The Mila Definitive Guide to IAQ Efficacy and Application of Air Purifiers

4.3 — Air Purifiers vs. Biological Contaminants

Air purifiers play a proven role in controlling airborne biological pollutants. By capturing bacteria, viruses, allergens, and mold spores, they help limit infection spread and allergic reactions, serving as an effective complement to ventilation, cleaning, and moisture control.

Biological contaminants are a diverse class of indoor pollutants that include mold spores, allergens, bacteria, and viruses. Unlike chemical pollutants released from combustion or materials, these are airborne particles or particle-associated agents that interact with both the indoor environment and its occupants. They can trigger allergic and inflammatory responses, exacerbate respiratory disease, and, in the case of pathogens, spread infection between people. Because many circulate readily in air or are re-suspended during daily activity, air purifiers have been investigated as a practical means of reducing exposure.

Bacteria and viruses spread indoors on airborne droplets and fine particles. Portable air cleaners equipped with HEPA filters are designed to capture these particles, and both controlled and real-world studies confirm their effectiveness. Chamber experiments using viral surrogates such as bacteriophage MS2, as well as field measurements detecting SARS-CoV-2 RNA, have shown that they can markedly reduce airborne microbial loads. Residential studies during COVID-19 outbreaks likewise found fewer detectable viral aerosols when air purifiers were operating, and field measurements in homes and schools have demonstrated 20–50% reductions in viable bacteria along with smaller proportions of respirable bioaerosols. In more polluted or industrial environments, air cleaners have been shown to lower bacterial concentrations, though open windows can diminish their impact. Modeling studies reinforce these observations, documenting that proper device placement and airflow orientation can cut exposure to virus-laden aerosols by more than 90%.

Airborne allergens are among the most persistent indoor pollutants, arising from sources such as pollen, pet dander, dust mites, and fungal spores. These particles readily circulate in the breathing zone and can be re-suspended from settled dust by normal activity. While cleaning, humidity control, and source avoidance remain the foundation of allergen management, air purifiers provide an important complement. HEPA filters are well suited to this role, as allergen particles typically fall within the 0.1–40 µm size range. Several studies demonstrate this effect directly, including a controlled field experiment where a portable HEPA-based unit lowered pollen and fungal spore concentrations to 10–20% of initial levels compared to a control room. Evidence also extends to animal allergens: a home study documented marked declines in airborne cat, dog, and dust mite allergens, while a separate household chores study confirmed measurable though variable reductions in personal exposure. A broader review of the literature notes that while portable air cleaners can lower airborne allergen concentrations, individual studies suggest their effectiveness may be limited to a single room rather than the entire dwelling.

Mold is a common indoor contaminant, and its spores are nearly always present in the air. Mold spores and fragments readily become airborne, contributing to exposure and redistributing contamination, and can establish difficult-to-remove colonies when they settle on damp building materials. Moisture control and remediation remain the foundation of mold management, but air cleaning technologies serve as a valuable supplement, with HEPA-equipped units effectively removing airborne spores in the 1–40 µm size range. While there are relatively few scientific studies directly quantifying this effect, a two-year trial among children with mold allergies found that bedroom units lowered airborne mold counts and were associated with fewer respiratory symptoms and reduced medication use, particularly outside of winter months when mold levels were highest. In a separate field study, portable air cleaners decreased fungal concentrations 1.5–6 times faster than natural decay, with highly contaminated spaces showing reductions from over 50,000 to a few hundred colony-forming units per cubic meter (CFU/m3) within minutes. Units that pair particle filtration with activated carbon can also help limit musty odors and microbial by-products, while some systems incorporate ultraviolet light or photocatalytic processes to inactivate spores and fragments captured on filters.

Air purifiers provide real-time control of airborne biological contaminants by reducing concentrations and limiting exposure. Since underlying sources — including moisture, dust reservoirs, and infectious occupants — remain present, they perform best as one component of a comprehensive approach that also includes ventilation, cleaning, and moisture management.

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