Per Wayback machine as of 5/23/25 this webpage still had iffy statements like "may provide a less-snug fit than respirators with head straps such as N95s." web.archive.org/web/20250523...

FULL DETAILS come to our presentation at #AAAR2025 in Buffalo, NY at 2:30pm on Wednesday, October 15, 2025

aaarabstracts.com/2025/view_ab...

CONCLUSION: The 3 filters in middle school retained filtration efficiencies like MERV 15 or higher, while 2 filters in the elementary school degraded to MERV 12 or lower => replace annually for school use (home use maybe more often). Anecdotally newer filters lasting longer.

MIDDLE SCHOOL (test #2): Filtration efficiencies were retained (>85%), in the same size range, for filters deployed in the middle school over the four-month period.

MIDDLE SCHOOL (test #1): filters from the middle school (4 months of use) yielded CADRs ranging from 190–213 cfm.

ELEMENTARY SCHOOL (test #2): The filtration efficiencies of the filters deployed in the elementary school degraded to <50% for particles in the 0.3 – 0.4 μm size range, which includes the most penetrating particle size (0.3 μm), over the 16-month period.

ELEMENTARY SCHOOL (test #1): Filters from the elementary school (16-months of use) yielded CADRs between 109–145 cubic feet per minute (cfm)

TEST #2: The size-dependent filtration efficiencies, for particles between 0.3 to 10 µm in diameter, were evaluated per an initial ASHRAE 52.2 test for each filter.

TEST #1: Each filter was tested in a SAFE DIY air cleaner, a 20” box fan paired with a fan shroud and safety screen. The clean air delivery rate (CADR) for particles between 0.3–2.685 µm was evaluated using aerosolized sodium chloride (NaCl) particles operating on low fan speed.

BACKGROUND: Supplementary Air Filtered Exchanges (SAFE) is a peer-reviewed Do-It-Yourself (DIY) air filtration system used in at least 4 San Francisco schools and several households thanks to @SaferTogetherSF @ValerieJay16 @anthrop_inst @crosscutanne.bsky.social
patientknowhow.com/safe.html

@crosscutanne.bsky.social

WHAT’s NEXT in 2026:

In Fall 2021, SFUSD installed “black box” HEPA purifier in 3000+ classrooms. Filter lasts 5 years (~$400). Needs replacement in 2026 @ 2.5 ACH.

For < $400 we get 3 DIY purifiers @ 6+ ACH.

What should SFUSD do in 2026?

www.sfchronicle.com/sf/article/S...

(4/4)

TEST PROCEDURE (6/5/2025):
1. Open windows to equalize aerosol concentration with outside
2. Close windows
3. Turn on air purifiers under test
4. Track aerosol concentration at 0.3 um every minute

(3/4)

DETAILS:
Classroom on 2nd floor of SF middle school
6600 cubic feet
Height 11' x Length 24' x Width 25'
No HVAC/ventilation system but it has 4 windows

(2/4)

Good question. What about same daily dose half in AM and half before bed? Blood pressure generally rises at wake up.

Pay attention to *all* variables. If you get in trouble but your cell phone has no service -- you're still in trouble. If you have cell service but EMS is 1 hour away -- you're still in trouble.

Interesting. Is main message here to stay in the green and avoid red by hydrating?

In hot climates, I found my thirst alone is an insufficient signal to hydrate so a conscious hydration strategy is needed to survive extremes. Sometimes electrolyte-drinks are also needed to recover from heat.