Catching the Flu: NIOSH Research on Airborne Influenza Transmission

Posted on by William G. Lindsley, PhD
A sneeze in progress. Need we say more? Cover your mouth!

As we enter another influenza season, one question continues to vex medical and public health professionals:  How do you stop people from catching the flu? The best way to prevent the flu is by getting an influenza vaccine every year. However, in the event of a large-scale influenza outbreak of a new virus strain or a pandemic, when influenza vaccine may not be promptly available, we will see tremendous demands on the health care system and its workers.  Thus, it’s critical to understand how influenza is transmitted from person to person so that we can determine the best ways to protect health care workers while still enabling them to do their jobs.

The typical incubation period for influenza is 1-4 days (average: 2 days). Adults shed influenza virus from the day before symptoms begin through 5-10 days after illness onset. However, the amount of virus shed, and presumably infectivity, decreases rapidly by 3-5 days after onset in an experimental human infection model. Young children also might shed virus several days before illness onset, and children can be infectious for 10 or more days after onset of symptoms. Severely immunocompromised persons can shed virus for weeks or months.

Experts think influenza may be spread to uninfected people in three ways: large-particle respiratory droplet transmission, airborne transmission, and contact (or fomite) transmission. Most experts think that influenza viruses are spread mainly by large-particle respiratory droplets produced when people infected with influenza cough, sneeze or talk. These droplets can land in the mouths or noses of people who are nearby or possibly be inhaled.  Transmission via large-particle droplets requires close contact between source and recipient persons, because droplets do not remain suspended in the air and generally travel only a short distance (usually less than 2 meters) through the air. Airborne transmission (via small particles suspended in the air for extended periods of time) also is thought to be possible, although data are limited. Less often, a person might also become infected with influenza by touching a surface or object that has influenza virus on it and then touching his or her own mouth or nose.

A better understanding of influenza transmission could help improve the infection control procedures and equipment used by health care workers. NIOSH has been conducting research for several years to learn more about the underlying science of influenza transmission, with a particular focus on airborne transmission and the protection of health care workers during a future pandemic. Here are some of the questions that are being addressed:

Is there a correlation between exposure to airborne influenza and illness among healthcare workers? A combination of three approaches is being used to better understand worker exposure to influenza and its consequences. First, aerosol sampling will be conducted in a health care clinic. Second, the amount of genetic material (RNA) from influenza virus on surgical masks, respirators and gloves worn by health care workers and on the surfaces of equipment and furniture in the clinic will be measured. Third, the health status and any occurrences of influenza among the workers in the study will be monitored. This project is part of a large multi-hospital study to compare the relative effectiveness of surgical masks and respirators in preventing influenza among health care workers.

Can better methods be developed to detect infectious airborne influenza virus?   The current methods for determining the infectivity of influenza aren’t sensitive enough to work with the small amounts of virus in a typical aerosol sample. NIOSH researchers are developing new, more sensitive methods of measuring influenza virus viability. One technique is a hybrid system called a “viral replication assay” that combines PCR with a more traditional culture method for increased sensitivity. A second technique uses genetically modified cells that glow faintly when they are infected with the influenza virus. This luminescence can be detected using standard laboratory equipment.

How well do different types of personal protective equipment perform under different exposure scenarios?  To explore this, NIOSH researchers constructed a simulated medical examination room containing a custom-built coughing machine that can cough an influenza-laden aerosol into the room much like a patient would, and a breathing machine that can simulate a health care worker treating the patient. The breathing machine can be outfitted with personal protective equipment (PPE), such as surgical masks, respirators, face shields, and powered air-purifying respirators (PAPRs). The simulated exam room is now being used to study how well different types of PPE and combinations of PPE protect from large spray droplets and small aerosol particles at shorter and longer distances.

Researchers at NIOSH have presented their work at scientific and public health conferences, and several articles have been published in peer-reviewed scientific journals. Information on NIOSH influenza research and influenza-related Health Hazard Evaluations can be found at and In addition, the CDC provides a comprehensive set of guidelines for preventing transmission of influenza in healthcare settings at

William G. Lindsley, PhD

Dr. Lindsley is a Research Biomedical Engineer in the NIOSH Health Effects Laboratory Division.


