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Surgical Masks May Provide Significant Aerosol Protection

By Eric Toner, M.D., February 15, 2007

The relative protection afforded by surgical and N-95 masks is an important issue that is prompting much debate in the process of planning for outbreaks of infectious respiratory diseases such as SARS or an influenza pandemic. In an article published recently in the American Journal of Industrial Medicine, Y. Li and colleagues from The Hong Kong Polytechnic University report the results of their study comparing the in vivo protective performance of surgical masks and N95 respirators [1]. The authors found that N95 respirators filtered out 97% of a test aerosol while surgical masks did almost as well, filtering out 95% of the aerosol.


The authors used a KCl-fluorescein solution aerosol as a viral simulant. KCl is the test challenge aerosol recommended by the National Institute for Occupational Safety and Health (NIOSH), and fluorescein was added as a visual marker to gauge the degree of KCl penetration. Each of 10 subjects (half men and half women) was tested wearing each kind of mask. The masks were fitted properly, and the subjects were tested at rest and while walking on a treadmill up to 6.4 km/hr (4 miles/hour). The KCl solution was sprayed on the mask twice every 10 minutes, for a total of 14 times, from a distance of 1 meter away using an atomizer.

The degree of filtration of the challenge aerosol was measured in two ways. First, the concentration of KCl in the 4 layers of the exposed N95 and the 3 layers of the exposed surgical mask was determined. Second, the degree of fluorescein staining on the portion of a subject’s face covered by the mask was quantified.


The estimated size of the most penetrating aerosol particles reaching the mask was 0.1-0.3µm. By each method, the N95 performed significantly better than the surgical mask, but the difference was small (2%).

Droplet vs. Aerosol

Surgical masks have long been the recommended respiratory protection for diseases transmitted by large droplets (>5µ) such as plague or meningococcal meningitis. For diseases such as TB that are transmitted by small droplet aerosols, an N95 (or better) respirator or powered air purifying respirators (PAPRs) with high efficiency particulate absorbing (HEPA) filter is recommended. It has been generally assumed that a surgical mask provided little protection from aerosols. With SARS and influenza, both droplet and aerosol transmission may occur, although the relative importance of each is debated.

During the SARS epidemic, in most circumstances, surgical masks were effective in protecting healthcare workers (HCW) from infection. In a case-control study of five hospitals in Hong Kong affected by SARS, W. H. Seto and colleagues found that consistent use of surgical masks was associated with a significant reduction in risk of infection. In fact, of 51 HCW with documented SARS exposure while wearing a surgical mask, none became infected. In contrast, 13 of 198 exposed HCWs (6.5%) who did not wear a surgical mask or N95 were infected. [2]

Similar results were found in Toronto during the SARS outbreak, where consistent use of surgical masks reduced the risk of infection by 50% among 32 critical care nurses who entered the room of a SARS patient. Consistent use of an N95 resulted in an 80% risk reduction. [3] 

The experience with the SARS epidemic clearly demonstrated a relationship between the risk of infection to HCWs and certain aerosol-generating medical procedures. The highest risk was associated with endotracheal intubation, airway suctioning, and non-invasive positive pressure ventilation [4]. In these settings, the highest degree of respiratory protection possible is warranted. 

Finite Supply of N95s

It has been argued that since influenza transmission occurs through both aerosols and large droplets [5], all personnel with direct patient contact should wear N95s or PAPRs. However, there are several problems with this approach: N95s cost approximately 10 times as much as surgical masks; they are much less comfortable to wear, especially for prolonged periods of time, which could result in under-utilization; and there is a finite national (and global) supply of N95s. The Strategic National Stockpile (SNS) has 105 million N95s on hand or on order [6]. Added to the stocks maintained by hospitals and distributors, this supply may be enough to provide an N95 for all HCWs in direct contact with flu patients during a relatively mild pandemic. This supply is probably not enough for all in a severe pandemic, in which case use of N95s would have to be restricted to those at greatest risk.

The HHS Guidelines

On October 17, 2006, the U.S. Department of Health and Human Services (HHS) released new interim guidelines for the use of surgical masks and respirators in an influenza pandemic. [7] These new interim guidelines are meant to augment and supersede previous recommendations located in Part 2 of the HHS Pandemic Influenza Plan ( This plan was released in November 2005 and may be updated or amended as new epidemiologic information becomes available.

Specifically, HHS now recommends that a properly fit-tested, NIOSH-certified N-95(or higher) respirator should be worn in lieu of surgical masks when:

  1. Participating in procedures that have a high likelihood of generating aerosolized particles (e.g., endotracheal intubation, nebulizer treatments, cardiopulmonary resuscitation, and bronchoscopy) performed on a patient with confirmed or suspected pandemic influenza.

  2. Providing direct care to patients with confirmed or suspected pneumonia due to pandemic influenza.  

Furthermore, HHS says that the use of an N95 is prudent in the direct care of all other patients with confirmed or suspected pandemic influenza if there is a sufficient supply.

With an insufficient supply of N95s, the guidelines recommend prioritization for use in high risk, aerosol generating situations. In addition, in the event of a shortage, other NIOSH-certified N, R, or P- class respirators or powered air purifying respirators (PAPRs) may be considered; however, re-usable respirators, such as PAPRs, must be decontaminated after each use. The guidelines reinforce that proper training in the use, removal, and disposal of respirators as well as the medical contraindications to respirator use is critical.

In the event that no respirators are available, the guidance points out that surgical masks will provide benefits against droplet exposure and should be worn by all HCW having direct contact with confirmed or suspected pandemic influenza patients.


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. In high risk settings, N95 respirators or PAPRs should be used if available. However, if N95s are not available, as might be expected in a severe pandemic, 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. Since surgical masks vary considerably in design, further study is needed to evaluate the aerosol filtering efficiency of various masks. The Institute of Medicine is hosting a meeting entitled “Workshop on Personal Protective Equipment for Healthcare Workers in the Event of Pandemic influenza: Next Steps and Research Agenda” on Feb. 22, 2007 in Washington, D.C. to explore these issues [].


  1. Li Y, Wong  T, Chung J , et al. In vivo protective performance of N95 respirator and surgical facemask, Am J Industrial Med 2006:49:1056 –1065.

  2. Seto W, Tsang D, Yung R, et al. Effectiveness of precautions against droplets and contact in prevention of nosocomial transmission of severe acute respiratory syndrome (SARS). Lancet 2003;361:1519-1520.  

  3. Loeb M, Mcgeer A, Henry B, et al. Emerg Infect Dis. 2004;10:251-255. Available online at Accessed February 8, 2007.

  4. Fowler R, Guest C, Lapinsky S, et al.  Transmission of Severe Acute Respiratory Syndrome during intubation and mechanical ventilation. Am J Respir Crit Care Med 2004;169:1198-1202. Available online at Accessed February 8, 2007.

  5. Tellier R. Review of aerosol transmission of influenza A virus. Emerg Infect Dis. 2006;12: Nov. Available from Accessed February 15, 2007.

  6. DHHS. Pandemic Planning Update III, A Report from Secretary Michael O. Leavitt November 13, 2006. Available online at Accessed February 8, 2007.

  7. Interim Guidance on Planning for the Use of Surgical Masks and Respirators in Health Care Settings during an Influenza Pandemic.