Influenza Virus Aerosols in Human Exhaled Breath: Particle Size, Culturability, and Effect of Surgical Masks

• Published in: PLoS pathogens Vol. 9; no. 3; p. e1003205
• Main Authors: Milton, Donald K., Fabian, M. Patricia, Cowling, Benjamin J., Grantham, Michael L., McDevitt, James J.
• Format: Journal Article
• Language: English
• Published: United States Public Library of Science 01.03.2013; Public Library of Science (PLoS)
• Subjects: Aerosols; Air Microbiology; Chemistry; Cough - virology; Cross Infection - prevention & control; Cross Infection - virology; Disease control; Disease transmission; Exhalation; Health aspects; Humans; Influenza; Influenza, Human - transmission; Masks; Medicine; Orthomyxoviridae - physiology; Particle Size; Physiological aspects; Respiration; RNA, Viral; Studies; Virus Shedding; United States
• ISSN: 1553-7374; 1553-7366; 1553-7374
• DOI: 10.1371/journal.ppat.1003205

The CDC recommends that healthcare settings provide influenza patients with facemasks as a means of reducing transmission to staff and other patients, and a recent report suggested that surgical masks can capture influenza virus in large droplet spray. However, there is minimal data on influenza virus aerosol shedding, the infectiousness of exhaled aerosols, and none on the impact of facemasks on viral aerosol shedding from patients with seasonal influenza. We collected samples of exhaled particles (one with and one without a facemask) in two size fractions ("coarse">5 µm, "fine"≤5 µm) from 37 volunteers within 5 days of seasonal influenza onset, measured viral copy number using quantitative RT-PCR, and tested the fine-particle fraction for culturable virus. Fine particles contained 8.8 (95% CI 4.1 to 19) fold more viral copies than did coarse particles. Surgical masks reduced viral copy numbers in the fine fraction by 2.8 fold (95% CI 1.5 to 5.2) and in the coarse fraction by 25 fold (95% CI 3.5 to 180). Overall, masks produced a 3.4 fold (95% CI 1.8 to 6.3) reduction in viral aerosol shedding. Correlations between nasopharyngeal swab and the aerosol fraction copy numbers were weak (r = 0.17, coarse; r = 0.29, fine fraction). Copy numbers in exhaled breath declined rapidly with day after onset of illness. Two subjects with the highest copy numbers gave culture positive fine particle samples. Surgical masks worn by patients reduce aerosols shedding of virus. The abundance of viral copies in fine particle aerosols and evidence for their infectiousness suggests an important role in seasonal influenza transmission. Monitoring exhaled virus aerosols will be important for validation of experimental transmission studies in humans.