Skip Navigation LinksInterim-Guidance-for-Ventilation-Filtration-and-Air-Quality-in-Indoor-Environments

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State of California—Health and Human Services Agency
California Department of Public Health

October 7, 2021

All Californians

Interim Guidance for Ventilation, Filtration, and Air Quality in Indoor Environments

Related Materials: More Employees & Workplaces Guidance | All Guidance 

Updates as of October 7, 2021:

  • Incorporate Cal/OSHA revisions to the COVID-19 Emergency Temporary Standards.
  • Clarify legal requirements pertaining to the guidance. 
  • Added additional resources.

CAL OSHA logoDepartment of Health Care Access and Information (HCAI) logoCDPH logo

This Guidance is intended to be used for buildings for which the state or local health department is permitting business, assembly, or other occupancy or use to occur indoors.

NOTE: On November 30, 2020, the California Division of Occupational Safety and Health (Cal/OSHA) implemented a mandatory emergency temporary standard (ETS) to prevent employee exposure to COVID-19 in California workplaces not covered by the Cal/OSHA Aerosol Transmissible Diseases standard; subsequently, a revised ETS was adopted and became effective June 17, 2021. Employers must become familiar with and implement all employee protection requirements covered in the ETS. Please see Section 9 of this document identifying the specific provisions of the ETS that pertain to ventilation.

The following guidance supplements the Cal/OSHA ETS by recommending practical steps building operators can take to promote better ventilation, filtration, and air quality in indoor environments for the purpose of reducing the spread of COVID-19. This interim guidance may change as scientific knowledge, experience, community transmission, and other conditions change. Other useful information on building ventilation and related issues is available from the Centers for Disease Control and Prevention (CDC) and Section 10 of this document, Resources.

The recommendations described below come with a range of initial costs and ongoing operating costs, which may affect decisions about which interventions to implement. Always consult with building engineering or maintenance staff prior to making changes to a mechanical ventilation system. 

The following protocols are based on experience and principles that have a wide application. This guidance does not supersede any other mandatory requirements. Workplaces must continue to meet the requirements of the Cal/OSHA ETS and all other local and state directives regarding COVID-19.

The guidance is intended for use by non-healthcare organizations, including many types of businesses, companies, offices, restaurants, schools, faith-based organizations, etc. Healthcare facilities, which are expected to have infectious patients, require higher ventilation rates and employ higher filtration in order to ensure sufficient infection control; these requirements are not addressed in this guidance. Note that the recommendations contained in the guidance might not be applicable to your particular building or activity. Be aware that some of the recommendations could result in increased energy bills or increased wear and tear on ventilation system components. 

1. COVID-19 Basics 

COVID-19 is transmitted from person-to-person and may occur in the following scenarios:

  • Large droplets from coughing and sneezing are propelled directly into the face, nose, eyes, or mouth of someone nearby, usually within six (6) feet. These droplets are sometimes called "ballistic droplets" because they tend to travel in straight lines and fall out of the air rapidly.
  • Small particles (also known as aerosols) are released when a person breathes, talks or vocalizes, sings, coughs, or sneezes. These small particles can remain suspended in the air for a period of time and can move beyond six feet on air currents. Other people might inhale these small particles even if they are farther than six feet away.
  • A person touches a surface that is contaminated and then touches a mucus membrane such as their nose, eyes, or mouth. Contaminated objects and surfaces are sometimes called "fomites." Aerosols deposited on surfaces may also be disturbed and introduced back into the air where they might be inhaled.

Effective ventilation is one of the most important ways to control small aerosol transmission.  However, ventilation and other indoor air quality improvements are an addition to, and not a replacement for mandatory protections required by the Cal/OSHA ETS and state or local directives. Individuals at higher risk for severe illness from COVID-19 should exercise more caution regarding the time they spend in indoor environments outside of their home.

2. Definitions

Aerosol means solid or liquid particles suspended in a gas (typically air).

Air Changes per Hour (ACH, also called Air Change Rate) approximates how many times the air within a space is replaced each hour. ACH is a calculated value that allows standards, guidelines, and comparisons for ventilation to be made for rooms of different dimensions and which have different ventilation systems.

