It was one of Daniel Gabriel Fahrenheit's most famous inventions, in 1714. But after nearly 300 years on the market, the still-common mercury thermometer now appears headed for extinction.

While many Maryland residents probably still have them in their medicine cabinets, or on their walls, the retail sale of mercury thermometers has been banned in Maryland since 2002 because of mercury's hazards as a powerful neurotoxin.

There are similar bans or restrictions in at least 17 other states, with more such legislation pending elsewhere, according to the Interstate Mercury Education and Reduction Clearinghouse.

Mercury thermometers are also on their way out in a wide variety of industries, along with a long list of other measuring devices, thermostats and switches that rely on mercury components.

And beginning March 1, the National Institute of Standards and Technology, in Gaithersburg, will no longer provide calibration services for manufacturers and users of mercury-in-glass thermometers — a critical service it had provided to American industry since 1901.

NIST and the Environmental Protection Agency are taking other regulatory steps that would limit the use of those mercury-based products, and provide alternatives.

"Due to elemental mercury's high toxicity, EPA seeks to reduce potential mercury exposures to humans and the environment by reducing the overall use of mercury-containing products, including mercury-containing thermometers," said EPA spokesman Dale Kemery.

Within five years, NIST officials expect the mercury thermometer will be officially obsolete. And none too soon.

Exposure to high levels of metallic mercury vapors can cause permanent damage to the brain, kidneys and a developing fetus. Brain damage can result in irritability, behavioral changes, tremors, changes in vision, hearing and memory problems, according to the Centers for Disease Control and Prevention.

Becoming obsolete

Many manufacturers and other industries have moved away from mercury devices, either out of concerns about the hazards and costs of breakage and cleanups, or because they have found something better.

"They have become obsolete in various industries as we work to remove them from the measurement stream, and find alternative thermometers," said Greg Strouse, leader of NIST's Temperature and Humidity Group.

If you were to compare the technologies available today, he said, "mercury is usually the least accurate of all current thermometers in the marketplace. Digital manufacturers have worked extremely hard to create products that work to meet the needs of end users, and usually better."

"We have yet to find an application that we can't solve with an alternate thermometer," Strouse said.

For those still using mercury devices, NIST is working with the EPA and private industry to revise more than 700 federal product standards that have long required the use of mercury thermometers, and find alternatives.

They will identify practical alternative thermometers, and write them into the new standards.

Almost half of those standards have already been amended to allow the use of non-mercury liquids in glass, or digital thermometers using electronic sensors. The process is expected to take several more years.

The EPA has also proposed new rules that would introduce more such flexibility into both the federal Clean Air Act and the Toxic Substances Control Act, where they currently require the use of mercury thermometers.

Accurate temperature readings are critical to many industrial processes, and commercial applications such as storage facilities for blood or vaccines.