Related Materials: More Employees & Workplaces Guidance | All Guidance | More Languages
Updates as of October 7, 2021:
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.
COVID-19 is transmitted from person-to-person and may occur in the following scenarios:
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.
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 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 "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.
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:
To help you in improving your building's ventilation, some of the following professionals may be able to assist:
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.
Consider implementing any of the following to improve the supply of outside air into a space, using caution on days with poor air quality:
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.
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:
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.
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 Prevention; 3205.1 Multiple COVID-19 Infections and COVID-19 Outbreaks; 3205.2 Major COVID-19 Outbreaks; 3205.3 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):
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.
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