            Using English units, the formula for ACH is:

                        ACH = (ventilation rate in CFM x 60 minutes/hour) / room volume in cubic feet

Air Cleaners are standalone devices that move air in a room through a filter. Some filters are capable of removing tiny particles, including virus particles and smoke. They are referred to in this document as Portable Air Cleaners (PACs) to differentiate them from filters and other devices in HVAC systems that provide air cleaning.

ASHRAE is the American Society for Heating, Refrigeration, and Air-Conditioning Engineers. Facilities staff, engineers, and health and safety professionals are familiar with this organization and its literature.

CADR, or Clean Air Delivery Rate, measures a Portable Air Cleaner's effectiveness based on room space and the volume of clean air produced per minute. Tested units have three CADR ratings; for COVID-19 purposes the "Smoke" CADR rating should be used.

CFM, or cubic feet per minute, is a measure of air flow into or out of a room. 

In order to calculate how many cfm are required to obtain a desired ACH,
the formula is:

                        CFM = (ACH desired) x (room volume in cubic feet) / 60 minutes/hour

            Room volume can be calculated by the following formula:

                        width x length x height to ceiling (all dimensions in feet)

Clean Air, for the purposes of this document, refers both to clean outside supplied air, and also to recirculated indoor supplied air that has been passed through a Portable Air Cleaner (PAC) with an appropriately rated CADR, or through an HVAC system equipped with a Minimum Efficiency Reporting Value (MERV) 13 or greater filter. Note that unfiltered outside air contaminated with wildfire smoke may not qualify as clean air.   

Fans are devices that pull or push air in one direction. Fans can be rectangularly shaped for placement in windows or doorways, they may be "pedestal type" for placement anywhere in a room, or they may be attached to ceiling fixtures. Some fans have switches that allow the user to change the direction of airflow of the fan; fans that do not have such switches must be physically turned to change the direction of air.

HEPA Filter refers to a High-Efficiency Particulate Air FilterThis type of air filter is designed to meet a standard of removing at least 99.97% of dust, pollen, mold, bacteria, and any airborne particles with a size of 0.3 micron (µm). They are tested with 0.3 micron-sized particles as a "worst case" scenario, as this particle size penetrates through a filter most easily. Particles that are larger or smaller are trapped with even higher efficiency.

HVAC stands for Heating, Ventilation, and Air Conditioning system. Also referred to as "Mechanical Ventilation" because of the system's use of fans to move air in and out of rooms, typically through ducts and plenums.

Mechanical Ventilation is the active process of supplying air to or removing air from an indoor space by powered equipment such as motor-driven fans and blowers, but not by devices such as wind-driven turbine ventilators and mechanically operated windows.

Outside Air (outdoor air) refers to clean air drawn from outside the building either by natural or mechanical ventilation. Also referred to as "Fresh Air" or for selected applications "Makeup Air."

PACs are Portable Air Cleaners, devices that can be moved within a building or room to provide air cleaning. PACs are generally sold with some form of highly efficient filter such as a HEPA filter. The portability of PACs allows them to be placed where air cleaning will be most beneficial to room occupants.

Natural Ventilation refers to ventilation that is accomplished by opening windows and doors to the outside.

Recirculated Air refers to air that has been drawn from the inside of the building, passed through filters, conditioned, and reintroduced into the building. Unless passed through MERV-13 or greater efficiency filters, recirculated air is not considered when assessing building ventilation for COVID-19 purposes.

3. General Considerations

Our understanding of the role that the built environment plays in the transmission of COVID-19 is evolving; recent literature has clearly demonstrated small aerosols can be carried well beyond the six (6) foot physical radius and remain suspended in room air where they can be inhaled. With the possible exception of hospitals, healthcare facilities, and research facilities that employ exhaust hoods, existing ventilation requirements, such as those established in the California Building Code and Title 24, were not intended to control exposures to small aerosols of hazardous infectious agents such as COVID-19. Consequently, code compliance should be considered as the baseline, or starting point, in creating more protective environments. Ventilation should be maximized to levels as far above code requirements as is feasible, particularly for areas where people are unmasked (e.g., while eating in restaurants) and/or where there is mixing of people from different households, regardless of mask use.

