OUR BIMAH REGULARLY HOSTS the highest concentration of people, and the lowest circulation of air, of any space within our sanctuary. Arguably this is where an air purifier (or two or three) can really make a difference in preventing the spread of infection.
In this post, I recommend a particular brand and model of commercially available air purifier for our bimah. And I report on the research that led me to this recommendation.
NOTE: Air purification on the bimah is one of three parallel avenues that are being explored to reduce the risk of airborne infection in the sanctuary, for the sake of public health during the pandemic. The other two avenues are: air conditioning filtration, and fans in the windows.
Executive Summary
RECOMMENDATION: Purchase 3 units of the Coway AP-1512 from Amazon.com (1-day Prime). Install them at tabletop height, placed according to the map below, and run them at medium speed.
- This appears to be the only commercial model for which there are published lab-tested noise-level and filter-performance results specifically for aerosols.
- Together, 3 units on the bimah in theory would achieve 5.9 air changes/hour on average, and very likely within an acceptable noise range.
- The cost for 3 units is $504 plus tax — which totals below the $600 expenditure limit that, when exceeded, requires a board vote in advance.
- The unit size is about 18" high x 17" wide x 10" deep. It weighs 12 pounds. Air intake is from the front, and the exhaust blows upward.
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| Top view |
NOTES:
- Reliable, useful data for the other models is markedly lacking; but even so they either clearly don’t meet the criteria or appear to be more expensive.
- These units will generate some heat on the bimah; at medium speed, my guess is that together they would produce about the same amount of heat as one adult person.
- Once their box is opened, we would own these units. Amazon’s return policy allows for the (free) return of an air purifier only if it is in “original condition” (new and unused) and within 30 days of receipt.
- What should be claimed as our achievement if the recommendation is adopted and implemented? That “we have good reason to believe that there is sufficient air filtration to reduce the risk of infection from airborne viruses, as might be inadvertently generated by people on the bimah.”
Design Considerations
Analysis
Volume of the Bimah Space
The following analysis treats the covered part of the bimah as if it were isolated from the larger sanctuary hall. That seems like a conservative assumption, because the bimah feels noticeably more stuffy than being in the main part of the hall. Thus the risk of infection from aerosols generated on the bimah itself seems greater than the risk of aerosols migrating there from the more diluted air of the main hall. However, it must be admitted that we really don’t know the effect of the fact that one side of the bimah space is open.
The inner (covered) part of the bimah measures 22’ wide x 13’ deep x 10’ high = 2860 cu. ft. (See the plan view, below.)
Target Rate of Air Treatment
A target treatment rate of 5–6 air changes per hour (ACH) seems to be optimal. That is, it appears to achieve a substantial benefit at reasonable cost. As reported in the L.A. Times and elsewhere:
“Changing the air in an indoor space just five times an hour … can cut the risk of COVID transmission in half, researchers have found. In Italy, one analysis found that ventilation systems could reduce the risk of coronavirus infection in schools by more than 80% if the air was changed six times an hour.”
Given the size of the bimah space, we can calculate that a filtration unit (or pair of units) needs to be able to process a minimum quantity of cubic feet per minute of air. That is, the effective Clean Air Delivery Rate (CADR) for 0.3 micron aerosols should be at least:
5 ACH x 2,860 cu. ft./AC x 1 hr./60 min. = 238 cfm
Target Noise Level
For the bimah, I am aiming for a purifier fan’s noise level of 45 dBA or less, because 45 is said to be the boundary between “quiet” and “moderate noise,” or somewhere in between a babbling brook and a refrigerator. (See charts HERE and HERE.) Every 3-dBA increase is said to represent double the amount of noise.
(ABOUT THE MEASUREMENT OF NOISE: The human ear is less sensitive at low audio frequencies than at high ones. In order to account for this, a weighting adjustment “corrects” an objective decibel, or dB, measurement, to reflect how the human ear actually perceives sound. Corrected values are usually given in units of dBA.)
