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Data Analysis Tutorials. Review step-by-step guidance on using NHANES data. Low Resolution Video. Low Resolution Video. Last Reviewed: May 30, 2024. Source: National Center for Health Statistics.
Science at CDC. To make science and data easier for broad audiences to interpret, CDC is translating science into practical, easy to understand policy by clarifying and presenting scientific language so that anyone can understand it and standardizing guideline development across the agency. CDC Moving Forward.
2024年3月15日 · Find links to guidance and information on all topics related to COVID-19, including the COVID-19 vaccine, symptom self-check, data, and other topics.
The Advisory Committee on Immunization Practices (ACIP) is a group of medical and public health experts that develop recommendations on how to use vaccines to control diseases in the United States.
- Summary of Recent Changes
- Ventilation
- What You Need to Know
- Ventilation Mitigation Strategies
- How Much Ventilation Is Enough?
- Ventilation Frequently Asked Questions
- Previous Updates
Updates as of May 12, 2023
•Added Key Strategies, a simplified summary of the most important recommendations on this page.
•Added discussion on “How much ventilation is enough?” with a recommendation to get at least 5 air changes per hour of clean air in occupied spaces.
•Updated the minimum filter recommendation to Minimum Efficiency Reporting Value (MERV) 13.
•Updated the guidance on post-occupancy flushing of building air.
•Included more information on up-front, maintenance, and energy cost considerations for ventilation strategies.
Ventilation is a term with different meanings to different people. For the purpose of this webpage, “ventilation” includes:
•Indoor air movement and dilution of viral particles through mechanical or nonmechanical (also called natural) means.
•Filtration through central heating, ventilation and air conditioning (HVAC) systems and/or in-room air cleaners (portable or permanently mounted).*
•Air treatment with Ultraviolet Germicidal Irradiation (UVGI) systems (also called Germicidal Ultraviolet or GUV).*
•An important principle to remember when contemplating mitigation strategies is to “First do no harm.” Building owners and operators should strongly consider seeking expert consultation when considering mitigation strategies if they lack knowledge and experience in implementing those strategies.
•Airborne viral particles spread between people more readily indoors than outdoors. Indoors, the concentration of viral particles is often higher than outdoors. Protective indoor ventilation practices can reduce the airborne viral concentrations and the overall viral exposure to occupants.
•Ventilation system upgrades or improvements can increase the delivery of clean air and dilute potential contaminants. Buildings that provided healthy, code-compliant indoor air quality prior to the pandemic can be improved for pandemic and post-pandemic occupancy using less costly interventions.
•While the mitigation strategies can be universally applied across many indoor environments, applying them to different building types, occupancies, and activities under environmental and seasonal changes can be challenging. The building owner or operator should identify which strategies are appropriate for each building throughout the year.
•Implementing multiple building-level mitigation strategies at the same time is consistent with CDC’s layered approach and will increase overall effectiveness of ventilation interventions.
•Building owners and operators can participate in the Clean Air in Building Challenge to improve indoor air quality and protect public health.
When indoors, ventilation mitigation strategies can help reduce viral particle concentration. The lower the concentration, the less likely viral particles can be inhaled into the lungs (potentially lowering the inhaled dose); contact eyes, nose, and mouth; or fall out of the air to accumulate on surfaces. Although it isn’t known exactly how much the concentration of viral particles in air needs to be reduced to start reducing risk of viral infection, ventilation mitigation strategies still provide a reasonable approach to reducing risk. Not all interventions will work in all scenarios and their selection must be carefully evaluated prior to adoption.
These ventilation interventions can reduce the spread of disease, but they will not eliminate risk completely. These interventions are intended to lower transmission risk by lowering the concentration of infectious aerosols in a room. However, the overall transmission reduction is less likely to apply to people who are very close (e.g. face-to-face) to the infectious source. Some of the following interventions are based on COVID-19 Technical Resources published by ASHRAE (a professional organization formerly known as the American Society of Heating, Refrigerating, and Air Conditioning Engineers).
In addition to buildings, vehicles – including public transportation such as buses, subways, trains, school buses, carpools, and rideshares – are also areas where ventilation improvements can be applied to reduce the spread of airborne viruses and lower the risk of exposure.
