While people worldwide search for ways to protect themselves from the novel coronavirus, some are testing the old saying that sunlight’s ultra­violet wavelengths are a good disinfectant against pandemic germs.

President Trump speculated about sunlight and ultraviolet rays as a panacea at a White House news conference on Thursday. Hospitals already have begun deploying artificial ultraviolet-C light for the first time to sanitize scarce N95 respirators for reuse. At home, fearful consumers are snapping up UVC sanitizing gadgets such as PhoneSoap to disinfect everything from cellphones to credit cards. Meanwhile, some are hanging gloves in the sun, while tinkerers are creating their own UVC gizmos.

Not only are artificial ultraviolet techniques ineffective and likely deadly for treating an infected person, scientists say, some of them can be extremely dangerous used at home for disinfecting.

“From nurses to some guy building a UVC box in their basement, I’m getting calls every day” asking for help with setup, said Brian Heimbuch, molecular biologist principal investigator and engineering sciences division head at Applied Research Associates, a high-end private research-and-development company. “It scares me that people are going to hurt themselves with UVC.”

Of the three types of ultraviolet light, UVC is the deadly one, ­long-established for water and air sanitizing, but also one that people should never fool with. The invisible light is highly carcinogenic, with disinfecting results that vary widely in professional settings depending on the setup.

“The devil is in the details,” Heimbuch said.

Sunlight has a dual nature. Wavelengths from 10 to 400 nanometers are known as ultraviolet. The parts of this spectrum that reach Earth’s surface are the longer UVA and UVB rays, vital for all life and for humans to produce vitamin D and other essential body processes, but also causing tanning, sunburn, skin cancer and wrinkles depending on skin type.

Less famous, but more dangerous UVC light never appears on sun block labels because these solar rays are too short to penetrate Earth’s atmosphere. From about 254 to 270 nanometers, these deadly wavelengths, labeled UVGI for germicidal irradiation, rapidly deactivate viruses such as H5N1, swine flu and severe acute respiratory syndrome coronavirus.

Single stranded RNA viruses such as coronaviruses are highly sensitive to ultraviolet radiation, which causes mutations that disrupt replication. But natural UVB from the sun and constant artificial UVC act at vastly different speeds.

“Sunlight definitely kills things. It’s just really slow,” said Andrea Silverman, environmental engineering and global health professor at New York University, who studies the role of sunlight in virus decay such as in natural wastewater treatment pond systems.

How long would it take to disinfect, for example, a glove hung outside in the sun? Retired biophysicist David Lytle has done modeling on sunlight’s impact in epidemics or in a potential bioterrorist attack with viruses such as Ebola or smallpox in a well-known research partnership with scientist Jose-Luis Sagripanti, then director of the Army’s high-level biosafety lab at the Edgewood Chemical Biological Center in Aberdeen, Md.

Lytle calculated that on May 1, two to three hours of midday sunshine would be needed to kill microscopic coronavirus particles (not including larger globules) in Washington and New York City, which receive similar amounts of sunlight in their respective positions near the 39th and 41st parallel north. On June 21, the summer solstice, it would take 1.3 to 2 hours.

“That’s impractical in the middle of a city with tall buildings” that cast shadows, Lytle said. Plus, sunlight hits only surfaces such as the tops of parcels left on the porch, but not the other sides or the bottom. And the notion that sunshine disinfected great-grandma’s white sheets hanging on the line, Lytle warned, ignores the boiling wash water before, then wind wafting on fabric, with textures and folds that harbor nanoparticles.

“The sunlight is going to be helpful, but it’s not the total answer,” Lytle said. “You still need disinfecting.”

This expert sunlight analysis is hardly comforting for people imagining covid-19 germs coating every blade of grass, tree branch, sidewalk and lamppost. Such worries are vastly overblown, said Krista Wigginton, environmental engineering professor at the University of Michigan, who studies emerging viruses and how humans release germs into water and the land from urine and feces.

