It has become an iconic image of the coronavirus outbreak in China: a masked official aiming what appears to be a small white pistol at a traveler’s forehead.

For weeks, this ominous-looking device has been deployed at checkpoints across China — tollbooths, apartment complexes, hotels, grocery stores, train stations — as government officials and private citizens screen people for fevers in an effort to prevent the spread of the deadly coronavirus.

Sometimes described as a “thermometer gun,” the device is equipped with an infrared sensor that can quickly measure surface temperature without making any contact with a person’s skin. In recent years, it has become an important tool for countries scrambling to contain viral outbreaks. It was widely used to try to slow the spread of SARS in China in the early 2000s and to curb the Ebola outbreak in West Africa a decade later.

But for all of its powerful sensing technology, the thermometer has ultimately proved to be an ineffective defense mechanism, according to medical officials and experts on infrared devices. Like the surgical masks that have become ubiquitous in China, thermometer guns tend to be unreliable outside carefully controlled health care settings.

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Credit…Kevin Frayer/Getty Images

The thermometers determine temperature by measuring the heat emanating from the surface of a person’s body. Often, however, those wielding the tools don’t hold them close enough to the subject’s forehead, generating unusually low temperature readings, or hold them too close and get a high reading. The measurements can be imprecise in certain environments, like a dusty roadside, or when someone has taken medication to suppress a fever.

“These devices are notoriously not accurate and reliable,” said Dr. James Lawler, a medical expert at the University of Nebraska’s Global Center for Health Security. “Some of it is quite frankly for show.”

When he traveled in West Africa during the Ebola outbreak, Dr. Lawler was frequently tested with infrared thermometers outside hospitals or at roadside checkpoints. The results suggested he was dying of hypothermia.

“My temperature was often 35 degrees Celsius or lower, which starts to become incompatible with life,” Dr. Lawler said. “So I’m not sure those were accurate.”

In theory, the thermometers, as well as more complex cameras that can also measure the heat coming off a person, allow local officials to quickly determine who may have a fever and then pull those people aside for further testing. So far, tens of thousands of people have contracted the coronavirus, and this week the death toll passed the 1,000 mark. Last month, the World Health Organization said temperature screenings could reduce “the risk of importation.”

But on social media in China, people passing through the checkpoints have complained that the thermometers are producing unrealistically low readings in some situations and artificially high readings in others, like when a traveler is tested from inside a hot car.

The Coronavirus Outbreak

  • What do you need to know? Start here.

    Updated Feb. 10, 2020

    • What is a Coronavirus?
      It is a novel virus named for the crown-like spikes that protrude from its surface. The coronavirus can infect both animals and people, and can cause a range of respiratory illnesses from the common cold to more dangerous conditions like Severe Acute Respiratory Syndrome, or SARS.
    • How contagious is the virus?
      According to preliminary research, it seems moderately infectious, similar to SARS, and is possibly transmitted through the air. Scientists have estimated that each infected person could spread it to somewhere between 1.5 and 3.5 people without effective containment measures.
    • How worried should I be?
      While the virus is a serious public health concern, the risk to most people outside China remains very low, and seasonal flu is a more immediate threat.
    • Who is working to contain the virus?
      World Health Organization officials have praised China’s aggressive response to the virus by closing transportation, schools and markets. This week, a team of experts from the W.H.O. arrived in Beijing to offer assistance.
    • What if I’m traveling?
      The United States and Australia are temporarily denying entry to noncitizens who recently traveled to China and several airlines have canceled flights.
    • How do I keep myself and others safe?
      Washing your hands frequently is the most important thing you can do, along with staying at home when you’re sick.