We use some necessary cookies to make this website work properly.
We want to set additional cookies to understand how you use GOV.UK, remember your settings and improve government services.
We also use cookies set by other websites to help us provide content from their services.
You can change your cookie settings at any time.
Search the department to understand what the government is doing
Departments, institutions and public institutions
News reports, speeches, letters and announcements
Detailed guidance, regulations and rules
Reports, analysis and official statistics
Government data, free release of information, and company reports
Unless otherwise noted, this publication is licensed under the terms of Open Government License v3.0. To view this license, please visit nationalarchives.gov.uk/doc/open-government-licence/version/3 or write to the Information Policy Team, The National Archives, Kew, London TW9 4DU, or send an email to: psi @nationalarchives.gov. U.K.
If we have determined any third-party copyright information, you will need to obtain permission from the relevant copyright owner.
This publication is available at https://www.gov.uk/government/publications/emg-simple-summary-of-ventilation-actions-to-mitigate-the-risk-of-covid-19-1-october-2020 Get on/emg-simple-summary-of-ventilation-actions-to-mitigate-the-risk-of-covid-19-1-october-2020
SAGE Environment and Modeling Group
The SAGE Environment and Modeling Group (EMG) has previously considered the potential for aerosol transmission [footnote 1] and the role of ventilation [footnote 2] and air purification [footnote 3]. This article aims to summarize in an easy-to-understand way why ventilation is important and what key practical steps can be taken to improve ventilation to reduce the risk of SARS-CoV-2 transmission. Although this article focuses on ventilation, maintaining other measures is also important, including maintaining distance, hand hygiene, cleanliness, wearing masks when appropriate, and limiting social interaction.
The virus that causes COVID-19 is spread through very small aerosols and droplets released in the exhaled breath. There is evidence that, in some cases, these aerosols can spread more than 2 meters in the air and may cause infection if inhaled [footnote 1]. When the room is poorly ventilated, this situation is most likely to occur in an indoor environment. If people stay in the room long enough, the virus will accumulate in the air, and people will inhale enough virus to cause infection. When people engage in activities that may cause them to exhale more aerosols, such as high-intensity exercise, singing or speaking loudly, the risk seems to increase [footnote 1], [footnote 4]. The risk may also be higher in places where face masks or masks are not worn, because they reduce the amount of virus released into the air [footnote 5], [footnote 6].
In addition to COVID-19, ventilation is also important to human health. Studies have shown that good ventilation is associated with improved health, increased concentration, increased satisfaction with the environment, lower absenteeism, better sleep quality, and reduced exposure to various air pollutants [footnote 7], [footnote 8] , [Footnote 9], [footnote 10].
Ventilation is the process of introducing fresh air from the outside and removing indoor air, which may contain contaminants such as virus particles. The ventilation method depends on the building. Naturally ventilated buildings do not use any fans and rely on openings to provide ventilation air. A certain level of background ventilation is provided through small openings, usually trickle vents (small vents are usually at the top of windows), sometimes air bricks or grilles. These ensure that there is always some airflow, so it is important to ensure that the drip opening is open. However, the flow through these may be quite low and cannot be increased, so it is necessary to use other openings, such as windows, to increase ventilation. These are usually manually operated, so the occupant will depend on the occupant to open and close them.
Mechanically ventilated buildings use fans to move air in and out of the room to provide ventilation. In small spaces and buildings, these may be in the room, but larger buildings may use a network of ducts and fans to blow clean air into the room and/or extract stale air. This can range from simple systems (such as bathroom extracts in houses) to very complex and sophisticated systems in large commercial buildings. Small systems can be controlled by the occupants (using controls or switches in the space), but larger systems can be controlled centrally or automatically. The advantage of mechanical ventilation is that if it works properly, it can provide more consistent ventilation, but it can be more expensive, requires energy to run the system and requires proper maintenance.
Many buildings mix natural ventilation and mechanical ventilation, using different systems in different spaces.