“There’s not a lot of evidence that viruses would be on the street and kicked up,” Wigginton said. “Unless you fall down on the street and lick the surface, there’s not a big route there” for transmission.

Today’s fascination with natural and artificial ultraviolet light for fighting disease hearkens to the early 20th century when Danish physician Niels Ryberg Finsen won the 1903 Nobel Prize for inventing light therapy, including for treating painful skin tuberculosis. UVA light is still used today to treat skin conditions including eczema, psoriasis, vitiligo and a type of skin cancer called T-cell lymphoma. The once-sensational advancements faded from the spotlight with the 1928 discovery of penicillin leading to the 1940s era of mass-produced antibiotics. Then came today’s new infectious viruses such as HIV, unaffected by antibiotics, and the evolution of cleanser- and drug-resistant bacteria, implicated in 1.7 million hospital infections and some 98,000 deaths per year.

On the consumer front, marketing has latched onto the gee-whiz ultraviolet factor with sometimes wild, unverified claims, including for gizmos that may not even contain actual disinfecting UVC. The Federal Trade Commission, for example, began proceedings against two companies selling a UV vacuum cleaner, shoe disinfectant and cleaning wand, alleging they falsely advertised devices as killing dangerous microorganisms without providing evidence. Any moving gadget, scientists advise, is prone to operator error, plus humans should never be exposed to UVC light, which also causes burns and discolors materials.

Some companies offer evidence.

PhoneSoap hired independent laboratories to confirm its claims that its sold-out UVC disinfecting devices kill dangerous microbes. A 2018 Bioscience Laboratories study found that the PhoneSoap 3 disinfects three viruses — Influenza A (flu), Rhinovirus (colds) and Rotavirus (diarrhea) — to 99.99 percent. But the report has not been published in any journal. Testing of PhoneSoap devices on a virus like the novel coronavirus is underway, chief executive Wesley LaPorte said.

UVC is too dangerous to tinker with at home, scientists said. Plus, disinfecting with it is a technical process, requiring knowledge of each virus’s UVC sensitivity and required light dose, which depends on bulb wattage, shape and distance from the object. One Web advertisement for a “UVC germicidal bulb,” for example, gave time and distance instructions for disinfecting, but failed to list bulb manufacturer or model type. Without such basic information, efficacy cannot be verified, said Wladyslaw Kowalski, author of the “Ultraviolet Germicidal Irradiation Handbook” and chief scientist for PurpleSun, which manufactures UVGI disinfecting systems for major health-care organizations.

Apart from its impact on viruses, UVGI also degrades different model respirators at different rates, which is another reason for caution in its use, experts said.

Owning a newfangled UVC sanitizing gadget may sound comforting during a pandemic. But it’s not necessary. Wiping — not spraying — with 70 percent isopropyl alcohol on your phone disinfects just as well, while alcohol or bleach solutions work much better on 3-D objects such as groceries or packages.

And the simple act of washing hands with old-fashioned soap, which readily breaks down the coronavirus, dissolving its lipid — fat — coating, remains among the best ways to disinfect. “Soap works,” Wigginton said. “It really works.”

Sunshine does not zap germs on short notice nor can it be inserted beneath the skin, as Trump speculated. It does promote health in many ways in measured doses, from setting the body’s sleep rhythms to releasing happy hormones such as serotonin, creating vitamin D and possibly lowering blood pressure. (Trump has since walked back his comments, particularly his suggestion that perhaps disinfectants could be used internally, saying Americans “have to work with their doctors” about treatment for the coronavirus and “of course” he was not encouraging the internal use of disinfectants. He said he does believe disinfectants can be helpful on the hands and touted the benefit of sunlight.)

“We have proven that the sun kills germs” on surfaces and in water, though slowly, said Sagripanti, who believes sunlight — more than temperature — plays a role in the seasonality of influenza outbreaks. He cautioned, however, that epidemics such as the coronavirus have been recorded throughout history in all climates and seasons.

“The sun helps, but it is not a magic bullet,” he said.