Questions raised about study on masks as aerosol barrier

Feb 28, 2007 (CIDRAP News) – A recent study has raised expectations that simple surgical masks might offer a reasonably good substitute for N-95 respirators for healthcare workers seeking protection from airborne viruses, but others say the study is seriously flawed.

In the study, Y. Li and associates, from Hong Kong Polytechnic University, measured how much of a potassium chloride spray penetrated masks and respirators worn by people walking on a treadmill. They found that the surgical masks filtered out at least 95% of the material, while the N-95 respirators filtered out 97%.

This suggested that "surgical masks and N-95 respirators can provide effective protection in a relatively low viral loading environment," says the report, published in the December 2006 issue of the American Journal of Industrial Medicine. Such a finding, if true, could be a blessing for healthcare facilities during an influenza pandemic, since masks are less costly than respirators and likely to be in better (though still limited) supply.

The Li study drew more attention when it was described in a Feb. 15 report by the Clinicians' Biosecurity Network, part of the Center for Biosecurity at the University of Pittsburgh Medical Center (UPMC). In that report, Erick Toner, MD, of UPMC wrote, "The current study, supported by the clinical observations from the SARS epidemic, provides evidence that a surgical mask may provide significant protection from aerosols as well as droplets."

While affirming that N-95s should be used in high-risk settings, Toner wrote that if N-95s are not available, "use of a surgical mask along with other routine barriers (gown, gloves, and goggles) may afford significant protection from infection, especially in low risk settings."

However, experts at 3M Co. in St. Paul, which makes both surgical masks and N-95 filtering facepiece respirators, told CIDRAP News the Li study has serious methodologic defects that undermine its conclusions. The researchers did not assess the size of the particles used or employ an accepted method for counting the particles sprayed onto the mask and the particles that penetrated it, among other problems, the 3M experts said.

Questions also were raised by Raymond Tellier, MD, a University of Toronto microbiologist who says he is not an expert on aerosols but who has monitored and written about research on the risk of airborne transmission of flu viruses. Tellier said the Hong Kong researchers did not appear to use methods consistent with those recommended by the US National Institute of Occupational Safety and Health (NIOSH).

Surgical masks are designed mainly to protect other people from respiratory droplets or particles expelled by those wearing the masks, whereas N-95 respirators are designed to reduce the wearer's exposure to potentially harmful particles in the surrounding air. N-95 filtering facepiece respirators are designed to be sealed against the face to limit exposure to airborne particles, and manufacturers recommend testing the fit. Surgical masks, in contrast, typically allow significant air leakage around the edges.

Whether healthcare workers should use masks or respirators to protect themselves from flu viruses has been increasingly debated in the face of the threat of a pandemic. Scientific evidence is unclear on how long flu viruses can remain in the air after being expelled in a cough or sneeze, though the traditional view has been that they usually come in relatively large droplets that quickly fall to the ground. Surgical masks are not designed to protect wearers from airborne particles.

Until last fall, the US Department of Health and Human Services's (HHS's) pandemic flu plan recommended that healthcare workers wear surgical masks during routine care of pandemic flu patients. But in view of the uncertainty about airborne transmission of flu, in October HHS issued new guidelines saying that N-95 respirator use is "prudent" for medical workers caring for pandemic flu patients.

Meanwhile, a committee of the Institute of Medicine of the National Academy of Sciences is currently examining evidence about masks, respirators, and other personal protective equipment for medical workers. The committee is expected to issue a report next September, an IOM official told CIDRAP News last week.

Hong Kong study protocol
The Hong Kong researchers used 10 volunteers to test the filtration effectiveness of an N-95 respirator made by 3M and a surgical mask made by Winner Medical Group of Hong Kong. In a series of tests, each volunteer wore a mask or a respirator while a solution containing potassium chloride and a fluorescent stain (Fluorescein) was sprayed at them from an atomizer 1 meter away. The tests were conducted with the volunteers at rest and walking on a treadmill at three different speeds.

The amount of material that penetrated the masks was measured in two ways. The researchers separated the used masks and respirators into layers, placed the layers in water, filtered out the fibers, and determined the weight of the potassium residue in the water. The results showed that 0.56 mg of potassium reached the innermost layer of the respirators, versus 0.79 mg of potassium for the surgical masks.

In the second measurement, the volunteers' faces were photographed, and the photos were used to assess the amount of Fluorescein stain on their faces, using a 7-point scale. The researchers said this qualitative test confirmed that the respirators were more efficient filters.

"The in-vivo filtration tests show that both N-95 respirators and surgical masks have 95% or greater filtration efficiency, and N-95 respirators have about 2% filtration efficiency higher than surgical masks," indicating that both can provide protection in "a relatively low viral-loading environment," the investigators wrote.

But they added, "It is important to note that 3-5% of the solution can still penetrate both types of masks, which may become critical when the viral loading is high." They also cautioned that they tested only one brand of respirator and one brand of mask, and the findings are limited to those types.