The ventilation rate refers to the amount of air provided to the room over a period of time, and is usually stated as a recommended value in the building guide. Some guidelines recommend ventilation rates in liters per second per person—many guidelines recommend 10 liters per second per person as a value suitable for most commercial buildings [footnote 11]. Evidence to date indicates that when the ventilation rate is very low, the risk of COVID-19 transmission increases; some super-transmission events quote values in the range of 1 to 3 liters per second per person [footnote 2].
Some documents describe the ventilation rate based on the amount of air exchange per hour, which is a measure of the air flow rate relative to the size of the room. This measure helps to understand how quickly ventilation removes pollutants from the air. A ventilation rate of 6 air changes per hour means that the ventilation system provides 6 times the room volume per hour. However, this does not mean that all the air is replaced 6 times in an hour-the new air mixes with the air already in the room, causing dilution over time. Air changes 6 times per hour, which can remove 95% of the pollutants in the air within 30 minutes.
Regular ventilation in the home is widely regarded as important for maintaining the health of people and families. Many people will find that they have to strike a balance between ventilation and thermal comfort, especially in winter, when the home needs to open windows for ventilation.
For COVID-19, if someone in the home is infected with the virus, the most important thing is to ventilate the space, as this helps prevent spread to other family members. Providing additional ventilation when there are visitors in the home and when they have just left may also reduce their risk of infection.
Ventilation can be provided by ensuring that any background ventilation devices are turned on. For most people, opening windows will be the easiest way to increase ventilation. Simple operations, such as ensuring that the background trickle vents remain open and avoid blocking the vents, can ensure continuous background ventilation. Wider openings will provide more air circulation, but this does not mean that the windows must be open all the time. The rate of ventilation through the opening depends on the wind speed and temperature difference between indoor and outdoor [footnote 12]. In colder weather, opening a small amount of windows can produce ventilation that is almost as effective as opening windows completely in summer. If the windows have openings at high and low places (such as window sashes), it is helpful to use only the top openings in cold weather, because the incoming cold air will mix with the warm indoor air and help ease the cold airflow. In warm weather, when cold wind is not a potential problem, using both the lower and upper openings at the same time will help provide more airflow. If noise or safety is an issue, or it is uncomfortable to open windows and vents for a long time, or you are worried about heating costs, opening the windows to regularly ventilate the room for a shorter period of time may be effective in reducing the concentration of viruses in the air [footnote 14].
After someone has used up the room, let the exhaust fans in the bathroom, toilet and kitchen area run longer than usual and close the door to increase ventilation. In households with mechanical ventilation systems, it is important to ensure that these systems are functioning properly, and the filters should be replaced regularly. When there are visitors or someone in the house is sick, you can use the enhanced mode to increase ventilation. If someone is isolated in a room, keeping the door closed and opening the vents and windows slightly may help separate the room from the rest of the home.
Ventilation in workplaces or public places should be considered an important part of the COVID risk assessment [footnote 14]. In larger buildings with mechanical ventilation systems, the facility management team is usually responsible for ventilation. If there is user control, the workplace should be able to provide employees with guidance on how to operate the system. In accordance with CIBSE [footnote 15] and REHVA [footnote 16] recommendations, the settings of many buildings have been adjusted where possible to ensure adequate external air ventilation.
In workplaces that rely on natural ventilation, it is important to keep vents open and open windows regularly, especially in spaces that are shared with others. Opening windows (and sometimes doors) intermittently, for example for 10 minutes every hour, can effectively reduce the risk of viruses in the air. This can be more effective if this is combined with the rest time for the occupants to leave the room (for example in a meeting room or classroom) [footnote 17].
The Chartered Institute of Building Services Engineers (CIBSE) provides detailed guidelines for workplaces and public buildings. The guide provides information on different ventilation systems and explains methods for assessing and managing COVID ventilation [footnote 16].
Air conditioning refers to changing the temperature and humidity of the air, usually to cool the air when it is too hot to make people feel uncomfortable. Air conditioners are sometimes connected to the ventilation system, but many rooms have independent air conditioners, which simply recirculate indoor air. If recirculating air conditioners are operated in rooms with very low outdoor flow rates, they may pose a risk-air conditioners can make the occupants comfortable, but they will mask poor air quality and may allow any viruses to accumulate in the air. Many studies have linked transmission to recirculating air conditioning. The high speeds generated by these devices may allow larger virus aerosols to remain airborne over longer distances [footnote 18], [footnote 19]. A directional flow from a desktop fan may have a similar effect.
