CDC: Use N95 respirators in event of pandemic flu

Airborne transmission trumps ineffective masks

Revising guidance on the use of surgical masks and respirators during a pandemic, the Centers for Disease Control and Prevention (CDC) now advises that it is "prudent" for health care workers to use N95 respirators during "direct care activities involving patients with confirmed or suspected pandemic influenza." Health care facilities should plan now to prioritize respirator use.

"Because the interventions available during seasonal influenza are absent during a pandemic, we're calling for this higher level of precautions," says Max Kiefer, MS, CIH, associate director for emergency preparedness with the National Institute for Occupational Safety and Health, which is part of the CDC.

However, the CDC doesn't recommend full airborne precautions with influenza. Isolation rooms are not required for routine patient care, but should be used "when performing high-risk aerosol-generating procedures," the guidance states.

"Strategically, [this] uncouples the use of N95 respirators with having to use a negative pressure room," says Deborah Levy, PhD, MPH, senior advisor for health care preparedness at the CDC in the Division of Healthcare Quality Promotion.

The CDC also recommends the use of other measures to limit the staff exposed to influenza, such as segregating patients in a particular ward and designating certain entrances and passageways for influenza patients. Receptionists could be protected by a transparent barrier, notes Kiefer.

With the new guidance, the CDC acknowledges short-range airborne transmission of influenza. "With influenza, particularly in the near range transmission, it's epidemiologically indistinguishable whether [transmission] is droplet or aerosol. The relative contributions of the different modalities are uncertain," says Kiefer.

The question of whether to use surgical masks or respirators with pandemic influenza created controversy and potential confusion. While the Department of Health and Human Services guidance previously called for surgical masks to protect against infectious droplets, some states, such as California and Minnesota, stated that respirators should be used.

"The lack of scientific consensus has led to conflicting recommendations by public health partners," the CDC guidance states. "Moreover, a large amount of incorrect, incomplete, and confusing information about surgical mask and respirator use has been disseminated on the Internet and by other popular media."

In their pandemic planning, hospitals should provide education to health care workers about when and if they will need to wear the respirators, says Levy. Widespread use of respirators among employees who have no direct contact with influenza patients may lead to a shortage — and those who need it most won't have the protection, she says.

Hospitals can include priorities for respirator use in their pandemic plans, but they will need to be flexible, she says. "Until the pandemic arrives, you really don't know what it's going to be like."

In a related development, researchers have found surgical masks were much less effective against airborne particles than N95s. In a study of elastomeric and N95 filtering-facepiece respirators and surgical masks, the surgical masks fared poorly, according to scientists with the National Institute for Occupational Safety and Health (NIOSH). The wearer would breathe in 53% of contaminated air, the researchers found.1 An N95 is designed to filter 90% of any air contaminant.

"Surgical masks don't provide much protection at all," says lead author Brent Lawrence, MS, an engineering technician at NIOSH in Morgantown, WV. "They don't have good fitting characteristics because they weren't designed to protect the wearer."

Elastomeric masks performed better than the N95 filtering-facepiece masks, but both were improved by fit-testing, the authors found.

Lawrence and his NIOSH colleagues tested 15 elastomeric N95 respirators, 15 N95 filtering-facepiece respirators, and six surgical masks. They found significant variation in the fit characteristics of the respirators and their performance with fit-testing.

When tested without fit-testing, only two of the N95 filtering-facepiece respirators typically used in hospitals had good inherent fit characteristics. Seven of the elastomeric respirators had good fit characteristics, while none of the surgical masks did.

The respirators varied widely in their fit-testing performance as well. Six of the N95 filtering-facepiece respirators failed to adequately fit anyone in the 25-person panel with at least one fit-test method. One filtering-facepiece respirator, the MSA FR200 Affinity, failed all fit-tests. All of the elastomeric respirators had at least some successful fit-tests.

"From this study and previous studies we've done, [we found that] not all respirators are created equal. Fit-testing is an important component of protection," says Chris Coffey, PhD, chief of the laboratory research branch.

In the hospital setting, that could translate into a lot of frustration over fit-testing. In fact, the authors suggest that employers might find more success with quantitative fit-testing because more respirators failed the qualitative fit-tests.

