When a new virus blasts out of the animals that harbored it and into people, experts can usually say, thank goodness it’s not like measles. That virus is more contagious than any others known to science: Each case of measles causes an astronomical 12 to 18 new cases, compared to about six for polio, smallpox, and rubella. Each case of the new coronavirus is estimated to cause two to three others.

The reason the measles is so, well, viral, is that the microbe is so small and hardy that it is able to stay suspended in the air where an infected person coughed or sneezed for up to two hours, making it one of the only viruses that can exist as a true aerosol.

Now there are conflicting reports on whether the new coronavirus can. The studies suggesting that it can be aerosolized are only preliminary, and other research contradicts it, finding no aerosolized coronavirus particles in the hospital rooms of Covid-19 patients.

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The weight of the evidence suggests that the new coronavirus can exist as an aerosol — a physics term meaning a liquid or solid (the virus) suspended in a gas (like air) — only under very limited conditions, and that this transmission route is not driving the pandemic. But “limited” conditions does not mean “no” conditions, underlining the need for health care workers to have high levels of personal protection, especially when doing procedures such as intubation that have the greatest chance of creating coronavirus aerosols. “I think the answer will be, aerosolization occurs rarely but not never,” said microbiologist and physician Stanley Perlman of the University of Iowa. “You have to distinguish between what’s possible and what’s actually happening.”

There are two ways a coronavirus can be transmitted via air.

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In droplet form, the coronavirus is airborne for a few seconds after someone sneezes or coughs. It’s able to travel only a short distance before gravitational forces pull it down. Someone close enough for the virus particles to reach in that brief period can therefore be infected. So can anyone who comes into contact with virus-containing droplets that fall onto a surface. The new coronavirus can survive on surfaces for several hours; hence the importance of hand-washing after touching a surface in a public place.

An aerosol is a wholly different physical state: Particles are held in the air by physical and chemical forces. Fog is an aerosol; water droplets are suspended in air. The suspended particles remain for hours or more, depending on factors such as heat and humidity. If virus particles, probably on droplets of mucus or saliva, can be suspended in air for more than a few seconds, as the measles virus can, then anyone passing through that pathogenic cloud could become infected.

There are strong reasons to doubt that the new coronavirus has anything close to that capability.

“If it could easily exist as an aerosol, we would be seeing much greater levels of transmission,” said epidemiologist Michael LeVasseur of Drexel University. “And we would be seeing a different pattern in who’s getting infected. With droplet spread, it’s mostly to close contacts. But if a virus easily exists as an aerosol, you could get it from people you share an elevator with.”

According to the Centers for Disease Control and Prevention, that is not happening. Earlier this month, CDC scientists reported that the rate of symptomatic infection among a patient’s household members was 10.5%. The rate among other close contacts was 0.45%. In the case of one particular patient, none of his five household members, although continuously exposed to the patient during the time he was isolated at home, tested positive for the virus.

Even if the virus infects only a small fraction of those who come into contact with it, the extremely low rate among close contacts and the absence of infections in some household members of patients suggests that it rarely exists as an aerosol in most real-world situations.

“It’s more evidence that [Covid-19] is predominantly spread through droplets and not as an aerosol,” LeVasseur said.

Physical evidence bolsters that epidemiological reasoning. When researchers in Singapore tested the air in the rooms of three Covid-19 patients, they found no virus particles on cleaned surfaces or in the air even when they took samples on days the patients were symptomatic and presumably shedding virus into the air, they reported this month in the Journal of the American Medical Association. In the room of the third patient, who shed more virus, virus particles were present on ventilation fans and numerous surfaces — but all air samples were negative.

That suggests that aerosolized virus particles are, at worst, rare in real-world conditions.

A study by virologist Ke Lan of Wuhan University and his colleagues found that “rare” does not mean “never,” however. They took 35 air samples at two hospitals as well as public areas in Wuhan, where the Covid-19 outbreak apparently started. They found no coronavirus in intensive care areas where Covid-19 patients were being treated, in general patient rooms, in hallways, or outside the hospitals.

But coronavirus aerosols were found near patients’ toilets in Wuchang Fangcang Field Hospital. That wasn’t a total shock: Receptors for coronavirus exist not only in the airways but also in the gastrointestinal tract, so cells there can become infected, shedding virus into fecal material. The paper, posted to a preprint site, has not been peer-reviewed.

