Health Threats from Airborne Pathogens
Airborne disease transmission occurs when pathogenic microorganisms hitchhike on dust particles or respiratory droplets.
Large droplets can settle out in a few minutes, smaller particles can suspend indefinitely and be windborne for some distance.
We are exposed by inhaling the contaminated air.
Of course air purifiers and sterilizers cannot protect us from bloodborne pathogens, or from airborne pathogens when close contact with someone who is ill is required.
Size Does Matter
Health-threatening bacteria and viruses range in size from .018 micron to as large as 1.325 microns.
I could drone on about relative air borne particle size and the micron, an important concept where most folks think in terms of visible particles - generally above 18 microns in size.
But a visual is worth a thousand words.
Here is a cool zooming graphic which helps us see what we are up against: the worst airborne pathogens are beyond microscopically small.
This opens in a new window, wait till it loads, then click on the words "start the animation." (sometimes it takes 2 clicks, worth the wait)
Click on the little blue arrows to zoom;
Notice that some smaller measurements there are in nanometers, but air purification is measured in terms of microns.
A micron is a thousand nanometers, or one millionth of a meter.
So the Ebola virus at 200 nanometers is .2 micron, the Rhinovirus at 20 nanometers is .02 micron.
Three general groups of health threat microbes are apparent from physical size; viruses, bacteria, and fungal/bacterial spores.
Filtration will remove spores and many bacteria, but some airborne viral pathogens are too small for filters.
Pathogenic microbes are initially distributed in droplets, about 10-100 microns diameter.
These dry quickly to droplet nuclei of 1-4 microns, averaging about 3 microns in size, containing viruses or bacteria.
Viruses are beyond microscopic, ranging in size from about .01 to .25 microns.
A few giant viruses go to .4 microns, overlapping the smallest bacteria, such as the Chlamydia and Mycoplasma.
A 0.1 micron pore membrane is required in the laboratory to retain Mycoplasma, known as the "stealth" health threat because the vast majority of medical lab tests cannot detect it.
Bacteria range in size from 0.2 to 5 microns, and most are caught by HEPA filters.
HEPA Air Fltration and Microbes
Viruses are too small to be captured in a HEPA air filter, which will trap most bacteria.
The HEPA specification calls for 99.97% removal of airborne particles in the most penetrating particle size, .3 micron.
Particles smaller than .3 micron are considered easier to capture from airflow.
It is important to realize that these filters are not like pore-sized laboratory filters used to culture microbes.
HEPA filters do not have holes this tiny, they rely on collisions with a network of fibers to trap particles.
No air purifier is 100% effective at removing airborne pathogenic microorganisms.
These unhealthy airborne microbes act differently than inanimate particles.
Some have mobility, and some have slippery surfaces and do not stick to fibers.
Some will find biological debris to eat and reproduce in the filter media.
This is one source of early filter replacement, especially in damp environments: bacterial odors from filters not clogged with dirt.
Tiny insects, such as Mediterranean Fruit Flies, hatching from citrus fruit, can cause this.
Antimicrobial filters are coming to market to combat filter media pathogen proliferation.
As detailed elsewhere on this site, airborne particle density is sporadic, clouds of dust raised by A/C blowers, foot traffic, and opening windows and doors.
Engineers can graph particulate levels and match peaks to these parameters.
Pathogenic microbes grow 24/7, in patterns unrelated to particulates.
Automated air purifiers can respond to chemical and particle levels, shutting down when sensors indicate air is cleaned.
Humans can manually regulate fan speed according to perceived need.
This type of cycling won't work where biologically healthy air is required.
Germicidal air purification means continuous full power operation to be effective against airborne pathogens, especially for sick or safe room applications.
Continuous high power operation means noise is a major factor.
Few HEPA aircleaners of the quality to be serious candidates for "germicidal" air purifiers are also quiet enough for continuous high speed filtering.
Many airborne microorganisms are carried on dust particles, so HEPA filtration should be the first building block of a health protecting system.