The recommendations presented here are not intended to replace guidance that may already exist in national, state, and local standards and guidelines. For example, some healthcare spaces have specified ventilation requirements intended to prevent and control infectious diseases. For spaces where existing standards and guidelines specify lower ventilation rates than the recommendations presented here, building owners and managers are encouraged to consider adoption of the more protective guidance.
Aim for 5 Air Changes per Hour (ACH)
When possible, aim for 5 or more air changes per hour (ACH) of clean air to help reduce the number of germs in the air. This can be achieved through any combination of central ventilation system, natural ventilation, or additional devices that provide equivalent ACH (eACH†) to your existing ventilation. Supplying or exhausting an amount of air (use the larger of the two values but do not add them together) that is equal to all the air in a space is called an air change. Multiplying that amount by 5 and delivering it over one hour results in 5 ACH.
Cost Considerations
The ventilation interventions listed above come with a range of initial costs and operating costs, which, along with risk assessment factors – such as community incidence rates and the adoption of other interventions – may affect the selection of ventilation mitigation strategies. The following are examples of cost estimates for different strategies: Intervention Strategy Intervention Strategy Intervention Strategy Up-front Cost Up-front Cost Up-front Cost Ongoing Daily Interaction Ongoing Daily Interaction Ongoing Daily Interaction Ongoing Maintenance Requirements and Incremental Energy Usage Ongoing Maintenance Requirements and Incremental Energy Usage Ongoing Maintenance Requirements and Incremental Energy Usage Opening windows Intervention Strategy Opening windows No Up-front Cost No Yes Ongoing Daily Interaction Yes •No ongoing maintenance requirements •Incremental energy usage varies, depending on ambient outdoor conditions Ongoing Maintenance Requirements and Incremental Energy Usage •No ongoing maintenance requirements •Incremental energy usage varies, depending on ambient outdoor conditions Expanded operation of dedicated exhaust ventilation Intervention Strategy Expanded operation of dedicated exhaust ventilation No Up-front Cost No No Ongoing Daily Interaction No •Periodic preventive maintenance required •Incremental energy usage varies, depending on exhaust system capacity and ambient outdoor conditions Ongoing Maintenance Requirements and Incremental Energy Usage •Periodic preventive maintenance required •Incremental energy usage varies, depending on exhaust system capacity and ambient outdoor conditions Repositioning HVAC outdoor air dampers Intervention Strategy Repositioning HVAC outdoor air dampers No Up-front Cost No No Ongoing Daily Interaction No •Periodic preventive maintenance required •Incremental energy usage varies, depending on HVAC system capacity and ambient outdoor conditions Ongoing Maintenance Requirements and Incremental Energy Usage •Periodic preventive maintenance required •Incremental energy usage varies, depending on HVAC system capacity and ambient outdoor conditions Switching thermostats from “Auto” to “On” or adjusting building HVAC control systems to disable Demand Controlled Ventilation (DCV) Intervention Strategy Switching thermostats from “Auto” to “On” or adjusting building HVAC control systems to disable Demand Controlled Ventilation (DCV) No Up-front Cost No No Ongoing Daily Interaction No •Periodic preventive maintenance required •Incremental energy usage varies, depending upon fan energy consumption Ongoing Maintenance Requirements and Incremental Energy Usage •Periodic preventive maintenance required •Incremental energy usage varies, depending upon fan energy consumption Using fans to increase effectiveness of open windows Intervention Strategy Using fans to increase effectiveness of open windows < $100 Up-front Cost < $100 Yes Ongoing Daily Interaction Yes •No ongoing maintenance requirements •Incremental energy usage varies, depending on HVAC system capacity and ambient outdoor conditions Ongoing Maintenance Requirements and Incremental Energy Usage •No ongoing maintenance requirements •Incremental energy usage varies, depending on HVAC system capacity and ambient outdoor conditions Repositioning supply/exhaust diffusers to create directional airflow Intervention Strategy Repositioning supply/exhaust diffusers to create directional airflow < $100 Up-front Cost < $100 No Ongoing Daily Interaction No •No ongoing maintenance requirements •No incremental energy usage Ongoing Maintenance Requirements and Incremental Energy Usage •No ongoing maintenance requirements •No incremental energy usage Adding in-room HEPA fan/filter systems Intervention Strategy Adding in-room HEPA fan/filter systems $500 (approximately) Up-front Cost $500 (approximately) Yes Ongoing Daily