The Centers for Disease Control and Prevention is addressing questions related to the Coronavirus Disease 2019 through CDC-INFO and on their webpage. As such, this blog has been closed to comments. Please visit You can find the most up-to-date information on the outbreak and get the latest answers to frequently asked questions. If you have specific inquiries, please contact CDC-INFO at or by calling 800-232-4636. If you have questions about PPE that are not related to Coronavirus Disease 2019, please contact us at

Posted on by William G. Lindsley, PhD

35 comments on “Catching the Flu: NIOSH Research on Airborne Influenza Transmission”

Comments listed below are posted by individuals not associated with CDC, unless otherwise stated. These comments do not represent the official views of CDC, and CDC does not guarantee that any information posted by individuals on this site is correct, and disclaims any liability for any loss or damage resulting from reliance on any such information. Read more about our comment policy ».

    I cannot find literature on PAPR vs. N-95 masks. Do you have any links? I hear of studies from the 70s or 80s

    Thank you for your question. There were no N-95 respirators in the 1970-1980’s because this class of respirators was created when NIOSH issued new respirator regulations 42 CFR 84 in 1995. There were predecessor filtering facepiece respirators, but they would not have been classified as N95.

    We have listed some articles from the 70s and 80s on the effectiveness of PAPRS below:

    1. Myers WR, Peach MJ 3rd. Performance measurements on a powered air-purifying respirator made during actual field use in a silica bagging operation. Ann Occup Hyg. 1983;27(3):251-9. .
    2. Lowry PL, Wheat LD, Bustos JM. Quantitative fit-test method for powered air-purifying respirators. Am Ind Hyg Assoc J. 1979 Apr;40(4):291-9.
    3. Myers WR, Peach MJ 3rd, Cutright K, Iskander W. Workplace protection factor measurements on powered air-purifying respirators at a secondary lead smelter: results and discussion. Am Ind Hyg Assoc J. 1984 Oct;45(10):681-8.
    4. Myers WR, Peach MJ 3rd, Allender J. Workplace protection factor measurements on powered air-purifying respirators at a secondary lead smelter–test protocol. Am Ind Hyg Assoc J. 1984 Apr;45(4):236-41.
    5. Que Hee SS, Lawrence P. Inhalation exposure of lead in brass foundry workers: the evaluation of the effectiveness of a powered air-purifying respirator and engineering controls. Am Ind Hyg Assoc J. 1983 Oct;44(10):746-51.
    6. Lowry PL, Wheat LD, Bustos JM. Quantitative fit-test method for powered air-purifying respirators. Am Ind Hyg Assoc J. 1979 Apr;40(4):291-9. Eneidi WL, Taylor RD. Air-purifying powered respirator pack utilizing a miniature two-stage air mover. Am Ind Hyg Assoc J. 1976 Aug;37(8):464-8.
    7. Burgess WA, Reist PC. Supply rates for powered air-purifying respirators. Am Ind Hyg Assoc J. 1969 Jan-Feb;30(1):1-6.

    A discussion of N-95s and Surgical masks can be found on the NIOSH blog .

    A more current article from 2003 may also be of interest:
    Zhuang, Z., Coffey, C.C., Jensen, P.A., Campbell, D.L., Lawrence, R.B., and Myers, W.R., Correlation Between Quantitative Fit Factors and Workplace Protection Factors Measured Under Actual Workplace Environments at a Steel Foundry. American Industrial Hygiene Association Journal 64:730–738, 2003

    Since the eyes are one of the routes of entry a respirator may not be an effective defense from a cough correct?

    According to the 2007 Guideline for Isolation Precautions: Preventing Transmission of Infectious Agents in Healthcare Settings ( , “protection for the eyes, nose and mouth by using a mask and goggles, or face shield alone, is necessary when it is likely that there will be a splash or spray of any respiratory secretions or other body fluids as defined in Standard Precautions.” Thus, eye protection is a standard precaution that should be used in caring for any patient who creates body fluid sprays that might strike mucous membranes, including patients who create sprays of respiratory or oral secretions by coughing or sneezing.

    I’m suprised to see that there is no mention of Home / Automotive air filtration that can help keep the spread down. What about the use of UV sterilizers?

    Thank you for the artical though, it was a good read.

