Apr 3, 2012 (CIDRAP News) – A new laboratory study supports the long-debated view that airborne viruses play a role in spreading influenza and that N95 respirators provide the best protection against airborne viruses, with surgical masks affording much less.

A US-Chinese team of researchers created a simulated examination room with mechanized mannequins spaced 6 feet apart to represent coughing and breathing humans. They found that flu viruses floated between the two and were "inhaled" by the breathing mannequin, but that an N95 respirator sealed to the mannequin's face stopped 99.8% of them.

A poorly fitted respirator or a loose-fitting surgical mask, by contrast, blocked only about two thirds of the virus particles.

The researchers say their findings suggest that "anyone present in a room with a patient who has influenza might be at risk of exposure" and that properly fitted N95 respirators provide maximal protection. Their report is an early online publication in Clinical Infectious Diseases.

Current guidance from the US Centers for Disease Control and Prevention (CDC) says that flu viruses spread primarily via virus-laden large droplets from coughs and sneezes, but that these generally travel in the air only about 3 feet.

As noted in an editorial accompanying the new study, the role of smaller aerosol particles that travel farther is uncertain and controversial, which has engendered confusion about the best infection control measures for healthcare workers.

The current CDC guidance for seasonal flu says that healthcare personnel should wear a surgical or procedure mask when in close contact (within 3 feet) of a patient who has symptoms of a respiratory infection. The guidance does not mention N95 respirators. During the 2009 flu pandemic, the CDC recommended that healthcare workers use N95 respirators in high-risk situation, such as aerosol-generating procedures.

Mannequin methodology

The new study was conducted by scientists from the CDC's National Institute of Occupational Safety and Health (NIOSH), West Virginia University in Morgantown, and China's Shen Zhen University. John D. Noti, PhD, of NIOSH is the first author.

In the experiment, the mannequins were placed in a room about 9 feet square. A nebulizer was used to aerosolize flu viruses, and a mannequin paired with a cough simulator "coughed" the virus preparation into the air five times at 2-minute intervals. The other mannequin, 6 feet from and facing the first one, was equipped with a breathing simulator.

In different phases of the study, a surgical mask or an N95 respirator was either tightly sealed over the mouth of the breathing mannequin with silicone sealant or simply attached with straps or elastic headbands. The latter two conditions were used to represent poorly fitted respirators and the usual way surgical masks are worn.

For an hour after the start of the coughing, NIOSH air samplers were used to collect samples at the breathing mannequin's mouth, a few inches to one side, and at three other points in the room, each a few feet from the mannequins. The samplers sorted the collected aerosols by diameter: more than 4 microns, 1 to 4 microns, and less than 1 micron.

The researchers determined that the average amount of virus collected by the five samplers over several runs of the experiment was about 13,500 copies per liter of air. About 75% of the viral copies came in particles in the 1- to 4-micron range, which are said to be small enough to stay airborne for a long time. Only about 5% of the virus copies were found in particles larger than 5 microns.

Regardless of the size of the particles involved, some of the viruses collected by all the samplers remained infectious after their air travel, the researchers found. On average, the report explains, only 6.3% of the virus was actually infectious before aerosolization, and this dropped to 2.2% after collection by the samplers.

Sealed respirator was highly effective

The team assessed the protectiveness of respirators and masks by comparing the volumes of virus collected by the sampler at the mannequin's mouth and the one a few inches to one side. In this way they determined that a respirator tightly sealed to the mannequin blocked 99.8% of viruses, including 99.8% of those that remained infectious.

Similarly, a tightly sealed surgical mask stopped 94.5% of the viral traffic, including 94.8% of the infectious viruses, the report says.

When a poorly fitted (unsealed) respirator was used, it blocked 69.9% of flu viruses from entering the mannequin's mouth, including 66.5% of infectious viruses, the team found. And a loose-fitting mask stopped 68.9% of the viral volume, including 56.6% of infectious viruses.

In this case, however, a second, enhanced infectivity assay showed that only 11.6% of infectious viruses were stopped by the mask. (For the other conditions, the enhanced assay agreed fairly closely with the primary assay.)

The authors comment that clinical studies so far have failed to clarify the relative importance of aerosol flu transmission and the need for using N95 respirators to prevent it. A large cluster-randomized trial in China, published in 2011, they note, suggested that respirators were more protective than masks, but the study was statistically underpowered to show that clearly.

The authors say their findings on the ineffectiveness of poorly fitted respirators and unsealed masks show that gaps between the wearer's face and the mask or respirator can have a "tremendous impact" on the protection afforded.

They add that the sealed respirators they used showed a filtering ability ("fit factor") similar to that of well-fitted respirators, and the unsealed masks had fit factors similar to those of masks in real-world use. "Our results support the use of properly fitted N95 respirators for maximal protection against infectious airborne influenza," they state.

In addition, they caution that the performance of the sealed masks in the study should not be interpreted to mean that masks can be relied on for respiratory protection. The filtration capacity of surgical masks varies tremendously by model, and much air leaks around them even when they are tied on tightly, they explain.

Surviving air travel

In the accompanying editorial, Benjamin J. Cowling, BSc, PhD, writes that the study demonstrates that "artificially aerosolized virus can remain viable while traveling across a room," though the air flow in the room, which could have affected the amount of virus reaching the breathing mannequin, was not reported. Cowling is with the School of Public Health, Li Ka Shing Faculty of Medicine, at the University of Hong Kong.

In the real world, Cowling adds, coughs and sneezes could aerosolize flu virus, but more research is needed to confirm whether the characteristics would be similar to the artificially aerosolized virus used in the study. He says the finding that viable aerosolized virus can float at least 6 feet may have implications for bed spacing.

Noting that unsealed masks and respirators stopped about two thirds of virus flow in the study, Cowling writes that the findings "suggest that a surgical mask could provide some degree of protection against exposures with low infectious doses, but a properly fitted N95-type respiratory would provide improved protection."

He concludes that the study improves the understanding of flu transmission and control, but asserts that large controlled studies in real-life settings will probably be needed "to confirm superior efficacy of N95 respirators over surgical masks in healthcare settings."

Lisa M Brosseau, ScD, a longtime researcher of respiratory protection and an associate professor at the University of Minnesota School of Public Health in Minneapolis, commented that many surgical masks have lower filtering capacity than the model used in the study, which could lead some to misinterpret the findings.

"Many surgical masks have very low filter efficiency, which would lead to even lower fit factors and less protection than measured in this study," she commented by e-mail. "These facts are addressed in the discussion, but I hope that someone reading the abstract will NOT be led to conclude that a surgical mask is just as good as an unfitted respirator.

"Neither of these provided much protection—but I would expect even an unfitted respirator to provide more protection than a surgical mask," she added. "I think their data for the viral replicate analyses illustrate this, where the poorly fitted respirator blocked 66% of the total infectious virus while the surgical mask blocked only 11%. This is likely a result of the differences in the efficiencies of the filters (not the fit)."

Noti JD, Lindsley WG, Blachere FM, et al. Detection of infectious influenza virus in cough aerosols generated in a simulated patient examination room. Clin Infect Dis 2012 (early online publication). [Abstract]

Cowling BJ. Airborne transmission of influenza: implication for control in healthcare and community settings. (Editorial) Clin Infect Dis 2012 (early online publication). [Excerpt]

See also:

CDC guidance on the use of masks to prevent flu transmission

Oct 14, 2009, CIDRAP News story on CDC guidance on use of N95 respirators during the 2009 pandemic