ACH map of the US

Below, is a map of minimum hourly air exchanges (ACH) recommended in 19 states (did not find any others), showing how they differ across public health agencies that made a recommendation (2 to 12 ACH).

Every effort will be made to keep this up-to-date however these recommendations are subject to change. If you have any questions or would like to suggest updates to any of the ACH recommendations listed here please reach out to me at sri at patientknowhow dot com or @sri_srikrishna on Twitter.

What does CDC say about minimum ACH?

Many states reference CDC's ”Ventilation in Buildings” which does not have explicit ACH recommendations. But oddly it recommends equivalent of 5 ACH (for HEPA air cleaners) based a “2/3 rule” from Association of Home Appliance Manufacturers (AHAM). Specifically, on this CDC web page there is a question, “What is a HEPA filter…” In the answer to this question, it references a so-called “2/3 rule.” For a 300 sq. ft. room with 8 ft. ceilings = 2400 cu. ft. requires 200 cfm, which is equivalent of 5 ACH = 200 x 60 / 2400. For any area and height, they page says to scale the 2/3 rule by height so it works out to 60 * (2/3) / 8 = 5 ACH.

What does ASHRAE say about minimum ACH?

Some states refer to ASHRAE guidance on schools. The 2020 version recommended a minimum of 2 ACH ("2 rotations/hour") as part of "Filtration design criteria." The 2021 version simply says "Target highest achievable air change rate." For example, Michigan updated their guidance in April, 2022 and linked to the 2020 version (not 2021). Like Michigan, many others also link to the older 2020 ASHRAE.

What does California say about minimum ACH?

As shown in the image below, California appears to be unique in recommending at least 6 ACH, ideally 12 ACH for schools in July 2021. This is the highest ACH recommendation I have seen among all states and the CDC combined. In July 2022, California reaffirmed this guidance for schools (see "technical considerations"). For other poorly ventilated indoor spaces, California recommended 4 to 6 ACH.

What do other states recommend for minimum ACH?

• California recommends 6-12 ACH for schools, and only recommends 4-6 ACH for other poorly ventilated indoor spaces
• Hawaii refers to CDC (5 ACH)
• Illinois refers to CDC (5 ACH)
• Kentucky recommended 6 ACH for schools, "All day: Program system to run terminal unit fans (classroom unit fans) continuously (don’t cycle them on and off) to provide more than six air changes per hour of MERV-13 filtered air."
• Maryland recommended "~3-5 per hour" or 3-5 ACH for schools
• Michigan refers to ASHRAE (2020) for schools (2 ACH based on 2020 guidance)
• Minnesota refers to CDC (5 ACH)
• Missouri (St. Louis) recommended 6 ACH
• Montana refers to ASHRAE for schools (2 ACH based on 2020 guidance)
• New Jersey refers to CDC (5 ACH)
• New Hampshire refers to CDC (5 ACH)
• New York refers to CDC (5 ACH) for schools
• Oregon recommends 3-6 ACH
• Rhode Island recommends 4-6 ACH
• Tennessee recommends 6 ACH
• Virginia recommends 4-6 ACH for schools
• Vermont recommends 6 ACH for schools
• Washington recommends 5-6 ACH
• Wisconsin refers to CDC (5 ACH)