It can be very difficult to figure out how to ventilate a place, but there are some clues. If there are vents (and sometimes ducts) on the ceiling or high on the wall, then this place is likely to have mechanical ventilation. Certain spaces (such as theaters) usually have these vents on the floor or under the seats.
Schools usually install ventilation devices under windows [footnote 20]. If a space has doors or windows open and there are no signs of other vents, then it is likely to be naturally ventilated. If a space feels stuffy or smelly, its ventilation rate may be low. Any room with openings for occupants can be naturally ventilated.
Being in a poorly ventilated space for a short period of time is unlikely to pose a major risk, especially if people are wearing masks. However, if you spend a long time in a poorly ventilated room with many people, this may be an environment with a much higher risk of transmission.
It is very difficult to accurately measure ventilation, but in some spaces, a carbon dioxide (CO2) meter can be used to estimate the effectiveness of ventilation. We all exhale CO2 when we breathe, and the concentration of the air in the room depends on the number of people in the room and the ventilation rate.
Most building guidelines recommend that the carbon dioxide concentration should be kept below 1000 ppm for effective ventilation. Previous analysis indicated that CO2 concentrations that usually exceed 1500 ppm indicate that the room may be poorly ventilated and may pose a greater risk to COVID-19 [footnote 2]. When performing aerosol-releasing activities such as high-intensity exercise, singing, and prolonged loud speech, it is recommended to take additional precautions and keep CO2 levels below 800 ppm [footnote 4]. CO2 meters can be used to identify poorly ventilated spaces, but they are less effective in showing good ventilation.
Using a CO2 meter to estimate ventilation needs to be done with caution, as you may get erroneous or inaccurate readings. When there are usually many people in the space, always measure it, preferably at least 1 hour to get a reliable reading. Use CO2 meters only if there is no other source of CO2-gas stoves or gas/solid fuel fires will produce CO2 and may give false readings. Measurements are much more reliable in spaces where there are a large number of people and measurements need to be taken away from open windows or vents. Meters that use infrared gas sensors (NDIR) are more likely to provide accurate readings than other types of sensors.
Evidence from laboratory research shows that the virus that causes COVID-19 can survive better in colder and drier conditions [footnote 21], while evidence from other viruses shows that relative humidity is a particularly important parameter [footnote 22 ]. Humidity may also have an impact on the physiological response of the human body, and it is more likely to be affected in the dry environment experienced in winter in the UK [footnote 23], [footnote 24]. In most indoor spaces, the effect of ventilation rate on transmission risk may be more important than the effect of temperature and humidity. However, keeping the space at a comfortable temperature and keeping indoor humidity between 40% and 60% RH may help to further reduce the risk.
Recently SAGE EMG [footnote 3] has conducted a detailed review of air purifiers, which may be suitable for spaces with insufficient ventilation and cannot be improved. There is little evidence that air purifiers are an effective control measure to prevent COVID-19, but the principles of air purification indicate that they may be useful in certain situations.
There are a large number of different air purifiers on the market, and it is difficult to choose an effective one. Air purifiers based on filtration (with HEPA filters) may be the most effective, and air purifiers that include ultraviolet lamps may also be effective. It is best to avoid any equipment that generates ozone or other chemicals, as they may irritate the respiratory tract. It is also important to consider how much air the equipment can clean; small equipment in a large room has little effect. Noise is also an important consideration, especially for large equipment with high fan speeds, which may cause noise. Never use an air purifier instead of ventilation.
