Photox Air Purification Systems

Dust, mold spores, volatile organic compounds (VOCs), hazardous airborne pollutants, and other contaminants can accumulate indoors, creating health problems for occupants and affecting the performance of other HVAC equipment. Filtering out contaminants is challenging, however, since different filtration methods are required to reduce particulates and gases across a variety of building types, ventilation systems, and end uses.

The most common mechanical filters use membranes to trap airborne particles, and include panel, pleated, box, and other varieties. ASHRAE 52.2 Test Procedure for filters measures particle size efficiency (PSE) at 0.3–1.0 micrometers (E1), 1.0–3.0 micrometers (E2), and 3.0–10.0 (E3) micrometers, to create an overall minimum efficiency reporting value (MERV). MERV ratings are based on a scale from 1 to 16, with 1 capturing <20% of E3 particles and 16 capturing 95% of all particles. High-efficiency particulate air (HEPA) filters must remove 99.97% of particles >0.3 micrometers.

Simple filters may be enough to protect HVAC equipment from contamination and damage, but disposable filters with MERV ratings between 8 and 13 are most often used to capture contaminants found in residential and commercial buildings. HEPA and higher MERV filters are usually used in hospitals, laboratories, and other applications that require control of pathogens or other contaminants. A balance has to be maintained between adequate filtration and ventilation, however, as higher MERV filters typically create a pressure drop that forces the HVAC equipment to work harder and requires additional energy.

Electrostatic precipitators can also be effective and have less pressure drop. In these systems, air passes through an ionizer where the particles are charged; the particles then attach to other particles and fall to the ground, attach to objects in the room, either to a charged filter (which is recommended). Electrostatic precipitators sometimes produce a small amount of ozone, but should not be confused with ozone generators, which can cause indoor air quality problems and should be avoided.

Filtering gases, such as VOCs, vehicle exhaust, and cooking odors, requires an adsorbant filter media, typically activated carbon. While not often used in residential applications, they are used as part of pollution control devices for restaurants in many urban areas.

UV lights alone or in combination with a catalyst in photocatalytic oxidation (PCO) systems can be effective for breaking down VOCs and other contaminants into less harmful forms, but most ducted UV systems are limited in applicability by cost, maintenance requirements, and limited air exchanges.

GreenSpec lists a variety of equipment, filters, and pre-filters that remove particulates and contaminants, minimize pressure drop, or reduce pollution from operations. Products include MERV 13 or higher filters from major manufacturers as well as proven technologies that remove harmful or nuisance gasses, and those that protect HVAC equipment from contaminants. GreenSpec does not list products that produce ozone, either using an ozone generator or as a byproduct of other systems, because of indoor and outdoor air quality problems associated with ozone.