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​​​This Guidance is intended to be used for buildings in which business, assembly, other occupancy or use occurs indoors.

Employers should familiar with and implement all employee protection requirements in the Cal/OSHA COVID-19 Prevention Non-Emergency Regulations. Please see Section 9 of this document to identify the specific provisions of the Cal/OSHA COVID-19 Prevention Non-Emergency Regulations that pertain to ventilation.

The following guidance supplements the Cal/​​OSHA regulations by recommending practical steps that 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 and other aerosolized infectious agents.

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 should continue to meet the requirements of the Cal/OSHA COVID-19 Prevention Non-Emergency Regulations​.

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 this 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. 

Local health jurisdictions and other entities may continue to have requirements in specific settings based on local circumstances, including in certain higher-risk settings or during certain situations that may necessitate mask requirements (for example, during active outbreaks in high-risk settings).

1. COVID-19 Basics 

COVID-19 is transmitted from person-to-person and may occur in the following ways (the categories are not mutually exlcusive):

• 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. 
• Inhalation of airborne particles suspended in the air is now considered the primary route of transmission for COVID-19 infection. More guidance can be found in the CDC's SARS-CoV-2 Transmission research brief.  
• Large droplets from coughing and sneezing are propelled directly into the face, nose, eyes, or mouth of someone nearby, usually within six feet. These droplets are sometimes called "ballistic droplets" because they tend to travel in straight lines and fall out of the air rapidly. 

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 COVID-19 Prevention Non-Emergency Regulations and any applicable 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 and use a respirator when COVID-19 is circulating, especially when transmission levels are high.

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 Imperial 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. This is also referred to as the Non-infectious Air Delivery Rate. 

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)

CFM/person or cubic feet per minute per person, is a measure of the air flow provided per each person in a room. It is equal to the total airflow of the room (in CFM) divided by the number of occupants.

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.   

Equivalent Outdoor (Clean) Air Changes per Hour  (eACH, also called Equivalent (Clean) Outdoor Air Change Rate) approximates how many times the air within a space is replaced each hour by any combination of outdoor air ventilation (OA) provided by a mechanical ventilation system or natural ventilation, recirculated air (RA) that is filtered and then returned to a space, and/or air supplied within a space after being filtered by portable air cleaners (PACs) equipped with a HEPA filter. eACH is a calculated value that allows standards, guidelines, and comparisons for ventilation to be made for rooms with different dimensions and different ventilation systems.

Using Imperial units, the formula for eACH is:

eACH =

*specified for a given particle size range

**use CADR value for “smoke" if different CADRs are presented

Equivalent Outdoor (Clean) Air flow rate per person (CFM/person), or cubic feet per minute per person), is a measure of equivalent flow rate of pathogen-free air per occupant of an indoor space.

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

CFM/person = ​

Fans are devices that pull or push air in one direction. Fans can be rectangularly shaped for placement in windows or doorways, they ma​y 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 airflow.

HEPA Filter refers to a High-Efficiency Particulate Air Filter. This 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 "Make-up Air." Note that outside/outdoor air may not always be considered clean, such as during times of wildfire smoke.

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.

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 has evolved; studies have clearly demonstrated that small virus containing aerosols can be carried well beyond a six 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 SARS-CoV-2 the virus that causes COVID-19. 

Consequently, code compliance should be considered as the baseline, or starting point, in creating more protective environments. Ventilation (i.e., equivalent ventilation, achieved by a combination of outdoor air, adequately filtered recirculated air, and air filtered by PACs) 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.

Occupant density of an indoor space is a crucial factor to consider when setting ventilation targets, as spaces of the same volume but with different numbers of occupants will require different total ventilation rates to provide the same level of protection for each occupant. One target metric that considers density of occupancy is the equivalent outdoor air flow rate per person (in units of CFM/person).

In contrast, target metrics of eACH (in units of /hr), do not factor in the number of occupants in an indoor environment and are thus more approximate; that is, eACH ventilation target levels, while simpler to apply broadly without knowing the number of occupants, may under- or over-protect individuals relative an evidence-based requirement of ventilation per person.

For example, REHVA, the European organization of ventilation engineers, has developed a strategy (PDF) for determining minimum ventilation airflows needed to control risk of indoor airborne infections, based on relevant mathematical models and the best available assumptions. The corresponding U.S. organization of ventilation engineers, ASHRAE, has also developed an approach to setting recommended airflows to control airborne infection risk.

A recent report from the Lancet Commission (PDF) drawing on available scientific evidence on airborne infection control has recommended 21 cfm per person as “good," 30 cfm per person as “better," and greater than 30 cfm per person as “best." These recommendations for Non-infectious Air Delivery Rates are based on a broad review of the available evidence, provided in their document. For selected building occupancies and sizes, the Lancet Commission also reported their “good" and “better" target flow rates as 4-6 eACH. (Because density of occupancy may vary across buildings, these eACH targets are approximations based on assumed density of occupancy.)

