Detection of infectious influenza virus in cough aerosols generated in a simulated patient examination room

Abstract

Background: The potential for aerosol transmission of infectious influenza virus (ie, in healthcare facilities) is controversial. We constructed a simulated patient examination room that contained coughing and breathing manikins to determine whether coughed influenza was infectious and assessed the effectiveness of an N95 respirator and surgical mask in blocking transmission.

Methods: National Institute for Occupational Safety and Health aerosol samplers collected size-fractionated aerosols for 60 minutes at the mouth of the breathing manikin, beside the mouth, and at 3 other locations in the room. Total recovered virus was quantitated by quantitative polymerase chain reaction and infectivity was determined by the viral plaque assay and an enhanced infectivity assay.

Results: Infectious influenza was recovered in all aerosol fractions (5.0% in >4 μm aerodynamic diameter, 75.5% in 1-4 μm, and 19.5% in <1 μm; n = 5). Tightly sealing a mask to the face blocked entry of 94.5% of total virus and 94.8% of infectious virus (n = 3). A tightly sealed respirator blocked 99.8% of total virus and 99.6% of infectious virus (n = 3). A poorly fitted respirator blocked 64.5% of total virus and 66.5% of infectious virus (n = 3). A mask documented to be loosely fitting by a PortaCount fit tester, to simulate how masks are worn by healthcare workers, blocked entry of 68.5% of total virus and 56.6% of infectious virus (n = 2).

Conclusions: These results support a role for aerosol transmission and represent the first reported laboratory study of the efficacy of masks and respirators in blocking inhalation of influenza in aerosols. The results indicate that a poorly fitted respirator performs no better than a loosely fitting mask.

Figure 1

Three-dimensional view of the aerosol exposure chamber. National Institute for Occupational Safety and Health samplers collected aerosols through the mouth (depicted as black oval in breathing mannequin's head), 10 cm beside the mouth of the breathing simulator, and in 3 other positions (P1, P2, P3), as shown. The mouths of the coughing and breathing simulators and sampler inlets at P1, P2, and P3 were located 152 cm above the floor (approximate height of a sitting patient and healthcare worker). For 3 experiments, fingertips from medical gloves were also placed on the manikin's forehead and alongside 3 of the aerosol samplers. All dimensions adjacent to white arrows within chamber are in centimeters.

Figure 2

Detection of infectious influenza on aerosolized particles. National Institute for Occupational Safety and Health (NIOSH) samplers drew aerosol samples from a port located ~1 mm above the mouth (through mouth) of the breathing simulator, 10 cm to the right of the mouth (beside mouth), and at 3 other positions (P1, P2, P3) within the environmental chamber. The amount of influenza virus detected in each fraction (>4, 1–4, and <1 µm) collected by the NIOSH sampler per liter of air collected is shown. A, B, Amounts of total virus (infectious and noninfectious) collected in each fraction was determined by quantification of the matrix gene by quantitative polymerase chain reaction (qPCR) (A) and by the plaque-forming unit assay (B). C, Viral replication assay (VRA) demonstrated the amount of infectious virus collected after amplification in Madin-Darby canine kidney cells to increase the sensitivity of detection. Data are means ± standard errors (n = 5).

Figure 3

Tightly fitting (sealed) N95 respirators efficiently block exposure to airborne infectious influenza. An N95 respirator was sealed over the mouth of the breathing mannequin with silicone caulk. Amounts of infectious and noninfectious virus collected are as described for Figure 2. Data are means ± standard errors (n = 3); qPCR, quantitative polymerase chain reaction; VRA, viral replication assay.

Figure 4

Tightly fitting (sealed) surgical masks efficiently block exposure to airborne infectious influenza. A surgical mask was sealed over the mouth of the breathing manikin with silicone caulk. The amount of infectious and noninfectious virus collected is as described for Figure 2. Data are means ± standard errors (n = 3); qPCR, quantitative polymerase chain reaction; VRA, viral replication assay.

Figure 5

Poorly fitting (unsealed) N95 respirators are less efficient at blocking exposure to airborne infectious influenza. An N95 respirator was fitted over the mouth of the breathing manikin with the mask's tie straps. The amount of infectious and noninfectious virus collected is as described for Figure 2. Data are means ± standard errors (n = 3); qPCR, quantitative polymerase chain reaction; VRA, viral replication assay.

Figure 6

Loosely fitting (unsealed) surgical masks are less efficient at blocking exposure to airborne infectious influenza. A surgical mask was fitted over the mouth of the breathing manikin with the mask's tie straps. The amount of infectious and noninfectious virus collected is as described for Figure 2. Data are means ± standard errors (n = 2); qPCR, quantitative polymerase chain reaction; VRA, viral replication assay.