In general, the greater the number of people in an indoor environment, the greater the need for ventilation with outdoor air. Efforts should be focused on providing fresh air ventilation to the spaces with the highest density of occupants, as well as where occupants may be unmasked. Decrease occupancy in areas where outdoor ventilation cannot be increased. Other changes that can be considered in buildings with specific ventilation features include:

  • For buildings with mechanical ventilation systems, see Section 5. Improving Mechanical Ventilation.
  • Inspect and maintain exhaust ventilation in support areas such as laundry rooms or kitchens.  
  • Ensure exhaust fans in restrooms and other areas are functioning properly and operating continuously or as needed. Since the virus can be present in fecal matter, closing toilet lids (if available) during flushing is advised.
  • Keep windows and other sources of natural ventilation open to the greatest extent possible.
  • Consider adding Portable Air Cleaners (PACs) in areas where fresh air ventilation cannot be increased.

To help you in improving your building's ventilation, some of the following professionals may be able to assist:

  • Facilities ("Stationary") Engineers,
  • Building Maintenance and Repair Staff,
  • Mechanical Engineers,
  • Mechanical (HVAC) Contractors,
  • General Contractors,
  • Architects, and/or
  • Indoor Air Quality or Industrial Hygiene Consultants

Schools and other interested parties are encouraged to read the Yale School of Public Health's web page Ventilation Key to Reducing Risk, part of Yale's Public Health Guidance for Reopening Schools in 2020.

4. Improving Natural Ventilation and Proper Use of Fans 

Consider implementing any of the following to improve the supply of outside air into a space, using caution on days with poor air quality:

  • When weather and air quality conditions allow, increase fresh outdoor air by opening windows and doors. Do not open windows and doors if doing so poses a safety or health risk to anyone using the facility.
  • Use fans to increase the effectiveness of open windows. Position fans securely and carefully in or near windows. Take care with electrical cords; look out for tripping or wet conditions, which can create electrocution hazards. Position fans so that air does not blow from one person to another. Window fans placed in exhaust mode can help draw fresh air into a room via other open windows and doors without generating strong room air currents. NOTE: For buildings with both operable windows and mechanical ventilation systems, the interactions between the two need to be carefully considered.
  • Some rooms have high ceiling fans. There is not enough scientific evidence supporting their effectiveness in diluting potentially contaminated air with cleaner air in the higher parts of the room. Ceiling fans do not bring additional fresh air into an indoor space and are not considered to be equivalent to fresh air ventilation. Given this uncertainty about their effect, ceiling fans should be turned off unless necessary for the thermal comfort of building occupants. Ceiling fans may result in improved air mixing, provided outdoor air is being introduced into the space.
  • For information on the use of portable air cleaners, please see Section 7. Portable Air Cleaners ("HEPA Air Filters").

5. Improving Mechanical Ventilation 

Consider mechanical ventilation system upgrades or improvements and other steps to 1) increase the delivery of clean air and 2) remove or dilute concentrations of COVID-19 or other contaminants in the building air. The amount of outdoor air brought into the mechanical system should be maximized.   

MERV 13 or greater filtration is efficient at capturing airborne viruses and should be the target minimum level of filtration. If the air handling system cannot function with such a high level of filtration, increase the filtration in the equipment to the maximum allowable for the system.

Note that (regardless of COVID-19) CCR Title 8, Section 5142, requires that mechanical ventilation systems be maintained and operated to provide at least the quantity of outdoor air required by the State Building Standards Code, Title 24, Part 2, California Administrative Code, in effect at the time the building permit was issued.

Obtain consultation from experienced HVAC professionals when considering changes to HVAC systems and equipment. Some of the recommendations below are based on ASHRAE's Guidance for Building Operations During the COVID-19 Pandemic. Review additional ASHRAE Guidelines for Schools and Universities for further information on ventilation recommendations for different types of buildings and building readiness for occupancy. Not all steps are applicable for all scenarios.