    Thank you for your question. In its infection control guidance for influenza, CDC does not recommend use of special air handling measures in most settings, since available data indicate that influenza aerosols typically transmit infection only over relatively short distances (up to about six feet). CDC does have one exception to this policy: it recommends air filtration in health care settings where aerosol generating medical procedures are performed on patients with suspected or confirmed influenza. [See Prevention Strategies for Seasonal Influenza in Healthcare Settings at

    In the last decade, scientists and policy-makers began to examine questions around flu transmission as emergency preparedness and the emergence of the H1N1 virus stimulated further evaluation of our knowledge about infection control. NIOSH is conducting research on air filtration to help inform the ongoing dialogue in regard to potential occupational risks for health-care workers. For example, in a recent peer-reviewed paper, NIOSH researchers found that air ventilation/filtration will gradually reduce the concentration of cough-generated aerosol particles in a room; however, the risk of exposure to particles is likelier to be related to the initial concentrated bolus of aerosol from a patient, the study showed. Further study is needed to understand the implications of the findings for deliberations on infection-control policies. (WG Lindsley, WP King, RE Thewlis, JS Reynolds, K Panday, G Cao & JV Szalajda (2012). Dispersion and Exposure to a Cough-Generated Aerosol in a Simulated Medical Examination Room, Journal of Occupational and Environmental Hygiene, 9(12): 681-690,

    CDC does not recommend air sterilization with ultraviolet germicidal irradiation (UVGI) in any setting as a standard method to prevent flu transmission. Available data suggest that UV lamps must be kept far enough away from a person to prevent harmful exposure to UV radiation, and so UVGI might not be effective in preventing short-range transmission of influenza between individuals. More research is needed to better understand this and other questions about UVGI. For those interested in UVGI, more information can be found in a NIOSH document on using UVGI to combat tuberculosis

    PAPR’s were meant to be used in industry. They were never intended nor designed for infectious particles.
    How does one disinfect them? the motor? the tubing? the hood? If the healthcare worker is him/herself
    infected, and asymptomatic carrier/spreader, the positive pressure from the hood will spread the infectious particles outwards towards patients! will not prevent spread as would an FFPR or half/full-face respirator.

    Thank you for your questions. The U.S. Centers for Disease Control and Prevention (CDC) provides guidance for preventing the transmission of seasonal influenza. The guidance recommends strategies for minimizing risks of transmission from an infected person to an uninfected person. Under the guidance, risks for exposure from an infected health-care worker to an uninfected patient would be minimized at the outset even before the employer makes any decisions about respirator use. As examples: If a worker is already home with the flu, CDC recommends that the employer direct the worker not to report to work. If the worker begins to show symptoms after he or she is already at work, CDC recommends that the employer direct the worker to stop patient-care activities, don a facemask, promptly notify their supervisor and the infection-control specialist at the facility, and leave work. For further information, the CDC guidance can be found at

    The CDC guidance recommends the use of facemasks for most patient-care activities and NIOSH-certified respiratory protection (which could include PAPRs) during aerosol-generating procedures, as part of a larger strategy to protect health care workers from infection. As with the use of respirators in other industries, the guidance recommends that employers follow a hierarchy of controls under which NIOSH-certified respirators would be used only after the employer has taken administrative or other steps to minimize circumstances in which respirators would be necessary. Procedures for selecting, using, and maintaining respirators in those instances for reducing work-related risks of exposure are subject to Occupational Safety and Health Administration (OSHA) requirements under 29 CFR Part 1910.134:

    Procedures for cleaning and disinfecting respirators under the standard appear in 1910.134, Appendix B-2:

    As the blog notes, NIOSH’s research supports the advancement of work-related flu prevention by building on existing guidance, better understanding ways in which flu is transmitted, and using that knowledge to refine existing interventions and potentially develop new ones.

    Just a short think note, “The breathing machine can be outfitted with personal protective equipment (PPE), such as surgical masks, respirators, face shields, and powered air-purifying respirators (PAPRs). ” setting up a cohort of regular urban city bus/train travelers with a filtered and unfiltered breathing machine/s to monitor virus levels for two virus events, “Adults shed influenza virus from the day before symptoms begin through 5-10 days after illness onset.” I’m sure the travellers names/contacts. etc. would be forth coming if a seat on their usual commuting conveyance was guaranteed for say, 15 working days. Further, observers placed on-board could pick their quarry quite easily once infection set in. Then just maybe, those in the cohort who did not contract the virus could be coaxed into a prospective study to find reasons for their immunity, not just from their travel to work, but their whole described environment. Maybe make a good comparative study from the semi-sterile (hospital/ clinical) environment.

