#filtration

How viruses lose infectivity in the air

Decrepit Filtration.

An old filtration pipe overgrown by wet looking vines, algae and mold. One side of the pipe seems to be broken open. The big grilles inside the pipe are partially broken and blue light is shining in from the top. Old stickers and graffiti can be seen on metal the inner walls.

I wonder what used to be filtered through these pipes and why they're not used anymore. What happened?

Two-thirds of the Earth's surface is awash with the stuff, but water -- specifically, the clean and drinkable kind -- is inaccessible to billions of people. A new purification system developed by researchers at the Beckman Institute for Advanced Science and Technology uses an electrified version of dialysis to separate salt and other unnecessary particles from the potable product. Successfully applied to wastewater with planned expansion into rivers and seas, the method saves money and saps 90% less energy than its counterparts. The study appears in ACS Energy Letters. If only stripping salt from water was as simple as waving a giant magnet above the Pacific or sifting liquid through a super-fine sieve. Once the shifty mineral dissolves, the separation process -- christened desalination in scientific circles -- becomes more expensive and uses more energy.

*Outdoors is better than indoors

*Indoors requires either greater ventilation or scrubbing of the air (filtration)

*Most buildings have poor ventilation, and cold or hot weather mat require less ventilation

*In those cases, you need to scrub (filter) the air alongside ventilation

*Air duct filtration/UV may have limited value

*Much better to filter "in the room where it happens" using portable filters and Upper-room germicidal UV (GUV) fixtures (not personal UV scrubbers)

*Upper room germicidal is most cost effective for schools and businesses

*Ionization ineffective and causes irritants

TLDR: Mechanical ventilation, upper room UV, and portable room air cleaners

"Outdoors, dilution of any aerosols is infinite—though the time it takes to dilute clouds of aerosol, depends on air movement. Think, for example, of how a cloud of cigarette smoke outdoors lingers or dissipates depending on whether there’s a breeze or not.

Indoors, however, aerosols almost always linger longer than outdoors—often long enough to be inhaled by someone sharing the same space. Put another way, if you breath in an indoor setting where other people are also breathing, you will almost surely breath in some amount of air that has been recently exhaled by someone else. That recycled air—the so-called rebreathed air fraction—estimated by room carbon dioxide measurements, is a good predictor of the risk of infection, given an infectious person generating infectious aerosol in the same room....

Think About Ventilation

Ventilation, natural or mechanical, is the main way that the risk of airborne infection indoors is reduced. For hospital airborne infection isolation and procedure rooms, the U.S. Centers for Disease Control and Prevention (CDC) recommends 6 to 12 room air changes per hour (ACH) with infection-free outdoor air, or air that has been filtered or otherwise decontaminated. One ACH occurs when a volume of air equal to that of the room enters and leaves over a period of one hour. As fresh air enters and mixes with contaminated room air, not all the contaminated air is removed by one air change. Under well-mixed conditions, one air change removes approximately 63% of room air contaminants, and a second air change removes about 63% of what remains, and so on.

But under real world conditions, the protection achieved by ventilation also depends on the amount of contaminant (virus in this case) being added over time, i.e. by an infected person, and on the contagiousness of the infection. The greater the infectiousness of the virus, greater the infection-free ventilation needed to keep concentrations low.

For Omicron, for example, 6-12 ACH ventilation, or equivalent air disinfection, may not be enough to prevent transmission. Unfortunately, not all transmission is preventable by air disinfection—for example, transmission at very close range where there is no time to remove or inactivate viruses generated by one person before they are inhaled by someone else.....

Many residential and older buildings without mechanical ventilation may have about one ACH or less due to air leakage around doors and windows—but when windows are open, depending on building design, orientation, and outside weather conditions, may enjoy significantly higher ACHs. For economical heating and cooling, however, windows are normally closed, especially in larger mechanically ventilated buildings, by design, or closed by occupants in response to outside temperatures. Automated mechanical ventilation systems often bring in a minimum amount of outside air under very cold or hot outside conditions, resulting in most air being recirculated within the building, thereby recirculating air contaminants rather than removing them...

....because mechanical ventilation may not be sufficient to reduce the risk of infection, mechanical ventilation in public buildings should be supplemented by other methods of air disinfection. For current and future viral pathogens like SARS-CoV-19, relatively high levels of “equivalent” ventilation by supplemental air disinfection will be needed.....

....the value of high-efficiency filters or germicidal UV in recirculating ventilation ducts for preventing spread is speculative and limited at best....

A more effective air disinfection strategy is to rapidly decontaminate the air within the room where person-to-person transmission occurs.

“In the room where it happened” is a song from the Hamilton musical, but it could also be a guide to the application of air-disinfection technology....