The role of NERVTAG/EMG aerosol transmission in COVID-19, July 22, 2020 ↩ ↩2 ↩3
EMG: The role of ventilation in controlling the spread of SARS-CoV-2, September 30, 2020 ↩ ↩2 ↩3
EMG: Potential applications of air purification equipment and personal purification in managing the spread of COVID-19, November 4, 2020 ↩ ↩2
PHE/EMG: Aerosols and droplets from singing, wind instruments and performance activities, August 13, 2020 ↩ ↩2
Liang, NHL, etc. (2020) "Respiratory virus shedding in exhaled breath and the efficacy of masks", Nature Medicine 2020. Springer US, pages 1-5. doi: 10.1038/s41591-020-0843-2 ↩
WG Lindsley, FM Blachere, BF Law, DH Beezhold, JD Noti (2020) The effectiveness of masks, neck protectors and face masks in reducing aerosol discharge from simulated cough, aerosol science and technology↩
J Sundell, H Levin, WW Nazaroff, WS Cain, WJ Fisk, DT Grimsrud, F Gyntelberg, Y Li, AK Persily, AC Pickering, JM Samet, JD Spengler, ST Taylor, CJ Weschler (2011) Ventilation rate and health: more Scientific literature review of the subject, Indoor Air Jun; 21(3):191-204. ↩
MJ Mendell, EA Eliseeva, MM Davies, M Spears, A Lobscheid, WJ Fisk, MG Apte (2013) Association for Classroom Ventilation to Reduce Absenteeism: A Prospective Study of California Elementary Schools, December Indoor Air; 23(6):515- 28. doi: 10.1111/ina.12042. Epub 2013, April 22. ↩
P. Strøm-Tejsen D. Zukowska P. Wargocki DP Wyon (2015) The effect of bedroom air quality on sleep and performance the next day, Indoor Air 26: 679-686 https://doi.org/10.1111/ina.12254 ↩
WJ Fisk (2018) How Home Ventilation Rate Affects Health: A Literature Review, Indoor Air 28: 473-487 ↩
CIBSE Guide B Heating, Ventilation, Air Conditioning and Refrigeration (2016) ↩
CIBSE AM10: Natural ventilation for non-residential buildings (2005) ↩
HSE ventilation and air conditioning during the coronavirus (COVID-19) pandemic, December 3, 2020 ↩ ↩2
Melikov, AK, Ai, ZT and Markov, DG (2020) "Combination of intermittent occupancy and ventilation: effective strategies to reduce indoor air transmission", Total Environmental Science, 744(2). ↩
J. Lu et al., "The COVID-19 outbreak related to restaurant air conditioning, Guangzhou, China, 2020", Emerge. Infect. Sub-volumes, rolls. 26, no. 7, pp. 1628–1631, 2020, ↩
Keun-Sang Kwon, Jung-Im Park, Young Joon Park, Don-Myung Jung, Ki-Wahn Ryu, and Ju-Hyung Lee (2020) Evidence of SARS-CoV-2 spread by direct airflow long-distance droplets Restaurant J Korean Med Sci. November 30, 2020; 35(46):e415 ↩
BB101: School Ventilation, Thermal Comfort and Indoor Air Quality Guidelines (2018) ↩
Dabisch P, Schuit M, Herzog A, etc. The influence of temperature, humidity and simulated sunlight on the infectivity of SARS-CoV-2 in aerosols. Aerosol Technology 2020; 0:1-15. ↩
LC Marr, JW Tang, J. Van Mullekom, and SS Lakdawala (2019) Insights into the mechanism of the impact of humidity on the survival, spread and incidence of airborne influenza viruses, Journal of the Royal Society Interface, 16 (150) ↩
Moriyama M, Hugentobler WJ, Iwasaki A. Seasonality of respiratory virus infections. Annu Rev Virol 2020; 7: 83–101 ↩
Kudo E, Song E, Yockey L, etc. Low environmental humidity will weaken the barrier function and the innate resistance to influenza infection. -Summary-European PMC. Proceedings of the National Academy of Sciences; 116: 10905–10 ↩
Do not include personal or financial information, such as your National Insurance number or credit card details.
To help us improve GOV.UK, we would like to learn more about your visit today. We will send you a link to the feedback form. It only takes 2 minutes to fill in. Don't worry, we will not send you spam or share your email address with anyone.