    That’s an interesting idea, although the logistics would be very challenging! The breathing machine is not portable in its current design, but it might be possible to design a portable version.

    I like what you guys are up too. Such intelligent work and reporting! Carry on the superb works guys I have incorporated you guys to my blogroll. I think it will improve the value of my web site :).

    In acute care, efficacy of N95 respirator vs. plain isolation barrier mask protection is questionable in current flu situation: we would need to be very explicit about use, handling, disinfection. There is a significant non-work related component to influenza to consider. Hence the push to vaccinate HCW against flu: so real and so valid. Masks did not demonstrate a long term effectiveness against flu exposures, before the January surge. Our recent experience in flu surge demonstrated 18% positive rate of all patients swabbed since Jan 1, 2013 (a quick test) positive for influenza A, with 2 samples also positive for B strain, and many flu admissions. Public holding areas are positive pressure. Walk-in patients had separate holding area. Exposed, non-immune staff had no flu cases of secondary type occurred. Contrast this in January with December 2012 when during Dec 20-30 flu peaked in staff caring for inpatient flu cases; self-reported, with several people went out on leaves! (flu confers a holiday) We’ve vaccinated and asked for proof if given elsewhere-with over 60% receiving vaccine. RPP stations everywhere with tissues, signs, handwash. If ILI: droplet precautions-we say Isolate first, look for diagnosis second. Third, discharge flu cases ASAP. Vaccinate-vaccinate. Not mandatory as yet…

    Great Article and Very Informative. I really loved your work. And I also know bits about flu, but article increased my knowledge to a much better level.


    Thanks for a very interesting article. I’m curious why the flu ‘vaccine’ is being pushed so hard?? From what I’ve read, about 2.7% of the population get the flu in any given year, and that number might go down to 1.2% if they get the shots…

    That means that the vaccine is meaningless for more than 98% of the people out there! Meanwhile, there are side-effects.

    I do agree that vaccines are the most powerful medical treatment that we currently have to prevent the flu. However, I believe the benefits are over stated. I wonder what you think about this Cochrane study, which seems to be the most neutral one I can find on the topic:
    I believe it is an important question, because the vaccine does come with risks, rare, but very real (encephalitis, narcolepsy, etc). And it seems to work the worst in those subgroups that are touted to benefit the most (elderly and young children).
    I know it is heated topic, and I believe we in the medical community should be open to both sides of the picture.
    Dr. Gietzen at Healthy Mind Body

    Frequent traveller mostly by plane that travel from one country to another has very high risk of carrying flu and in the initial phase it is hard to detect. Recently ebola should taken care of and all the passengers must be checked carefully. Also train travellers should be careful about the threat. WHO should increase fund for this disease.

    Good Article and Informative. Influenza actually has been familiar for us but by this article let us know more how to prevent & treat.

    If ILI: droplet precautions-we say Isolate first, look for diagnosis second. Third, discharge flu cases ASAP. Vaccinate-vaccinate. Not mandatory as yet.

    Thanks for the article. I’ve always been curious to know how influenza is transmitted from person to person. It’s complicated …


    Certainly flu can spread three major ways:
    1.-transmission through respiratory droplets,
    2. transmission through the air,
    3. Contagion by physical contact.
    Most flu virus is mainly spread through respiratory droplets or sneezing.


    Convincing people that going outside with wet hair will not cause one to catch the flu, which is a virus, is my mission in life.

    What we know that flu can spread infection for about 6 feet, but I saw others telling that flu can spread infection for 3 meter in normal breathing and more when sneezing. Can you enlighten us ?

    The “6-foot rule”, which says that infectious diseases like influenza can spread by airborne droplets to about 6 feet (or about 2 meters) from a patient, has been a common rule of thumb for many decades. However, recent work has shown that small infectious particles can be propelled much further than 6 feet from an infected person, which suggests that the 6-foot rule may not be appropriate for diseases that can be transmitted by small airborne particles. Aerosol disease transmission is reviewed in a recent article by Jones and Brosseau (RM Jones and LM Brosseau (2015). Aerosol transmission of infectious disease. J Occup Environ Med 57(5): 501-8.)

