As their name suggests, HEPA filter negative air machines combine two technologies: HEPA filters and negative air machines. High Efficiency Particulate Air (HEPA) filters are responsible for removing harmful particulate from the air. Negative air machines are responsible for keeping the filtered air contained, so that nothing above the specified particle size goes in or out. The need to combine the precision of a HEPA filter with the power of a negative air machine was somewhat of a rarity in pre-pandemic life, but now this is changing.

Air barriers and the limitations of existing structures

An air barrier refers to any situation where you have air on one side of a structure that is kept separate from the air on the other side. The classic example would be cooler indoor air that is kept separate from warmer air outside. Although it may work well to have housing material serve as the main device for creating air barriers in most applications, it is not good enough in others. For example, in a hospital ward where some patients are infected with a contagious airborne virus, it is not always possible to erect a structure around them to create the necessary air barrier. Even if this could be done, you also have the problem of ventilation – what kind of duct work would have to be present to keep the air moving? Where would the contaminated air go once it leaves the structure itself?

Because this kind of solution is infeasible, the most common approach for many years was to simply filter the air where it was needed, and if the filtration was robust enough, hope that it was sufficient to keep people safe. Usually this involved using an air scrubber device. An air scrubber is essentially a HEPA filter with fan blades underneath it. The idea is that the fan can operate at differing speeds. For a room that is small, you can expect the air in the room to be cleaned on a lower fan speed. If the room is larger, the same level of air purity will require a higher fan speed. This kind of solution is hardly adequate. The main issue is that it often takes an air scrubber one hour to clean the air in the whole room. During that one hour, there is plenty of opportunity for an airborne virion to travel to the other side of the room and get inhaled by somebody else. Increasing the fan speed may lower the amount of time that a particle has to travel, but this is surely not enough to stop the migration of particles altogether. This is where the magic of HEPA filter negative air machines comes in.

Why negative air machines are the gold standard

The general idea behind a HEPA filter negative air machine is that it is a portable structure with its own ducts, filters and fans. Often the walls of the structure are made of a soft material, like a tarp or tent. This is partly a matter of convenience – it is practical for a non-permanent structure to be easily taken down and put up again. It is also ideal from an engineering standpoint. The soft structure has the ability to act like a balloon when the air pressure in the structure is higher than outside, and act like a vacuum when the inside pressure is below the outside level. This means that if you want to prevent the particulate from dissipating, making the unit’s pressure lower than outside will serve to keep everything contained because of the vacuum effect. Analogously, when the pressure inside is higher than outside, the balloon effect will serve to push the harmful particulate away, keeping the space isolated and safe.

Effectiveness and Reliability

Pre-COVID there was not as much demand for negative air machines that had HEPA filters on them. HEPA filters are usually used to remove particulate down to 0.3 microns in size, which means they can catch airborne bacteria and viruses. The rise of COVID has led to a surge in demand for HEPA filter negative air machines because they are the only equipment out there that can remove the COVID virions in the air, and also create either a balloon or vacuum effect to preserve the necessary air barrier to stop transmission. These machines were pivotal in stopping transmission within intensive care settings throughout North America and Europe. One can only hope they will remain just as reliable in the presence of Omicron.