President Trump has long pinned his hopes on the powers of sunlight to defeat the Covid-19 virus. He returned to that theme at the White House coronavirus briefing on Thursday, bringing in a science administrator to back up his assertions and eagerly theorizing about treatments involving the use of household disinfectant that would be dangerous if put inside the body, as well as the power of sunlight and ultraviolet light.

After the administrator, William N. Bryan, the head of science at the Department of Homeland Security, told the briefing that the agency had tested how sunlight and disinfectants — including bleach and alcohol — can kill the coronavirus on surfaces in as little as 30 seconds, an excited Mr. Trump returned to the lectern.

“Supposing we hit the body with a tremendous — whether it’s ultraviolet or just very powerful light,” Mr. Trump said. “And I think you said that hasn’t been checked, but we’re going to test it?” he added, turning to Mr. Bryan, who had returned to his seat. “And then I said, supposing you brought the light inside the body, either through the skin or some other way.”

Apparently reassured that the tests he was proposing would take place, Mr. Trump then theorized about the possible medical benefits of disinfectants in the fight against the virus.

“And then I see the disinfectant where it knocks it out in a minute — one minute — and is there a way we can do something like that by injection inside, or almost a cleaning?” he asked. “Because you see it gets in the lungs and it does a tremendous number on the lungs, so it would be interesting to check that.”

Experts have long warned that ultraviolet lamps can harm humans if used improperly — when the exposure is outside the body, much less inside. The link between ultraviolet light and skin cancer is well established. Bleach and other disinfectants may kill microbes but they also can kill humans if swallowed or if fumes are too powerful. That is why bottles of bleach and other disinfectants carry sharp warnings of ingestion dangers.

Mr. Trump’s comments prompted an explosion of warnings about the dangers of any improvised remedies. Emergency management officials in Washington State posted a warning on Twitter. “Please don’t eat tide pods or inject yourself with any kind of disinfectant,” they wrote, before urging the public to rely only on official medical advice about Covid-19. “Just don’t make a bad situation worse.”

The maker of the disinfectants Lysol and Dettol also issued a statement on Friday warning against the improper use of their products. “As a global leader in health and hygiene products, we must be clear that under no circumstance should our disinfectant products be administered into the human body (through injection, ingestion or any other route),” the company said.

By Friday morning, the White House issued a new statement. “President Trump has repeatedly said that Americans should consult with medical doctors regarding coronavirus treatment, a point that he emphasized again during yesterday’s briefing,” said Kayleigh McEnany, the White House Press Secretary. “Leave it to the media to irresponsibly take President Trump out of context and run with negative headlines.”

At the Thursday briefing, Mr. Trump had assailed a reporter who expressed concern that people might “think they would be safe by going outside in the heat considering that so many people are dying in Florida.”

“I hope people enjoy the sun, and if it has an impact, that’s great,” Mr. Trump said.

Mr. Trump then turned to Dr. Deborah Birx, the White House coronavirus response coordinator, and asked if she had heard of the success of sunlight as an effective tool against viruses, and more specifically the coronavirus.

“Not as a treatment,” Dr. Birx replied. “I mean, certainly fever is a good thing when you have a fever. It helps your body respond. But not as — I have not seen heat or ….”

Mr. Trump cut short her answer.

“I think that’s a great thing to look at,” he said. “I mean you know, OK?”

On Friday the White House also sent a corrected briefing transcript, which initially misrepresented Dr. Birx’s response. The Thursday transcript quoted Dr. Birx as saying, “That is a treatment”; the corrected version clarified that she indeed said, “Not as a treatment.”

Mr. Trump has long touted various ideas against the coronavirus despite a lack of scientific evidence, from sunlight and warmer temperatures to an array of drugs, including the malaria drug hydroxychloroquine, which he has promoted as a “what have you got to lose” remedy.

But some of his recommendations, however, have had disastrous effects. Last month, an Arizona man died and his wife was hospitalized after the couple ingested a chemical found in hydroxychloroquine.

As the pandemic has spread to countries experiencing hot weather, including Australia and Iran, some groups have investigated whether the warmer summer season would slow the virus. Early this month, a committee of the National Academy of Sciences looked exclusively at humidity and temperature and found that they would have a minimal impact on the virus.

At the Thursday briefing, Mr. Bryan said that the novel coronavirus dies rapidly when exposed to sunlight, high temperatures and humidity. He cited experiments the agency had conducted at a high-security laboratory in Frederick, Md.

“Our most striking observation to date is the powerful effect that solar light appears to have on killing the virus — both surfaces and in the air,” Mr. Bryan said. “We’ve seen a similar effect with both temperature and humidity as well, where increasing the temperature and humidity, or both, is generally less favorable to the virus.”

The sunlight finding was no surprise to life scientists who, for many decades, have reported that ultraviolet light — an invisible but energetic part of the sun’s electromagnetic spectrum — can damage DNA, kill viruses and turn human skin cells from healthy to cancerous.

For public health, the big challenge is widening such narrow laboratory findings so they take into account how the global environment and its changing weather and endless nuances can impact the overall result — most especially on the question of whether the virus that causes Covid-19 will diminish in the summer. This week, a pair of ecological modelers at the University of Connecticut reported evidence that balmy weather may indeed slow the coronavirus, but not enough to do away with the social-distancing measures advised by public health officials.

The inherent limitations of lab studies were driven home on April 7 in a letter to the White House from a National Academy of Sciences panel looking into research on the coronavirus. “With experimental studies,” the panel said, “environmental conditions can be controlled, but almost always the conditions fail to adequately mimic those of the natural setting.”

Katie Rogers contributed reporting.