“The virus aerosol,” the Wuhan scientists concluded, “is a potential transmission pathway.” Since aerosols can come directly from patients as well as from stirring up droplets that landed on surfaces, “effective sanitization is critical in minimizing aerosol transmission of SARS-CoV-2,” the Covid-19-causing coronavirus.

Because real-world studies like these have numerous confounding conditions, scientists at the National Institute of Allergy and Infectious Diseases examined what can happen under controlled lab conditions, although somewhat artificial, worst-case ones.

NIAID virologist Vincent Munster and his colleagues used a nebulizer — a device that creates an aerosol from liquids — to release samples into the air of both the new coronavirus and the one that caused the SARS outbreak in the early 2000s. They reported detecting viable virus in aerosols for up to three hours. That compared to the four hours that active virus particles were found on copper, up to 24 hours on cardboard, and up to two or three days on plastic and stainless steel.

Both the Covid-19 and the SARS viruses had an aerosol half-life of 1.1 hours, meaning half the particles drop out of the air after that amount of time, and half of what remains drop out after another 1.1 hours. After a day, roughly nine half-lives, 0.002 (0.2 of 1%) of the original particles remain. As a result, the scientists said, “aerosol … transmission of [the new coronavirus] is plausible, since the virus can remain viable and infectious in aerosols for hours.”

Scientists not involved in the study raised a couple of concerns: whether the mechanical nebulizer simulates coughs or sneezes, and whether the lab conditions reflect the real world.

The NIAID study “is measuring virus under ideal conditions and with a lot of virus,” said microbiologist Benjamin tenOever of the Icahn School of Medicine at Mount Sinai. “So their results are all likely to be overestimates. That said, I think those values should at least be used to let people know that things like subway poles can harbor virus for more time than I would have considered possible,” because an aerosol that encounters a solid object can stick to it. “Washing hands is more important than ever.”

“We’ve seen no evidence that aerosolized virus is the primary transmission risk for everyday people in everyday settings,” said Dylan Morris of Princeton University, a co-author of the study. “One should not rule anything out categorically with a novel, still-poorly-understood virus, [but] based on what we know about coughing and sneezing, one should be cautiously optimistic that aerosolization may not play a big role in everyday transmission.”

Iowa’s Perlman said the nebulizer may mimic what occurs during procedures such as intubation. That’s why the World Health Organization recommends the use of respirators, gowns, and other extensive protection for medical workers performing such procedures on Covid-19 patients.

Doctors at Kaiser Permanente in California, in a paper published in JAMA on Friday, also note that while the new coronavirus is “primarily spread by droplets,” certain medical procedures can make it airborne — as an aerosol — and therefore require extra protection for health care workers and, ideally, negative pressure rooms.

The behavior of the SARS virus during the 2003 epidemic offers some clues about any risk from aerosol forms of its cousin.

One puzzling outbreak, with 329 cases, occurred at a Hong Kong apartment complex whose residents had not been in close contact with each other. A 2014 analysis concluded that “airborne spread was the most likely explanation, and the SARS coronavirus could have spread over a distance of 200 meters,” or about 600 feet, apparently starting with a SARS patient who had diarrhea.

The 329 Hong Kong cases, another analysis concluded, suggest that although SARS was primarily spread by droplets, “to a much limited degree [it was also spread] by aerosols.” The aerosolization likely originated “from malfunctioning sewers in the building.”

The much-discussed hope that warmer, more humid weather will strangle the Covid-19 pandemic may or may not pan out, but there are solid data that it will make a difference to any aerosol transmission. The SARS virus survived better at 30% to 50% relative humidity than at 80%, with a half-life of only three hours rather than 27 hours at 30% humidity. Other research has also found that coronaviruses have much more difficulty existing in aerosol form in warm, humid conditions.

That reflects the fact that the SARS virus has an “envelope” that falls apart in warmer, more humid conditions. The new coronavirus has a similar envelope.

The World Health Organization has studied the emerging data on coronavirus aerosols to see if it needs to change its current recommendations, including that that healthy people do not need face masks. “, and healthcare workers should take extra precautions during procedures that can generate aerosols. “From the available studies that we have seen, we are confident that the guidance we have is appropriate,” Maria Van Kerkhove, WHO’s technical lead for the coronavirus response, told reporters on Monday.

This story has been updated with response from the WHO and calculations.