biologicalcontrols.com - Omnidirectional vs. Unidirectional

What makes air purification systems effective?

The CDC Guidelines for TB Control address the need for dilution of airborne bacteria in rooms which are occupied by known or suspected TB patients.  Dilution is a key ingredient as an engineering control. Obviously, the fewer particles in the air, the less risk of anyone breathing them in.  But even with its importance, it is often overlooked and misunderstood.  This is for one main reason - we can't see either the bacteria in the air or the dilution occurring.  For this reason, the CDC explains what is known to ventilation experts around the country; that is, room air does not mix well in order to get the expected dilution.  The air must be moving if dilution is to occur, and it must be moving throughout all areas of the room.  The guidelines discuss the need to test rooms and visualize air movement with smoke sticks. Corrective actions should be taken if there is a lack of air movement anywhere in the room. 

It is important to understand why the air doesn't move well.  Air handling systems built since the
1970' s were designed to conserve energy while maintaining patient comfort.  Most air remains in
the building, and supply air in the room is typically delivered over the bed so that the patient will get the benefit of the conditioned air entering the room.  Exhaust ducts are typically placed near the door in order to try to avoid "short circuiting" of the air.  Since relatively small amounts of air are brought in, except when there is an exceptionally great need for cooling, there is not the capability of the incoming air to displace all the air occupying the room.  It helps to think of the air as basically lazy, since it will only move when forced to.  This lack of movement leads to dead areas which can be heavily laden with pathogens, and released when air currents in the room change for one of any number of reasons.  These pathogens can be directed anywhere, including out a door if one is open. 

The CDC allows for the use of portable HEPA filtration systems to help in achieving the dilution
needed in a room.  They caution though, that in order to be effective, they must move all the air in
the room - remember the reason for their use in the first place.  They also mention that not every
device can achieve this.  Let's look at the reasons why, as they are inherent in the designs.  We'll
use some drawings to help in visualizing what is happening, but you can use your mind to picture the movement of a particle as it is affected by each type of system. 

A system designed with air exhausted out the front, either straight or at an angle, has a limitation in its ability to move all the air rapidly enough to be effective.  This is referred to as unidirectional exhaust.  The air is cleaned, and depending on placement, pushes contaminants away from the bed area and then into general distribution around the room.  As more and more filtered air is brought into the room, eventually a portion of the contaminants are drawn into the system and filtered out. This type of system will not be able to rapidly move all the air. Imagine trying to blow out candles on a cake when there are a large number of them, or a fan in a room blowing in one direction as opposed to an oscillating fan.  Unidirectional exhausts cannot achieve much better than 60% reductions in particle counts by the manufacturers' own tests and admission, and this fact has been verified in other tests as well.  The second major limiting factor is that air is very difficult to pull, but easy to push.  These systems rely nearly entirely on the system to pull air back to it in order to filter the air.  Tests with fans, the size of those put into these types of products, show that air can be pulled from only about 5 to 6 feet at best, if there are no other affects on the air,  which unfortunately is the case when air is being blown in the other direction.  This air delivery design (top exhaust in one direction or 'unidirectional' airflow) is consistent with every unit currently available on the market, except for the MICROCON®.  The differences between both systems are dramatic and conclusive. The difference translates into performance, the ability to clean an entire roomquickly, with an extremely high percentage of particle removal and maintaining a consistently low particle count anywhere in the room.

MICROCON® 800/400. Omnidirectional airflow pattern

360°

INTAKE                                 EXHAUST

By combining air intake and exhaust in 360 degree patterns, air patterns are able to be produced keeping air moving everywhere in the room and moving it toward the air intake.
The MICROCON® utilizes this airflow pattern, which we refer to as CIRCUMFLOW®. The intake is on the top, within the breathing zone, an area from 3 to 7 feet off the floor.
The MICROCON® continually extracts contaminated air from the very region that is most critical to individuals since it is the air we are breathing. Contaminants are filtered out and clean air is exhausted out the bottom in a 360 degree pattern. Air is pushed in a horizontal plane until it hits a vertical surface, at which time it moves up that surface. When it contacts another surface, the ceiling, it moves back toward the MICROCON®, which is pulling air toward it.This process of pushing and pulling air is continuous, assuring that the air will be cleaned rapidly. Anything expelled into the breathing zone will be removed before it can be scattered around the room. Air is constantly in motion, eliminating stagnant layers and pockets of contaminated airborne pathogens.  This has been demonstrated repeatedly with tests, some by filling a room completely with smoke particles and seeing them removed in 5 to 10 minutes.  Similar tests can be done for you.

The CIRCULFLOW® pattern is critical because respirable particles in the submicron size range, such as the Mycobacterium tubercle bacteria, will float around the room, as will those particles just over a  micron in size, such as droplet nuclei.  They are propelled by air currents in the room and will not fall to the floor. They are light enough to float for weeks or months with no air currents, so they will remain suspended indefinitely when air currents are present, unless there is a means to remove them. Particles that are most dangerous, because they are of a size that, when inhaled, will pass the body's defensive system and lodge deep in the lungs, are invisible to the eye thereby making them that much more dangerous.  They can become entrapped in stagnant air pickets which you can't see, but exist in any room.  Unidirectional air filtration systems, since they are blowing air in only one direction, can scatter these pathogens in all directions.  The air over the patient may be clean, but air in other parts of the room can be increasing with pathogens until some type of equilibrium is reached.  The key element for reducing the risk of breathing hazardous particles is not by scattering them around the breathing zone, but by removing them from it completely.  This is why the MICROCON® was designed as it is and is the only patented system for removal of airborne contaminants.

MICROCON® top inlet, bottom exhaust (4 directions)
Omnidirectional airflow pattern