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Table of Contents:

1. Executive Summ​​ary​ 

2. Respiratory Infections Basics ​

3. Definitions​

4. Recommendations for Reducing Respiratory Infections in Indoor Environments​

5. Improving Natural Ventilation and Proper Use of Fans​

6. Improving Mechanical Ventilation​

7. Determining Mechanical System Function​

​​​8. Portable Air Cleaners ("HEPA Air Filters")​

9. Ventilation During Wildfire Smoke Events Resources​ 

​10. Resources​

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This Guidance is intended to be used for non-healthcare settings and applies to buildings in which business, assembly, other occupancy or use occurs indoors. See the CDC infection Control Guidance: Respiratory Virus and CDPH Best Practices for 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​ for more information.

​1. Executive Summary

To prevent the spread of respiratory infections, the California Department of Public Health (CDPH) recommends providing at least 5 air changes per hour or 30 cubic feet per minute (CFM​)per/occupant of equivalent clean airflow, whichever is greater, in occupied indoor environments. These recommendations for non-healthcare settings are in line are in line with CDC recommendations. It also targets values recommended by experts. Equivalent clean airflow can be achieved through a combination of ventilation with outdoor air and filtration using heating, ventilation and air conditioning (HVAC) filters and/or portable air cleaners.

2. Respiratory Basics 

This guidance is aimed at reducing the risk of respiratory infections that are transmitted through the air via inhalation. Such respiratory infections include COVID-19, influenza, tuberculosis, and measles among others. 

Individuals with these infections generate infectious respiratory partic​​les  when they breathe, talk, vocalize, sing, cough, or sneeze. These particles can be inhaled by others, particularly in shared indoor spaces.

Transmission can occur from “close contact" interactions or from farther away and over a longer timeframe. The longer an infected person stays in a crowded indoor space with other people, the greater the risk of transmission. This is because after they are exhaled, some infectious respiratory particles can remain suspended in the air and “build up" in an indoor space over time, especially if the space is poorly ventilated.  When this happens, other people can inhale the small particles that have accrued in the air and become infected. This can happen even if they are farther away, or even if they are not in close contact with the infected person.   

Improving indoor air quality is one of the most important ways to control the spread of respiratory infections. However, ventilation and other indoor air quality improvements are an addition to, and not a replacement for, mandatory protections required by any applicable state or local directive. 

3. Definitions

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 = (rate of air flow 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 can remove 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 PAC'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 passed through a PAC with an appropriately rated CADR, or through an HVAC system equipped with a Minimum Efficiency Reporting Value (MERV) filter*. Note that unfiltered outside air contaminated with wildfire smoke may not qualify as clean air.   

*The rate of recirculated clean air can be calculated by multiplying the recirculated air flow rate by the filtration efficiency value for filter in use which can be found in ASHRAE 241 Table 7-1.​

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 PACs equipped with a  high-Efficiency Particulate Air Filter (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 =

*filtration efficiencies of Minimum Efficiency Reporting Values (MERV) filters for infectious respiratory particles are specified in ASHRAE 241 Table 7-1.​

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

Equivalent Outdoor (Clean) Air flow rate per person (CFM/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 = ​[(OA inCFM) + (RA in CFM x filtration efficiency*)+ (CADR from PAC**)] / person

*filtration efficiency of Minimum Efficiency Reporting Values (MERV) filters for infectious respiratory participles is specified in ASHRAE 241 Table 7-1

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

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 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. HVAC systems are 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 air drawn from outside a 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 when wildfire smoke is present.

PACs are portable 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 reducing transmission of respiratory infections.

4. Recommendations for Reducing Respiratory Infections in Indoor Environments

The understanding of the role that the built environment plays in the transmission of respiratory infections has evolved; studies have clearly demonstrated that infectious respiratory particles 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 infectious respiratory particles transmitted through the air. 

Code compliance should be the minimum needed to create 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.

The 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). To see the calculation for this, see the “Equivalent Outdoor (Clean) Air flow rate per person (CFM/person) equation" above in the “Definitions" section. 

