Five Common Air-Purification Technologies and How They Work

1. Amino-acid purification

Amino acids can remove formaldehyde efficiently, and their total removal capacity depends on how much amino-acid material is used. However, amino acids biodegrade readily. When they are loaded onto an activated-carbon filter, moisture can encourage microorganisms to break them down. This can reduce formaldehyde-removal efficiency and create unpleasant odors. Keeping the filter dry is therefore important.

2. Electrostatic purification

Electrostatic purification uses an electric field to capture airborne particles. Consumable-free air purifiers use the same basic principle as industrial electrostatic precipitators, a technology that has been used for decades.

High voltages above 10 kV inevitably generate some ozone. Manufacturers generally keep ozone concentrations low to protect indoor air quality. At these low levels, the concentrations of reactive oxygen species are also limited, so electrostatic collection alone usually removes formaldehyde poorly. Although it does not require replacement dust filters, an electrostatic purifier should be paired with a formaldehyde-catalyst filter so that indoor formaldehyde and the ozone generated by the equipment can be removed together.

3. Negative-ion purification

The concentration of negative ions produced by a purifier is far too low to remove formaldehyde effectively. At a voltage of 2–10 kV, a negative-ion generator typically produces about 5–20 million ions per cubic centimeter, while the molecular concentration associated with formaldehyde at the national limit is enormously higher.

Increasing the voltage can raise the negative-ion concentration, but it can also generate harmful ozone, deposits, and oxidized pollutants. Negative ions are mainly useful for charging and settling particles, making the air feel fresher, and providing some antimicrobial effect. For better control of formaldehyde and ozone, the technology should be combined with a formaldehyde-catalyst filter.

4. Activated-carbon adsorption

Activated carbon adsorbs many substances without strong selectivity, but its formaldehyde capacity is limited. One gram of high-quality activated carbon may remove only about 0.1–10 mg of formaldehyde.

As the filter approaches saturation, rising temperatures can cause some adsorbed formaldehyde and VOCs to be released again, creating odors. If the carbon becomes damp and the purifier is switched off, microorganisms on the filter may consume some VOCs under locally oxygen-deficient conditions and produce additional odors. The adsorption filter should then be replaced promptly. Keeping the purifier running continuously can help reduce odor formation.

5. Catalytic purification

Catalytic technologies mainly include room-temperature catalysis and photocatalysis. Photocatalytic air purifiers commonly use titanium dioxide. Under ultraviolet light, the catalyst produces highly oxidative hydroxyl radicals that break down formaldehyde and VOCs.

Because air purifiers move a large volume of air, a single pass may not fully oxidize every VOC. Partially oxidized by-products that leave the catalyst can become a source of sour or other unpleasant odors. Photocatalytic purifiers also use ultraviolet lamps; if the equipment is poorly designed, leaked UV radiation may generate ozone that can accumulate indoors and affect health.