  • Fully open outdoor air dampers and close recirculation dampers to reduce or eliminate air recirculation. Set economizers at 100% outdoor air. In mild weather, this will not affect thermal comfort or humidity, but in cold, hot, or humid weather this may result in changes to indoor air, so expect a need for adjustments regarding clothing and/or space heaters.
  • Improve central air filtration to as high as possible without significantly diminishing design airflow. Target air filtration should be MERV 13 or greater.
    • Inspect filter housings and racks to ensure appropriate filter fit and check for ways that air could bypass the filter.
    • Clean or replace filters and check filters to ensure they are appropriately installed, seated, functioning, and are not torn. Note that during poor air quality events caused by wildfire smoke, for example, higher efficiency filters will load faster and will need closer monitoring. Since filters may be contaminated with virus particles, anyone changing filters must wear, at a minimum, a fit-tested N95 respirator in accordance with the requirements of CCR Title 8, Section 5144 or Section 5199, as well as eye protection (face shield or goggles), and disposable gloves.
  • Disable "demand controls" and occupancy sensors on ventilation systems so that fans operate continuously, independently of heating or cooling needs. This is done by setting the fan on the system’s thermostat to the “ON” position instead of “AUTO.”
    • If HVAC systems operate on day/night or other pre-programmed cycles, consider running the HVAC system at maximum outside airflow for 1-2 hours before the building opens and for 2-3 hours after the building is closed.  
    • Consider running HVAC fans 24/7.
    • Continuous operation of the HVAC system is required regardless of COVID-19 when employees are present under CCR Title 8, Section 5142.
  • Generate clean-to-less-clean air movement by adjusting the settings of supply and exhaust air diffusers and/or dampers in higher risk areas, so that potentially contaminated air is moved away from occupants.
  • Typically, in-room, wall-mounted fan coil systems do not remove virus particles; this could allow virus particles to accumulate in a space. Such systems should not be operated in occupied rooms unless the fan coils have MERV 13 filtration as a minimum.
  • The amount of outdoor air brought into the mechanical system should always be maximized regardless of air filtration.

6. Determining Mechanical System Function 

  • Small pieces of ribbon or tissue paper can be affixed to ventilation supply registers to verify that the system is operating.
  • A lightweight (down) feather on the end of a stick or dowel can be used to trace air currents such as from fans or PACs to verify that air is not being blown from person to person.
  • Carbon dioxide (CO2) levels increase as mechanical ventilation systems fail to keep up with the occupancy of a space. Therefore, the measurement of CO2 levels in a space may be used to determine the effectiveness of the ventilation system in more densely occupied indoor spaces. However, COlevel is a lagging indicator since it takes time for it to increase after a space becomes occupied. Consultation with a knowledgeable professional mechanical engineer or industrial hygienist on how to best to use CO2 monitoring technology in a facility is recommended.
  • If you need assistance in evaluating your system, see the professionals listed in Section 3. General Considerations.

7. Portable Air Cleaners ("HEPA Air Filters")

Portable Air Cleaners (PACs) should be considered in rooms and areas where mechanical and passive ventilation cannot be improved. PACs come in a range of sizes, features, and prices; higher-priced units may not necessarily provide greater improvements to air quality. Depending on the quantity, quality, and condition of existing ventilation, PACs providing 2-5 additional ACH may be needed. Review these key points about effective use of PACs:

