When considering the purchase of an air purification system, consumers should understand the different technologies available, what pollutants each technology can remove, and the limitations of the system. A useful air purifier should be selected according to the particular indoor-air problem, the size of the room, the amount of air it can effectively clean, and the manufacturer’s maintenance requirements. No single air-cleaning technology removes every type of indoor pollutant.

Some air purifiers use ionization or other technologies that electrically charge particles. These devices can cause airborne particles to become charged and either collect on surfaces or be captured by a collection system. However, some ion-generating air cleaners can also produce ozone as a by-product. Ozone is itself an air pollutant and can irritate the lungs and worsen respiratory symptoms. For this reason, consumers should be cautious about air cleaners that intentionally generate ozone or produce significant ozone as a by-product. The U.S. Environmental Protection Agency does not recommend the use of ozone generators in occupied spaces for controlling indoor air pollution.

Ionization technology should also not be confused with ordinary mechanical filtration. An ionizer may cause particles to settle onto walls, floors, furniture, and other surfaces rather than actually removing them from the indoor environment. The performance of individual products varies considerably, so claims that an air cleaner “kills germs” or “purifies the air” should be evaluated carefully.

Mechanical filtration is one of the better-established methods for removing airborne particles. High-Efficiency Particulate Air (HEPA) filters are designed to remove very small particles from an airstream with high efficiency. Properly designed and operated HEPA filtration can therefore reduce concentrations of airborne particulate matter, including many particles that carry microorganisms such as bacteria and viruses. HEPA filtration is widely used in healthcare facilities and other environments where control of airborne particles is important.

However, a HEPA filter should not be described as a sterilization system. A HEPA filter primarily captures particles; it does not necessarily kill every microorganism that becomes trapped in the filter. Captured microorganisms may remain viable for some period of time depending on the organism and environmental conditions. The important benefit of HEPA filtration is that microorganisms attached to or contained within airborne particles can be removed from the circulating air when those particles are captured by the filter.

The effectiveness of a portable air purifier depends on more than the filter itself. Airflow through the filter, the efficiency of the filter, the size of the room, how long the unit operates, and whether air bypasses the filter all affect performance. The Clean Air Delivery Rate (CADR), where available, can be useful when comparing portable air cleaners because it takes both particle-removal efficiency and airflow into account.

Consumers should also distinguish between particulate and gaseous pollution. HEPA filters are designed primarily to remove particles; they do not remove most gases or volatile organic compounds. Activated carbon and other sorbent materials can be used to remove some gaseous pollutants, but their effectiveness depends on the amount and type of material used, the pollutant, airflow, and how quickly the sorbent becomes saturated.

When purchasing an air purification system, consumers should therefore consider the specific pollutants they want to reduce and choose a system that is appropriately sized for the room. For reducing airborne particles, a well-designed mechanical air cleaner using a high-efficiency filter, operated at an adequate airflow rate, is a well-established option. Consumers should also avoid products that intentionally generate ozone in occupied spaces and should be cautious about unsubstantiated claims concerning the ability of an air purifier to eliminate all pollutants, bacteria, viruses, or allergens.

Air purification should also be regarded as one component of indoor-air-quality management rather than a substitute for controlling pollution at its source. Removing or reducing the source of pollution, providing appropriate ventilation, controlling moisture, and maintaining heating and cooling equipment can often be more important than relying on an air cleaner alone.

By Michael Chiu, MWS, CI
Previously published in Ming Pao Daily News – Updated and Revised in 2026