small amounts of the metal. A standard mercury thermometer can provide enough raw mercury (500 mg) for about 100 bulbs, says Greg Strouse, the head of NIST’s Temperature and Humidity team.)

NIST has been pushing to phase out mercury since 2004, says Strouse. But it took six years for NIST to prove to scientists that electronic thermometers were as accurate and durable in certain applications where mercury is still popular, like autoclaves, high-pressure ovens used to sterilize equipment in biological and medical settings. Strouse says the last real hurdle to eliminating mercury thermometers now is the oil industry, which is very large and still uses them for various tests.

But oil scientists aren’t the only holdouts. NIST gets dozens of calls per month about switching to newfangled thermometers, and some of the callers have grumbled about the change. Some complain about the cost of changing over, others complain about the mere fact of having to change. Even around NIST there’s some nostalgia for the old quicksilver thermometers, Strouse admits. “Most people grew up with them,” he says. “We have pictures on the wall here of people calibrating them dating back to the early 1900s. So yes, there’s some nostalgia in seeing them go. But we actually look at [the change] as a success, because we’re finding a solution to a problem and moving to a new technology.”

That may be so, but for those of us who ever felt the cold prick of a mercury bulb beneath our tongues when we had a fever—and the shiver of illicit pleasure when a mercury thermometer slipped and fell and split open along a quicksilver vein—it’s still something of a sad day.