PHOENIX — An Arizona megachurch hosting President Trump on Tuesday misleadingly claimed that its new air purification system “kills 99.9 percent of Covid within 10 minutes” but then backtracked shortly before the president spoke.
Mr. Trump visited Dream City Church in Phoenix, one of the nation’s biggest megachurches, to speak to thousands of Arizona college students gathered to support his re-election. With coronavirus cases sharply increasing in the state, some public health experts said the gathering had the potential to be a disaster.
But the church offered a possible solution on Sunday: Technology installed in the building’s ventilation system that would clean the air and kill the disease.
The technology, the church’s pastor said in a Facebook post that has since been removed, was developed by a local company whose C.E.O. said he sometimes attends the church.
“So when you come into our auditorium, 99 percent of Covid is gone, killed, if it was there in the first place” the pastor, Luke Barnett, said in the video. “You can know when you come here, you’ll be safe and protected. Thank God for great technology and thank God for being proactive.”
In an interview on Tuesday, Tim Bender, the chief executive of CleanAir EXP, the company behind the technology, said the church officials did not fully understand how the system worked and were not precise enough in describing the company’s claims.
Church officials clarified their remarks on Tuesday afternoon, saying in a statement that they had used imprecise language.
“We have heard Coronavirus and Covid used interchangeably. Our statement regarding the CleanAir EXP units used the word Covid when we should have said Coronavirus or Covid surrogates,” the statement read. “We hope to alleviate any confusion we may have caused.”
Even as Arizona is seeing some of the steepest increases in cases and deaths in the country, thousands of residents have packed bars and restaurants in recent weeks, trying to escape both heat and boredom. Until last week, Gov. Doug Ducey, a Republican, prevented Democratic mayors in the state from requiring face masks.
After calls to restrict or cancel the Trump appearance, Mr. Ducey told reporters, “we’re going to protect people’s rights to assemble in an election year.” He attended the event on Tuesday.
Mayor Kate Gallego of Phoenix, a Democrat, repeatedly criticized the event, saying on Monday that “it does not abide by C.D.C. guidelines during Covid-19.”
“Public health is a group effort, not a partisan issue,” she added. “It requires the participation of every resident and every level of government.”
Photos of the event taken inside the church showed the crowd shoulder to shoulder, with very few people appearing to wear masks.
The event was sponsored by Students for Trump, a group affiliated with Turning Point Action, a pro-Trump group backed by the financier Charlie Kirk.
Using charged ions to remove airborne pollutants is not new, and such a system could help cleanse the church’s air, but certainly without the rapidity claimed, and it would not guarantee safety, experts said.
“The claims seem suspicious on several counts, but they don’t provide enough information to decipher what they are really doing,” said Jose L. Jimenez, a professor of chemistry and biochemistry at the University of Colorado at Boulder.
Church officials did not respond to requests for comment.
Mr. Bender said the technology, which would be installed in the ventilation system, generate positively and negatively charged oxygen ions. The ions then attach to particles in the air, adding electrical charge to the particles, causing them to clump together and fall out of the air. The ions could also react with the viruses to disable them.
Companies like CleanAir EXP base their claims on laboratory tests by outside firms but financed by the companies. A test of a CleanAir EXP device looked at a different type of virus in a test chamber about 900 cubic feet in volume — smaller than a box 10 feet on each side and tiny compared with the size of the church.
The test did not use the novel coronavirus that causes Covid-19 but a different virus that is often used as a stand-in for pathogens because it does not cause disease. “We do not, however, eliminate Covid-19 at this time,” the company said in a statement. “Our coronavirus surrogate testing results are significant for the future of clean air. We welcome the opportunity to collaborate with the CDC for additional laboratory testing and support the CDC’s guidelines on hygiene habits to prevent the spread of Covid-19.”
In the test, 99.9 percent of the virus was destroyed, but Mr. Bender said that level of effectiveness would “absolutely not” occur in a real-world setting like the church. However, the air purification system would “reduce the chances” for the transmission of disease, he said.
“The system could help reduce background levels of infectious virus in the air, but in a crowded situation such as a rally, it is most likely that any transmission that occurs is between people standing close to each other for prolonged periods,” said Linsey Marr, a professor of civil and environmental engineering at Virginia Tech.
William P. Bahnfleth, a professor of architectural engineering at Pennsylvania State University, who looked over the testing results, said, “Suffice it to say that, based on the evidence available, the scientific community is skeptical of performance claims for these devices.”
Anyone who registered for the event was required to sign a waiver.
“By attending this convention, you and any guest voluntarily assume all risks related to exposure to Covid-19 and agree not to hold Turning Point Action, their affiliates, Dream City Church, employees, agents, contractors, or volunteers liable for any illness or injury,” it said.