Interaction Yes •Must inspect/replace HEPA filter per manufacturer instructions •Low incremental energy usage Ongoing Maintenance Requirements and Incremental Energy Usage •Must inspect/replace HEPA filter per manufacturer instructions •Low incremental energy usage Adding upper room UVGI [Typical classroom requires 2-3 fixtures] Intervention Strategy Adding upper room UVGI [Typical classroom requires 2-3 fixtures] < $1500 (approx. per fixture) Up-front Cost < $1500 (approx. per fixture) No (unless manual activation) Ongoing Daily Interaction No (unless manual activation) •Must clean/inspect/replace UVGI lamps per manufacturer instructions •Low incremental energy usage Ongoing Maintenance Requirements and Incremental Energy Usage •Must clean/inspect/replace UVGI lamps per manufacturer instructions •Low incremental energy usage Adding in-duct UVGI to treat moving air Intervention Strategy Adding in-duct UVGI to treat moving air Varied, more cost-effective (<$0.25/cfm) with larger systems Up-front Cost Varied, more cost-effective (<$0.25/cfm) with larger systems No Ongoing Daily Interaction No •Must clean/inspect/replace UVGI lamps per manufacturer instructions •Low incremental energy usage Ongoing Maintenance Requirements and Incremental Energy Usage •Must clean/inspect/replace UVGI lamps per manufacturer instructions •Low incremental energy usage
1. Can COVID-19 be transmitted through HVAC (ventilation) systems?
Some SARS-CoV-2 viral particles might travel from one space to another through an HVAC system. However, this is not known to be a significant risk for disease transmission.
While airflow distribution within an occupied space is an important factor worth evaluation (see FAQ #11), outbreak investigations have predominantly found that COVID-19 transmission occurs between an infected person and uninfected persons in the same space. Viral RNA has reportedly been found on return air grilles, in return air ducts, and on heating, ventilation, and air conditioning (HVAC) filters, but detecting viral RNA alone does not imply that the virus was capable of transmitting disease. One research group reported that the use of a new air-sampling method allowed them to find viable viral particles within a COVID-19 patient’s hospital room with good ventilation, filtration and ultraviolet (UV) treatment (at distances as far as 16 feet from the patient). However, the concentration of viable virus detected was believed to be too low to cause disease transmission. There may be some implications for HVAC systems associated with these findings, but it is too early to conclude that with certainty. While airflows within a particular space may help spread disease among people in that space, there is no definitive evidence to date that viable virus has been transmitted through an HVAC system to result in disease transmission to people in other spaces served by the same system.
Healthcare facilities have ventilation requirements in place to help prevent and control infectious diseases that are associated with some healthcare environments. For more information, see the CDC Guidelines for Environmental Infection Control in Health-Care Facilities.
Non-healthcare (e.g., businesses and schools) building owners and managers should, at a minimum, maintain building ventilation systems according to state and local building codes and applicable guidelines. Ensuring appropriate outdoor air and ventilation rates is a practical step to ensure good indoor air quality. However, these codes do not address infection prevention in non-healthcare buildings and code minimum ventilation may be insufficient to protect indoor occupants under some circumstances (e.g., high incidence rates, occupants near one another, crowded spaces, etc.).
2. How long will it take to dilute the concentration of infectious particles in a room once they are generated?
Updates from Previous Content
As of June 2, 2021
•Added a new Frequently Asked Question on protective barriers and ventilation.
As of March 23, 2021
•Simplified language in the overall list of tools to improve ventilation.
•Added three new Frequently Asked Questions (FAQs) on the usefulness of carbon dioxide monitors to inform ventilation decisions, the usefulness of temperature and relative humidity to control the spread of COVID-19, and the use of fans indoors.
2024年3月14日 · Source: Centers for Disease Control and Prevention, National Center for Immunization and Respiratory Diseases (NCIRD) Flu vaccines cause antibodies to develop in the body about two weeks after vaccination. This page includes links to more information about the different type of flu vaccines available.
2023年8月1日 · Major changes to the best practice guidance in this section include 1) enhancement of the definition of a “precaution” to include any condition that might confuse diagnostic accuracy and 2) recommendation to vaccinate during a hospitalization if a patient is not acutely moderately or severely ill. General Principles.
