Temperature-scanning devices that check for fevers in schools, workplaces and public venues across the United States distort the results in a way that could overlook the telltale sign of a coronavirus infection, according to new research that casts doubt on the systems’ effectiveness in helping people resume normal life.

The thermal cameras and “temperature tablet” kiosks have been heralded as a critical first line of defense against new pandemic outbreaks. But in a new study of the scanners by the surveillance research organization IPVM, researchers warn that the tools are dangerously ineffective, raising the risk that infected people could be waved through medical screening checkpoints and go on to spread the virus unchecked.

The researchers found that seven widely used scanners attempt to compensate for the imprecisions of lower-cost sensors and the unpredictable factors of real-world tests by “normalizing” the readings of people’s temperatures.

But that “compensating algorithm,” they argue, severely undermines the devices’ medical usefulness. A feverish person with a core temperature of 100.4 degrees, their research found, could be assessed by the test devices as having a temperature of 98 degrees, well within the healthy range.

“The utility of these devices as fever screeners is now highly questionable, and arguably a risk to public health, because they actively report fevers as normal,” said Conor Healy, the lead researcher of the study set.

Representatives of the tested companies Certify, Dahua, Meridian and ZKTeco disputed the findings, saying their systems don’t manipulate temperature readings but in some cases use software techniques to “self-calibrate” to their environment.

“The deviation setting is not intended to ‘distort’ results. It’s designed to allow customers to receive alerts only when actual threats exist,” said Larry Reed, the chief executive of ZKTeco, which makes a “SpeedFace” thermal-imaging system that sells for about $3,800. “On a hot summer day in Arizona, [non-feverish] employees might trigger the device alarm all day if it’s set at 101 degrees and the employees are scanned upon immediately entering the building.”

Peter Plassmann, a thermography expert whose U.K.-based company Thermetrix designs thermal-imaging systems for medical use, said the research highlights how companies have sought to bolster their business by oversimplifying how well the devices are supposed to work.

“That’s generally the problem with infrared imaging: It’s so deceptively easy,” he said. “You point the camera at somebody, you get a nice colorful image and you get a temperature reading. Great. But in reality, it’s all rubbish. There are so many factors you need to consider.”

Companies have promoted the thermal-imaging systems for their ability to measure temperatures more quickly and at a safer distance than traditional and infrared thermometers that measure temperature from a person’s forehead, ear or mouth.

The scanners use infrared sensors to analyze the heat radiating from a person’s skin — a close but imperfect reflection of their core body temperature — and some systems are advertised as being able to assess multiple people in a passing crowd.

The FDA typically requires thermal scanners and other medical-use devices be tested for safety and effectiveness under a process known as 510(k) clearance. But in April, the agency said it would no longer require premarket reviews or object to unvetted devices that did not “create an undue risk.”

A surge of new thermal scanners followed, said the researchers, who counted more than 200 companies now making or advertising such devices. Many of the companies first jumped into the market last year without any previous experience in thermal-imaging or medical devices.

The FDA has said the change was a necessary move to address fears of device shortages. But the researchers said they worry that such untested systems are now widely distributed across the country and could fuel a “false sense of security” that could imperil public health.