  • Purchase PACs that are certified for ozone emissions and electrical safety by the California Air Resources Board (CARB).
  • Ensure PACs are appropriately sized for the room or area they are deployed in. One method for selecting the appropriate size unit is the Association of Home Appliance Manufacturer's (AHAM) Clean Air Delivery Rate (CADR). The authors of the CADR standard suggest that a unit should have a CADR at least 2/3 of the room's floor area (in square feet), with adjustments made if the room's ceiling is more than eight feet in height. If this method is used, the unit's CADRs for Smoke should be used. A list of all units with CADR ratings (with the rating values) can be found on AHAM's "Verifide" website. It's possible that a room may need more than one PAC.
  • PACs are very efficient at capturing coronavirus-size particles, but the particles must first physically travel to the filter. The faster a PAC can cycle air through the filter, the better its chances of catching virus particles. CADR reflects, in cubic feet per minute, the volume of clean air the PAC produces at its highest speed setting. (The efficiency of the PAC in cleaning the air decreases at lower speeds). PACs have three CADR ratings; smoke, dust and pollen, which represent small, medium and larger particles, respectively. For purposes of COVID-19, the smoke CADR rating should be used. Smoke particles are similar in size to the smallest virus droplets, while larger virus droplets are closer to the pollen size range. A PAC with a CADR of 250 for smoke reduces smoke particle levels to the same concentration that would be achieved by adding 250 cubic feet of clean air each minute.
  • For more in-depth help determining the correct size of PACs for COVID-19, Harvard University and the University of Colorado, Boulder have jointly developed a spreadsheet for identifying the correct PAC, using the CADR. If using this spreadsheet, please note that the PACs listed on the third tab are only examples of verified manufacturers and models; you can input your CADR (using the smoke value) for any unit on the second tab of the spreadsheet.
  • Manufacturer's specifications, CADR values, and the Harvard/CU spreadsheet all base their estimates on the PAC operating at maximum fan speed. Reducing fan speed may reduce the noise generated by the unit but will also decrease the amount of air filtration the unit will provide.
  • For effective air cleaning, a PAC should be placed towards the center of where people sit or gather with the unit exhaust directed so that it will not blow air from person to person. PACs that exhaust straight up should be used to avoid blowing air from one person to another. Placing air filtration units in unused corners of rooms or beneath tables will not effectively clean the air. Do not create a tripping hazard with the PAC or associated electrical cords.
  • Industrial air cleaners that use high efficiency particulate air (HEPA) filtration can be used and are particularly well-suited for larger rooms and areas:

Commercial/Industrial units, sometimes referred to as "Negative Air Machines (NAMs)" or "hogs," may already be available in larger facilities; check with Facilities/Maintenance personnel, who may also be able to order this type of unit through their equipment suppliers. All such units should be inspected for proper discharge of exhaust.

Industrial air cleaners typically do not have CADR ratings. Instead, the manufacturer's rated airflow (in CFM) is incorporated into the Air Changes per Hour calculation provided in Section 2. Definitions. Depending on the fresh air ventilation in the room, ACHs of 2.5-6 are needed, with lower values working for well-ventilated rooms, and ACHs of 4-6 for rooms with marginal ventilation.

8. Ventilation During Wildfire Smoke Events

  • Ventilation and filtration can be very effective in reducing indoor air concentrations of both wildfire smoke particles and coronavirus-sized particles; however, these strategies should be used in conjunction with physical distancing protocols, face coverings or masks, frequent hand washing, and other practices. 
  • When used with windows and doors closed, and when properly installed and maintained and operated, an HVAC system with MERV 13 filters will effectively reduce indoor exposure to both wildfire smoke and virus particles. 
  • Where buildings are not equipped with HVAC systems, PACs can effectively reduce the concentration of both smoke and corona virus-sized particles in indoor air. As noted above, more than one portable air filter might be needed to meet the air filtration rate recommended by the Association of Home Appliance Manufacturers (AHAM).
  • To filter wildfire smoke and coronavirus-sized particles, the PAC should have a CADR for tobacco smoke (0.9-1.0 μm).

9. Requirements on Ventilation in Cal/OSHA COVID-19 Prevention Standards

On June 17, 2021, Cal/OSHA implemented a revised version of the mandatory emergency temporary standard (ETS) originally implemented on November 30, 2020, to prevent employee exposure to COVID-19 in California workplaces, with the following exceptions: (1) when employees are covered by CCR Title 8, Section 5199, the Aerosol Transmissible Diseases standard; (2) when employees are working from home; or (3) in work locations with one employee who does not have contact with other persons; or (4) when employees are teleworking from a location of the employee’s choice which is not under control of the employer.

The ETS appears in CCR Title 8, Sections 3205 COVID-19 Prevention3205.1 Multiple COVID-19 Infections and COVID-19 Outbreaks3205.2 Major COVID-19 Outbreaks3205.3 COVID-19 Prevention in Employer-Provided Housing; and 3205.4 COVID-19 Prevention in Employer-Provided Transportation to and from Work.

The ETS requires covered employers to establish, implement and maintain an effective, written COVID-19 Prevention Program that includes elements pertaining specifically to ventilation and filtration, as follows:

  • Section 3205 (c)(2)(E): For indoor locations, the employer shall evaluate how to maximize ventilation with outdoor air; the highest level of filtration efficiency compatible with the existing ventilation system; and whether the use of portable or mounted High Efficiency Particulate Air (HEPA) filtration units, or other air cleaning systems, would reduce the risk of COVID-19 transmission.