"Instead of doing one quantitative test, you're doing two or three qualitative tests," says Coffey. "You're rejecting good-fitting respirators [because of fit-test errors]."

NIOSH is developing minimum criteria for the fit characteristics of filtering-facepiece respirators, called Total Inward Leakage. That would lead to less variation in respirator performance, Coffey says.

Other findings may reflect on respirator purchasing decisions. The elastomeric respirators had adjustable head straps, while some N95 filtering-facepiece respirators had only elastic, nonadjustable straps. "Adjustable straps may allow a more customized seal (i.e., having one strap tighter than the other strap) resulting in greater protection," the authors said.

Another relevant study was recently published by Raymond Tellier, MD, MSc, FRCPC, CSPQ, a medical microbiologist at the Hospital for Sick Children in Toronto and associate professor at the University of Toronto. He argues in a review of the literature that there is plenty of evidence of airborne transmission of influenza, which would indicate the need for respiratory protection against pandemic influenza. Tellier notes that coughing and sneezing increase the number of particles that behave as aerosols, and that both animal and human experiments have demonstrated the infectivity of aerosolized influenza.

He also cites a study during the 1957-58 pandemic of a tuberculosis ward with ceiling-mounted UV lights meant to kill viruses in the upper air of the room and tuberculosis units without the lights. Although the same personnel treated the two groups of patients, and 18% of those health care workers contracted pandemic influenza, only 2% of the patients in the UV-irradiated rooms became infected with influenza. UV irradiation is not effective in inactivating viruses in fomites or droplet, Tellier notes.2 "Despite extensive searches, I have not found a study that proves the notion that large-droplet transmission is predominant and that aerosol transmission is negligible (or nonexistent)," he wrote.

While infection control practitioners argue that surgical masks have effectively stopped nosocomial influenza outbreaks, Tellier asserts that the evidence is actually "inconclusive." Lacking careful lab work, cases of influenza-like illness may be incorrectly attributed to influenza and asymptomatic infections among health care workers are not counted. Immunity and vaccination also may have played a role, he says.

"Given the strong evidence for aerosol transmission of influenza viruses, in general, and the high lethality of the current strains of avian influenza A [H5N1], recommending the use of N95 respirators, not surgical masks, as part of the protective equipment seems rational," he concludes.

The CDC acknowledged that conclusion by changing its pandemic influenza guidance on respirator use. In that document, it states: "In contrast to tuberculosis, measles, and varicella, the pattern of disease spread for seasonal influenza does not suggest transmission across long distances (e.g., through ventilation systems); therefore, negative pressure rooms are not needed for patients with seasonal influenza. However, localized airborne transmission may occur over short distances (i.e., 3 to 6 feet) via droplet nuclei or particles that are small enough to be inhaled. The relative contribution of short-range airborne transmission to influenza outbreaks is unknown."

The two papers were welcomed by those who have pressed for adequate respiratory protection for health care workers, particularly in pandemic influenza planning.

"I'm very pleased to see that a lot of these issues are coming to the fore," says Gabor Lantos, MD, PEng, MBA, president of Occupational Health Management Services in Toronto. "It's been very political."

Lantos notes that surgical masks are not respiratory protection — even if they provide some partial protection. "The epidemiology [infection control professionals] keep quoting is based on the existing influenza strains, against which most everybody has partial immunity," he says.

Bill Borwegen, MPH, occupational safety and health director for Service Employees International Union (SEIU), says health care worker advocates feel vindicated in their insistence on better protection against infectious aerosols.

"We never understood how you could justify the use of a loose-fitting surgical mask to protect people from any kind of airborne biological agent," he says. "We know from the science that there's no such thing as a purely droplet spread."

(Editor's note: The CDC's Interim Guidance on Planning for the Use of Surgical Masks and Respirators in Health Care Settings is available at


  1. Lawrence RB, Duling MG, Calvert CA, et al. Comparison of performance of three different types of respiratory protection devices. J Occup Environ Hyg 2006; 3:465-474.
  2. Tellier R. Review of aerosol transmission of influenza A virus. Emerg Infect Dis (serial on the Internet) October 2006. Available from