    The CDC discusses the issue of disease transmission by airborne droplets and small particles in the “2007 Guideline for Isolation Precautions” (

    “Section I.B.3.b. Droplet Transmission. . . Historically, the area of defined risk has been a distance of <3 feet around the patient, based on epidemiologic and simulated studies of selected infections. Using this distance for donning masks has been effective in preventing transmission of infectious agents through the droplet route. However, experimental studies with smallpox and investigations during the global SARS outbreaks of 2003 suggest that droplets from patients with these 2 infections could reach persons located 6 feet or more from their source. It is likely that the distance that droplets travel depends on the velocity and mechanism by which respiratory droplets are propelled from the source, the density of respiratory secretions, environmental factors (e.g., temperature, humidity), and the pathogen’s ability to maintain infectivity over that distance. Thus, a distance of <3 feet around the patient is best considered an example of what is meant by ‘‘a short distance from a patient’’ and should not be used as the sole criterion for determining when a mask should be donned to protect from droplet exposure. Based on these considerations, it may be prudent to don a mask when within 6 to 10 feet of the patient or on entry into the patient’s room, especially when exposure to emerging or highly virulent pathogens is likely. More studies are needed to gain more insight into droplet transmission under various circumstances.
    . . .
    Section I.B.3.c. Airborne Transmission. Airborne transmission occurs by dissemination of either airborne droplet nuclei or small particles in the respirable size range containing infectious agents that remain infective over time and distance (e.g., spores of Aspergillus spp. and M. tuberculosis). Microorganisms carried in this manner may be dispersed over long distances by air currents and may be inhaled by susceptible individuals who have not had face-to-face contact with (or even been in the same room with) the infectious individual. Preventing the spread of pathogens that are transmitted by the airborne route requires the use of special air handling and ventilation systems (e.g., AIIRs) to contain and then safely remove the infectious agent. . . In addition to AIIRs, respiratory protection with a National Institute for Occupational Safety and Health (NIOSH)-certified N95 or higher-level respirator is recommended for HCWs entering the AIIR, to prevent acquisition of airborne infectious agents such as M tuberculosis.”

    For how long influenza viruses can survive on surfaces?
    I read in CDC for about 2-8 hours but I think this depends on study of Bean et al 1982 .

    In April 2009, the first cases of infection with pandemic A (H1N1) 2009 virus in the United States were reported ( 1 ). On 11 June, the World Health Organization (WHO) declared an influenza pandemic due to widespread global transmission ( 2 ). As of 13 September, the six WHO regions had reported some 296,471 cases of pandemic H1N1 influenza, including 3,486 deaths ( 3 ). On 29 June, the first case of pandemic H1N1 was confirmed in Kenya.

    Every year the Flu scenario continues to be the one-sided battle with mortality rates variable as it is reported but nevertheless impacting human mortality rates with increases and immensely causing not only loss of life but overall the national economic, social and behavioral health. Advances in overall health technology, genetic engineering discoveries as well as experimental and clinical innovation success move us an unstoppable potential source force that we are to enhance yet our struggle to eradicate the ever-evolving infectious and communicable disease. Still, we have been at mercy of one superbug, the flu virus that has continued to over and over stomped us untiringly every cold winter season. I wonder in my ever average but curious mind if we have planned our approach repeatedly to this menace that doesn’t measure on par with the ability of this virus to be at a pace above of the human mind In essence, are we looking at our interventions in the “same box ” approaches when the virus has inevitably been ready way ahead?
    How can I work @ a CDC doing research on FLU as a RN on a State level employment, or is this nonexisting inquiry

    With concern over this coronavirus, what is the latest information with small particle transmission through the eyes? Reading through the above, with common flu, info seemed to suggest 6 feet could transmit through nose and mouth and now we have masks. But the standard masks do not cover eyes and there does not seem to be much info on this. How pathogenic is this virus through the eye? Thanks for the informative info.

    While the transmissibility of the 2019 novel coronavirus (2019-NcoV) is still being studied, infectious diseases can be transmitted through various mechanisms, including through the mucous membranes of the eye (conjunctiva). Infectious agents can be introduced to the eye either directly (e.g., blood splashes, respiratory droplets generated during coughing or suctioning) or from touching the eyes with contaminated fingers or other objects.

    Eye protection provides a barrier to infectious materials entering the eye and is often used in conjunction with other personal protective equipment (PPE) such as gloves, gowns, masks or respirators. For more information about eye protection and infection control, please take a look at our Q&A’s located here:

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Page last updated: August 13, 2020