Methodologic weaknesses cited
Specialists at 3M evaluated the Hong Kong study at CIDRAP's request. "Overall, what was done by Li et al is not considered filtration efficiency testing and should not be used to evaluate filtration efficiency or compare media. They also did not use accepted methods for evaluating the fit to the face which is a very important component in reducing exposure to airborne particles," said Robert Weber, lab manager for technical service and regulatory affairs in 3M's Occupational Health and Environmental Safety division.

A key problem with the study is that the researchers didn't determine the size of particles that were generated by the atomizer they used, said Craig Colton, a 3M senior technical service specialist. "Filtration is very dependent on particle size, so if you're talking about efficiencies you need to know about particle size," Colton said in an interview. "Without that critical piece of information, it's hard to know what you're comparing."

"We know they used a nebulizer, but we don't know what particle size that nebulizer was designed to produce," he added. "Oftentimes they produce different particle sizes depending on the conditions under which they operate. Changing concentrations of different salts can change the particle size formed. They assumed they were testing in the most penetrating particle size range. This is not a safe assumption to make."

The Li study says that potassium chloride was used instead of sodium chloride as the test aerosol because NIOSH has said that potassium chloride content in human sweat is very low, which implies a reduced chance of confounding the results. But Colton said the standard NIOSH approach calls for sodium chloride.

He said the study also should have included a control group of volunteers wearing the masks and doing the same exercises but not being sprayed with the test solution. That would have made it possible to exclude the chance that potassium chloride from other sources, such as the volunteers' sweat or the masks themselves, would affect the results.

The use of a fluorescent stain to test the mask and respirator was also questioned by 3M. Weber commented via e-mail, "The use of scanning subjects with fluorescent light to determine the effectiveness of filtration is not a proven method to determine either filtration efficiency or fit. This method is not quantitative and does not take into account particles that do not settle onto the face (those that are deposited in the lung and those that remain airborne). This method only demonstrates that particles are entering the mask, most likely due to significant faceseal leaks."

Colton said the fluorescent stain method has never been calibrated to determine what degree of particle penetration is associated with what degree of staining.

The report by Li and colleagues says that the volunteers "followed the manufacturer's instructions including the pressure tightness test" when donning the N-95 respirator. However, Weber of 3M said in his e-mail, "Fit testing was not done prior to the testing to determine if the wearers had a good fit and would be qualified to wear the N-95 respirator."

A 2006 study published in the Journal of Hospital Infection exemplified a more accepted method of testing the effectiveness of respiratory protection, according to Colton. In that study, investigators from the Chinese University of Hong Kong used commercially available instruments to count the particles inside and outside three different types of protective equipment worn by volunteers: surgical masks, "laser masks" (special masks used during laser surgery), and a respirator called an FFP2, similar to an N-95. A particle generator was used to produce sodium chloride particles of a known size range. The researchers found that the respirator provided significantly better protection than either type of surgical mask, though the laser mask was marginally better than the other mask.

Toronto microbiologist raises questions
Tellier, a microbiologist at Toronto's Hospital for Sick Children and associate professor at the University of Toronto, also raised questions about the methods of Li and colleagues, though he took pains to point out that he is not an expert on filter testing. Tellier wrote an article in Emerging Infectious Diseases last year urging US and Canadian health agencies to strengthen their recommendations on respiratory protection for medical workers in a flu pandemic.

Commenting by e-mail, Tellier said the NIOSH test method for respiratory protection calls for using sodium chloride particles in a particular size range and an airflow rate of 85 liters per minute, because the difference between respirators and surgical masks diminishes at lower airflow rates. It's not clear whether Li and colleagues used that prescribed flow rate, he said.

Tellier also suggested that the masks in the Li study were hit by droplets, not solid particles, which would change the results. Spraying a potassium chloride solution from 1 meter away produces droplets of liquid "in the aerosol [size] range," but "if they remain liquid until they hit the mask, what we are measuring . . . is the penetration of the mask by an aqueous solution, which can be repelled in part by waterproofing and the remaining liquid would adsorb into the fabric; the latter would not occur with 'solid' particles.

"My understanding is that testing with solid particles is more in keeping with aerosol protection and this is what NIOSH recommends," Tellier stated.

See also:

Li Y, Wong T, Chung J, et al. In vivo protective performance of N-95 respirator and surgical facemask. Am J Ind Med 2006 Dec;49(12):1056-65 [Abstract]

Derrick JL, Li PTY, Tang SPY, et al. Protecting staff against airborne viral particles: in vivo efficiency of laser masks. J Hos Infect 2006 Nov;64(3):278-81

Oct 18, 2006, CIDRAP News story "HHS backs respirator use in caring for pandemic flu patients"

Sep 29, 2006, story "Airborne flu viruses threaten health workers, expert says"

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