These eACH recommendations are similar to a recent CDC recommendation target of 5 eACH in indoor spaces to help reduce the risk of COVID-19 transmission. Per CDC, this target value is likely to reduce infectious particle concentration and reduce infection risk by an unknown amount, but will not guarantee that infection risk is eliminated. The CDC document did not provide target ventilation airflows per person, and provided as justification for their specific eACH target two main points: that the Lancet Commission Report based on available scientific evidence proposed 4-6 eACH, and that 5 eACH is provided by portable air cleaners when properly sized following U.S. EPA guidance. 

Please note that even high levels of ventilation will have limited effectiveness in reducing any transmission through virus-containing particles to susceptible persons very near an infected person; greater distancing between occupants, or masking, are more likely to reduce short-range transmission. 

Additionally, greater ventilation rates may be necessary or required under regulations for higher-risk settings that are outside the scope of this document, such as healthcare. For additional guidance on ventilation for isolated areas in other settings, see the California Department of Public Health (CDPH) Ventilation of Isolation Areas to Reduce COVID-19 Transmission Risk in Skilled Nursing Facilities, Long-Term Care Facilities, Hospices, Drug Treatment Facilities, and Homeless Shelters guidance. Further research is needed to identify optimal ventilation and occupancy strategies for all spaces and contexts.

In general, the greater the number of people in an indoor environment, the greater the need for ventilation with outdoor air, in combination with filtration of the indoor air. Efforts should be focused on providing fresh air ventilation and filtration to the spaces with the highest density of occupants, as well as where occupants may be unmasked. 

Crowding in areas should be avoided where outdoor ventilation cannot be increased and respirators or masks are not worn. Reducing transmission risks near infected persons requires greater physical distancing, which may be assisted by reduced occupancy. 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 that 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 to supplement other control measures.

To help improving a building's ventilation, one or more 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
  
• Indoor Air Quality or Industrial Hygiene Consultants
  

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.

• Omit using ceiling fans as a form of improving air quality in rooms as a prevention measure. 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 the 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. Review additional resources at the end of this document 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.
• Increase central air filtration as much 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 can 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, CO2 level 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 enough to meet targets. 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 can be useful to provide 2-5 additional eACH. 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. 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). Note that in spaces where low noise is critical, such as in classrooms, identifying and using low-noise units is especially important, because keeping the noise level tolerable on a noisy PAC may require operation at a low speed, with low air cleaning efficiency. 
  
• PACs have three CADR ratings - smoke, dust, and pollen, which represent small, medium, and larger particles, respectively. For the purposes of filtering for 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.
• PACs require cleaning, inspection, and filter replacement. Be aware of operating parameters (such as recommended fan speed), placement, and maintenance practices that optimize the benefits provided by the unit.
  
• 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 and equivalent Air Changes per Hour calculation provided in Section 2.
  

8. Ventilation During Wildfire Smoke Events

• Ventilation and filtration can be very effective in reducing indoor air concentrations of both virus particles and other types of particles (e.g., wildfire smoke, pollens, spores, allergens); however, the strategies used should be adjusted appropriately (e.g., reducing the amount of fresh air being brought in during wildfire smoke events).
• 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 coronavirus-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 AHAM.
  
• To filter wildfire smoke and coronavirus-sized particles, the PAC should have be operated using the CADR for tobacco smoke (0.9-1.0 μm).

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

The Cal/OSHA COVID-19 Prevention Non-Emergency Regulations ​​​​appears in CCR Title 8 and requires covered employers to establish, implement and maintain an effective, written COVID-19 Prevention Program that includes elements pertaining specifically to ventilation and filtration. Covered employers should consult the Cal/O​SHA COVID-19 Prevention Non-Emergency Regulations and Cal/OSHA FA​​Qs for the specific ventilation and filtration requirements contained in these subsections:

• Section 3205 COVID-19 Prevention
• Subsection 3205 (h)(1)
  
• Subsection 3205 (h)(1)(A)
  
• Subsection 3205 (h)(1)(B)
• Subsection 3205 (h)(1)(C)
• Subsection 3205 (h)(2)
  
• Subsection 3205 (h)(3)
  
• Subsection 3205 (h)(4)
  
• 3205.1 Multiple COVID-19 Infections and COVID-19 Outbreaks
• Subsection 3205.1 (e)(1)
  
• Subsection 3205.1 (e)(3)
  
• Subsection 3205.1 (f)
• 3205.2 COVID-19 Prevention in Employer-Provided Housing
• Subsection 3205.2 (c)
  

Information on major o​utbreaks can be found in 3205.1.