My regular readers know the next line: I start with an IQAir for baseline health security, adding cheap HEPAs (Honeywell, Holmes) as dust collectors. Then I experiment with mid priced anti-microbial technology leaders, Sharp Plasmacluster for example.
Ultraviolet for Pathogenic Microbes
Air purifiers with UV-C light use a short section of ultraviolet wavelengths clustered around 253.7 nanometers.
UVC is a proven method of disinfection, but is frequently under applied in inexpensive air purifiers.
Exposure of airborne microbes to a sufficient power level for a long enough period ("dwell time") creates a dosage which damages the pathogen's DNA structure.
This makes reproduction impossible, reducing the microbial population.
Bugs cannot develop immunity to this cleansing power. Viruses, without cell walls, are particularly vulnerable.
Everybody is jumping on the UV bandwagon, adding $10 lamps to $100 air cleaners and bumping price $40.
Why doesn't this work?
First, all UV bulbs are not created equal.
A key feature is the actual emission spectrum of the bulb: cheap bulbs typically emit unfocused UVC, with bands other than 254 nm getting more of the energy.
Second, to deliver proper germicidal dosage, airflow must slow down in the UV chamber long enough.
Doing this would limit CADRs and reduce particle filtration.
Many "UV air purifiers" end up just sterilizing the filter, with little reduction in airborne pathogen density.
Repeated passes to get the air pure require noisy high speed fans, universally disliked.
Third, there are health issues which many UV installations ignore.
You should not see a purple glow, unless a special (expensive) glass port is present: eye damage from leaky chambers is possible.
Cheaper glass bulbs may break and discharge their unhealthy Mercury vapor.
Unbreakable quartz is safer, and costs more.
Uh, boss, am I starting to see a pattern here?
And when you add the replacement bulb costs, cheaper ultraviolet air cleaners often look expensive.
Photocat: UV Plus
Photocatalytic oxidation (PCO) is an emerging technology that offers UV powered, self cleaning, low maintenance, germicidal air filtration.
I am working on an air purifier purchase for my wife's office building, where pathogen exposure is a bigger risk. PCO is my first technology choice.
The problem here is also cost: air purifier builders frequently add a hunk of aluminum coated with Titanium dioxide (TiO2) to an existing air purifier, and then double the price.
TiO2 is cheap enough they could coat the entire machine in it without significant cost pressure.
UV powered PCO systems share the health concerns of plain UV purifiers, plus the possibility of oxidant and partial product emissions.
The oxidation process is also very effective against odors and chemicals.
Plasma ion technologies are claimed to remove pathogenic bioaerosols.
Ions of both positive and negative polarities are claimed to cluster around airbornes, generating oxidant radicals in the airspace outside the air purifier.
Lab tests have shown the ion clusters to be effective against pathogenic airborne microbes.
I doubted this could be true, but my experience with my Sharp Plasmacluster suggests it really works. There are oxidant health issues here too.
In a chronic state of poor health, I am an extreme environmental sensitive, and can feel the tiny pinpricks where the clusters hit my skin.
A Plasmacluster user has reported possible partial product residues where a Sharp Plasmacluster was installed near some freshly antiqued furniture.
Carbon VOC adsorbers have little direct value in a germicidal strategy, but I consider at least one big carbon filter essential for promoting health.
Air moisture is a big factor in the survival time of airborne germs, they will dry up and die in 40% or lower humidity.
The practice of adding moisture to sickrooms is often questionable.
A serious germicidal health protection plan requires the investment of time and money.
Overkill is the only sure method, unless cleanroom level monitoring is attempted.
I suggest a minimum combined capacity of 10 air changes per hour, using multiple air purifiers and sterilizers.
Air Purifiers for Biological Airborne Health Risks
Mitsubishi Biofresh, an example of biocidal filters.
Airfree Air Sterilizer P1000 Review, a hot air sterilizer.