Air filtration, covered previously, targets solid matter (particulates) such as dirt, dust, leaves, and any visible airborne debris. Gas-phase filtration is for the unseen pollution that is airborne gases that may have an odor–but not always. This is chemistry that came from a liquid or solid to become a gas at ambient temperature. Gases act very differently than particles since they are much lighter and tend to stay airborne longer and mix and swirl in a space in all directions.
Chemistry in the air comprises a very complex environment since gases can interact creating different gases and either reduce the problem or make it worse. Since there can be multiple chemicals present indoors this becomes a reason for concern. In industrial environments the chemical gases that are byproducts of production (such a hydrogen sulfide or formaldehyde) can harm humans exposed when there is a release. Many are not hazardous or even odorous (volatile organics in cleaners, carpeting, and building materials) but many are harmful to humans if exposure is over a long period of time. There is now agreement among organizations such as USGBC and ASHRAE that most common gases of concern in spaces not manufacturing, such as offices, buildings or schools, are relatively few. Compare that to buildings where smoking is allowed, like some casinos: smoke contains around 5,000 chemical gases including 60 that are confirmed to be carcinogenic.
Gas-phase filtration has become a key tool for achievement of indoor air quality. A wide range of HVAC filters can remove particulates. What is left are gases. We know that these gases trigger allergic reactions and can lead to lowered productivity or even illness. Combining high efficiency particle filters with gas-phase filtration media covers both types of air pollution and achieves a healthy, safer environment in which people can live and work.