Placement
For three commercial units—which typically draw air in from their sides and blow filtered air out their top—I would suggest a placement in the two back corners of the bimah and near one of the front corners, as shown. (Gray boxes represent the filtration units.)
Product Evaluation
The various units considered include:
• The DIY air filtration unit that I assembled, as discussed HERE. This is the least expensive option by far. Unfortunately, it is both too noisy (roughly 54 dBA, judging from Srikrishna’s tests) and too visually unappealing for use on the bimah. (That’s Rabbi Katzan’s judgment, and I would agree.) To surpass 5 ACH would require 3 such units on the bimah.
• Coway AP-1512 (recommended) — tested by Srikrishna, which calculated a CADR of 94 cfm for 0.3 micron-sized aerosols when run at medium fan speed. (This level was 44% of the CADR at the highest fan speed — a huge drop!) The tested noise level was 46 dBA at 9" away, which is at the edge of acceptably quiet. To surpass 5 ACH would require 3 units on the bimah, at a cost of $168 x 3 = $504 plus tax via Amazon.com (1-day Prime). The result would be 5.9 ACH. Cumulatively, 3 units might make too much noise, but that concern should be obviated by spacing them widely apart—which is desirable for purification effectiveness anyway. And if noise does prove to be an issue, one of these units could be turned off temporarily when only 1 or 2 people are on the bimah. Although Consumer Reports rated this model poorly, those results were due to two issues that do not matter to us: lackluster performance at low speed, and too noisy at high speed. User manual available on Amazon product page. 3-year limited warranty.
• Blueair Classic 605. CADR for 0.3 microns is unknown; at medium fan speed, which the manufacturer rates at 44 dBA, CADR is probably above 200 cfm. If so, 2 units would be needed to exceed 5 ACH target, but possibly 3 would be needed. Model recommended by Consumer Reports, despite being very loud at the highest speed. No pre-filter: difficult to clean. On sale from manufacturer for $332 ea. including shipping, with a 60-day trial period. Total cost $664 plus tax. I called Blueair re CADR and dBA at medium speed for another of their models and was told: “Our engineers in Sweden haven’t provided us with that data, even though we’ve been asking for it for months.” Even if they did provide it, it would be for smoke and not for 0.3 microns.
• Coway Airmega 300 — tested by Srikrishna only for the highest fan speed: CADR for 0.3 micron aerosols = 265 cfm, at 59 dBA — which is too loud. Medium-speed performance is unknown; CADR is probably above 200 cfm, which would require at least 2 units to exceed 5 ACH target, at a minimum cost of $412 x 2 = $824 via Amazon.com (2-day Prime). Seems highly regarded on various websites. Although Consumer Reports rated it poorly (NOT recommended), those results were due to two issues that are not of interest to us: lackluster performance at low speed, and relatively high filter costs. Given the relatively high cost, this option does not seem pursuing further.
• Honeywell HPA5300 — recommended by Consumer Reports, being relatively effective even at its lowest speed, although operating costs are relatively high. However, the manufacturer gives an indirect CADR rating online (and by phone) only for its highest fan speed, which is presumably too loud for our purposes. It has 4 speeds, but they actually reflect different modes of operation. Only the lowest speed is “optimized to capture small particles”; apparently that’s when all three of the unit’s HEPA filters are brought into play. (In contrast, the 2nd-highest setting is optimized for pollens, which are 16–36 times larger than the aerosols of interest to us.) An email query received this answer: “We only measure the CADR on the highest setting of the air purifier.… Smoke: 226 cfm.… Additionally, we do not measure the decibel level of our air purifiers.” Thus it is likely that to surpass 5 ACH at the lowest speed would require 3 such units on the bimah; at $296 via Amazon.com (2-day prime; versus $310 MSRP), total of $888 plus tax. There is a bit of a gamble here as to whether 5 ACH would be achieved, but also something of a safety margin.
• The Airthereal AGH550 was tested by Srikrishna and showed good results, as reported on in his article that I discussed HERE. However, the company website says: “Airthereal ozone generators do not meet CARB [California Air Resources Board] requirements and cannot be shipped to California.”
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