  • Section 3205 (c)(2)(F): The employer shall review applicable orders and guidance from the State of California and the local health department related to COVID-19 hazards and prevention. These orders and guidance are both information of general application, including Interim guidance for Ventilation, Filtration, and Air Quality in Indoor Environments by the California Department of Public Health (CDPH), and information specific to the employer’s industry, location, and operations.

  • Section 3205 (c)(7)(A): For buildings with mechanical or natural ventilation, or both, employers shall maximize the quantity of outside air provided to the extent feasible, except when the United States Environmental Protection Agency (EPA) Air Quality Index is greater than 100 for any pollutant or if opening windows or maximazing outdoor air by other means would cause a hazard to employees, for instance from excessive heat or cold.

Under Sections 3205.1, Multiple COVID-19 Infections and COVID-19 Outbreaks, employers have additional ventilation and filtration requirements in the event of a COVID-19 outbreak, as follows (note: these provisions also apply to major outbreaks, addressed under Section 3205.2):

  • Section 3205.1 (e)(3): Multiple COVID-19 Infections and COVID-19 Outbreaks. The employer shall implement changes to reduce the transmission of COVID-19 based on the investigation and review required by subsections (e)(1) and (e)(2). The employer shall consider moving indoor tasks outdoors or having them performed remotely, increasing outdoor air supply when work is done indoors, improving air filtration, increasing physical distancing as much as feasible, requiring respiratory protection in compliance with section 5144, and other applicable controls.
  • Section 3205.1 (f): In buildings or structures with mechanical ventilation, employers shall filter recirculated air with Minimum Efficiency Reporting Value (MERV) 13 or higher efficiency filters if compatible with the ventilation system. If MERV-13 or higher filters are not compatible with the ventilation system, employers shall use filters with the highest compatible filtering efficiency. Employers shall also evaluate whether portable or mounted High Efficiency Particulate Air (HEPA) filtration units or other air cleaning systems would reduce the risk of transmission and, if so, shall implement their use to the degree feasible.

Under Sections 3205.3 and 3205.4, employers must implement ventilation requirements for employer-provided housing and transportation, as follows:

Section 3205.3 COVID-19 Prevention in Employer-Provided Housing …

… (c) Ventilation. In housing units, employers shall maximize the quantity and supply of outdoor air and increase filtration efficiency to the highest level compatible with the existing ventilation system. If there is not a Minimum Efficiency Reporting Value (MERV) 13 or higher filter in use, portable or mounted High Efficiency Particulate Air (HEPA) filtration units shall be used, to the extent feasible, in all sleeping areas in which there are two or more residents who are not fully vaccinated.

Section 3205.4 COVID-19 Prevention in Employer-Provided Transportation …
…(f) Ventilation. Employers shall ensure that vehicle windows are kept open, and the ventilation system set to maximize outdoor air and not set to recirculate air. Windows do not have to be kept open if one or more of the following conditions exist:

(1) The vehicle has functioning air conditioning in use and excessive outdoor heat would create a hazard to employees.

(2) The vehicle has functioning heating in use and excessive outdoor cold would create a hazard to employees.

(3) Protection is needed from weather conditions, such as rain or snow.

(4) The vehicle has a cabin air filter in use and the U.S. EPA Air Quality Index for any pollutant is greater than 100.

10. Resources

State of California

Cal/OSHA (Division of Occupational Safety and Health, Department of Industrial Relations) workplace safety regulations

Centers for Disease Control and Prevention

AIHA (formerly the American Industrial Hygiene Association)

American Conference of Governmental Industrial Hygienists

American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE)

Association of Home Appliance Manufacturers

California Air Resources Board (CARB)

Environmental Protection Agency (EPA)

Harvard University School of Public Health and University of Colorado, Boulder School of Engineering

Johns Hopkins Bloomberg School of Public Health Center for Health Security

World Health Organization

Yale University School of Public Health

This Interim Guidance for Ventilation, Filtration, and Air Quality in Indoor Environments was adapted with permission from a similar document prepared by the San Francisco Department of Public Health (SFDPH): SFDPH COVID-19 Information and Guidance

Originally Published on February 26, 2021