In contrast, target metrics of eACH (in units of ACH), 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. To see the calculation for this, see the eACH equation above in the “Definitions" section.​

Different authoritative bodies have released recommended target values for equivalent outdoor air flow rate per person and eACH in order to control respiratory infection risk. ​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 in their “Standard 241- 2023 – Control of Infectious Aerosols.”​

A 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 infectious respiratory particles 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 does 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. 

Besides CDC eACH recommendations, other experts have recommended (PDF) a target  equivalent outdoor air flow rate per person of 30 CFM/person.

Given all of this information, CDPH is currently aligned with CDC and experts in recommending a minimum of 5 air changes per hour or 30 CFM/occupant of equivalent clean airflow, whichever is greater, in occupied indoor environments. This recommendation is subject to change as science and understanding of the relationship between ventilation and respiratory infection transmission evolves.

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 and/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 facilities. For additional guidance on ventilation for isolated areas in other settings, see the CDPH Best Practices for Ventilation of Isolation Areas to Reduce COVID-19 Transmission Risk i​n 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. 

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.
  
• Ensure that exhaust fans in restrooms and other areas are functioning properly and operating continuously or as needed. 
  
• Keep windows and other sources of natural ventilation open to the greatest extent possible.
• Add Portable Air Cleaners (PACs) in crowded 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
  

5. 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, or if doing so is against the policy of the facility.
• Ceiling fans should be used as they improve air mixing and can help distribute outdoor air in the indoor environment, provided it is being introduced into the space. Ceiling fans also help with the effectiveness of HVAC and PAC filtration (discussed later in Section 6 and 8).
• Use fans to increase the effectiveness of open windows. 
• Position fans securely and carefully in or near windows.
• 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. Having windows and doors open during very cold or very hot days can strain building HVAC systems trying to maintain a comfortable, stable indoor air temperature.​

6. Improving Mechanical Ventilation​ 

At a minimum, all facilities shall achieve minimum outdoor air ventilation rates set by the State Building Standards Code, Title 24, Part 6, California Administrative Code, in effect at the time the building permit was issued. The most recent ventilation requirements can be found in Section 120.1.

In general, if these outdoor air ventilation rates are being met and supplemented with HVAC filtration and PAC use, the facility's ventilation and filtration will play a meaningful role in reducing respiratory infection risk.

Note that CCR Title 8, Section 5142, requires that mechanical ventilation systems be maintained and operated continuously to provide at least the quantity of outdoor air required by the State Building Standards Code, Title 24, Part 6, California Administrative Code, in effect at the time the building permit was issued. Note also that this regulation requires HVAC systems be run continuously during work hours if the facility is a place of employment.

If ventilation needs to be improved to meet or exceed these targets, 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 infectious respiratory particles or other contaminants in the building air. 

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.

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.

• Ensure outdoor air dampers are open and not closed to promote minimum levels of outdoor air as required by Title 24.
• 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. 
• During work hours or hours building is occupied, operate HVAC fans 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.”
• Continuous operation of the HVAC system is required regardless of respiratory infection risk 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. These systems should be supplemented by additional filtration such as PACs.

7. 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 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 4. General Considerations.

8. 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 “filtration-based” (rely on HEPA filters), as opposed to units that advertise additive cleaning technologies such as ionizers. This is because according to U.S. EPA, ionizers have the potential to generate ozone and other potentially harmful by-products indoors​.
• 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 infectious 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 CFM, 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, multiple smaller PACs can be used together to achieve desired CADRs at lower noise levels. 
  
• PACs have three CADR ratings - smoke, dust, and pollen, which represent small, medium, and larger particles, respectively. For the purposes of filtering for infectious respiratory particles, the smoke CADR rating should be used. 
  
• 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 for schools, 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/Colorado University 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 with exhaust systems that point straight up should be used to avoid blowing air from one person to another. 
• Placing air filtration units in unused corners of rooms, near open windows or beneath tables will not effectively clean the air. 
• Becareful not to 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 3.
  

9. 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. 
• In buildings that are not equipped with HVAC systems, PACs can effectively reduce the concentration of both smoke and infectious respiratory 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.
  

10. Resources​