The Coronavirus Is Airborne Indoors. But We’re Still Scrubbing Surfaces.

HONG KONG — At Hong Kong’s deserted airport, cleaning crews constantly spray baggage trolleys, elevator buttons and check-in counters with antimicrobial solutions. In New York City, workers continually disinfect surfaces on buses and subways. In London, many pubs spent lots of money on intensive surface cleaning to reopen after lockdown — before closing again in November.

All over the world, workers are soaping, wiping and fumigating surfaces with an urgent sense of purpose: to fight the coronavirus. But scientists increasingly say that there is little to no evidence that contaminated surfaces can spread the virus. In crowded indoor spaces like airports, they say, the virus that is exhaled by infected people and that lingers in the air is a much greater threat.

Hand washing with soap and water for 20 seconds — or sanitizer in the absence of soap — is still encouraged to stop the virus’s spread. But scrubbing surfaces does little to mitigate the virus threat indoors, experts say, and health officials are being urged to focus instead on improving ventilation and filtration of indoor air.

“In my opinion, a lot of time, energy and money is being wasted on surface disinfection and, more importantly, diverting attention and resources away from preventing airborne transmission,” said Dr. Kevin P. Fennelly, a respiratory infection specialist with the United States National Institutes of Health.

Some experts suggest that Hong Kong, a crowded city of 7.5 million residents and a long history of infectious disease outbreaks, is a case study for the kind of operatic surface cleaning that gives ordinary people a false sense of security about the coronavirus.

The Hong Kong Airport Authority has used a phone-booth-like “full-body disinfection channel” to spritz airport staff members in quarantine areas. The booth — which the airport says is the first in the world and is being used in trials only on its staff — is part of an all-out effort to make the facility a “safe environment for all users.”

Such displays can be comforting to the public because they seem to show that local officials are taking the fight to Covid-19. But Shelly Miller, an expert on aerosols at the University of Colorado Boulder, said that the booth made no practical sense from an infection-control standpoint.

Viruses are emitted through activities that spray respiratory droplets — talking, breathing, yelling, coughing, singing and sneezing. And disinfecting sprays are often made from toxic chemicals that can significantly affect indoor air quality and human health, Dr. Miller said.

“I can’t understand why anyone would think that disinfecting a whole person would reduce the risk of transmitting virus,” she said.

A range of respiratory ailments, including the common cold and influenza, are caused by germs that can spread from contaminated surfaces. So when the coronavirus outbreak emerged last winter in the Chinese mainland, it seemed logical to assume that these so-called fomites were a primary means for the pathogen to spread.

Studies soon found that the virus seemed to survive on some surfaces, including plastic and steel, for up to three days. (Studies later showed that much of this is likely to be dead fragments of the virus that are not infectious.) The World Health Organization also emphasized surface transmission as a risk, and said that airborne spread was a concern only when health care workers were engaged in certain medical procedures that produce aerosols.

But scientific evidence was growing that the virus could stay aloft for hours in tiny droplets in stagnant air, infecting people as they inhaled — particularly in crowded indoor spaces with poor ventilation.

In July, an essay in The Lancet medical journal argued that some scientists had exaggerated the risk of coronavirus infection from surfaces without considering evidence from studies of its closely related cousins, including SARS-CoV, the driver of the 2002-03 SARS epidemic.

“This is extremely strong evidence that at least for the original SARS virus, fomite transmission was very minor at most,” the essay’s author, the microbiologist Emanuel Goldman of Rutgers University, said in an email. “There is no reason to expect that the close relative SARS-CoV-2 would behave significantly different in this kind of experiment,” he added, referring to the new coronavirus.

From Nairobi to Milan to Seoul, cleaners in hazmat suits have been fumigating public areas despite W.H.O. warnings that the chemicals could do more harm than good.

In Hong Kong, where 299 people died during the original SARS epidemic, elevator buttons are often covered in plastic that is cleaned multiple times a day. Crews in some office buildings and subways wipe escalator handrails with disinfected rags as commuters ascend. Cleaners have blasted public places with antimicrobial coatings and added a fleet of robots to clean surfaces in subway cars.

Several Hong Kong-based scientists insist the deep cleaning can’t hurt, and supported the government’s strict social-distancing rules and its monthslong insistence on near-universal mask wearing.

Procter & Gamble said sales of its personal cleansing products grew more than 30 percent in the quarter that ended in September, with double-digit growth in every region of the world, including more than 20 percent in greater China.

“People are removing masks for lunch or when they get back to their cubicle because they assume their cubicle is their private space,” said Yeung King-lun, a professor of chemical and biological engineering at the Hong Kong University of Science and Technology.

“But remember: The air you’re breathing in is basically communal.”

Mike Ives reported from Hong Kong, and Apoorva Mandavilli from New York.

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