How air filtration works
HEPA filters
True HEPA filters capture at least 99.97% of particles at 0.3 micrometers — the most-penetrating particle size. They are highly effective against dust, pollen, mold spores, pet dander, and PM2.5. Portable HEPA purifiers have been shown to reduce indoor PM2.5 by 40–80% depending on room size, air exchange rate, and unit capacity [2][5]. They do not, however, remove gases, odors, or VOCs.
Activated carbon
Activated carbon filters adsorb gaseous pollutants including VOCs, formaldehyde, and odors. The effectiveness depends on the amount of carbon (thin carbon pre-filters in cheap units do very little) and the specific compound. Carbon filters need periodic replacement because they become saturated. Units that combine HEPA and a substantial activated carbon bed address both particles and gases [1].
MERV ratings and HVAC filters
MERV (Minimum Efficiency Reporting Value) rates the effectiveness of furnace and HVAC filters on a scale from 1 to 20. Residential systems typically handle MERV 8–13 without airflow issues. MERV 13 filters capture roughly 85% of particles in the 1–3 µm range and are recommended by ASHRAE for improved indoor air quality [4]. Going above MERV 13 in a residential system can restrict airflow and strain the blower motor unless the system was designed for it.
Do houseplants clean the air?
The famous 1989 NASA study found that plants could remove VOCs in small sealed chambers. However, Cummings and Waring (2020) calculated that you would need approximately 10–1,000 plants per square meter of floor space to match the air-cleaning rate of simply opening a window or running a modest air purifier [3]. At the one or two plants per room that most people have, the effect on air quality is negligible. Plants have many benefits — air purification just isn't meaningfully one of them.
Ventilation strategies
Dilution with outdoor air is a fundamental strategy for reducing indoor pollutant concentrations. Mechanical ventilation with heat recovery (HRV or ERV systems) allows fresh air exchange without major energy loss. Even without such systems, strategic window opening — cross-ventilation with windows on opposite sides of a space — can dramatically reduce indoor pollutant levels when outdoor air quality is acceptable [1].
Evidence base
Portable HEPA purifier efficacy
A systematic review by Cheek et al. (2021) analyzed 30 studies on portable air cleaners and found consistent PM2.5 reductions of 40–82% in real-world residential settings [5]. The Clean Air Delivery Rate (CADR) is the standard metric for comparing units — it represents the volume of filtered air delivered per minute. The EPA recommends selecting a purifier with a CADR at least two-thirds of the room's area in square feet [1].
Barn et al. (2018) conducted a randomized crossover trial in Ulaanbaatar, Mongolia — one of the most polluted cities in the world — and found that portable HEPA purifiers reduced indoor PM2.5 concentrations by an average of 40% even in homes with very high baseline levels and significant infiltration from outdoors [2].
MERV filtration in HVAC systems
ASHRAE recommends MERV 13 as the minimum for adequate particulate filtration in commercial buildings and has increasingly recommended it for residential applications as well [4]. A key practical consideration is that higher-MERV filters increase static pressure in the duct system. MERV 13 filters are generally compatible with most residential HVAC systems manufactured after 2000, but homeowners should verify with their system specifications or HVAC technician.
The houseplant question
The disconnect between the popular narrative and the science is stark. The original NASA study by Wolverton et al. (1989) was conducted in sealed chambers of approximately 0.7 m³ — nothing like a real room. When Cummings and Waring (2020) modeled the results against realistic room volumes and air exchange rates, they found that the clean air delivery rate (CADR) of a single potted plant is orders of magnitude below what would be needed to meaningfully affect indoor air quality [3]. A typical plant provides a CADR of roughly 0.023 m³/hr — a modest portable air purifier delivers 100–400 m³/hr.
Carbon filtration limitations
Activated carbon is effective for many VOCs but performance varies significantly by compound. Formaldehyde, for example, is poorly adsorbed by standard activated carbon and requires chemically treated (impregnated) carbon media. The EPA notes that gas-phase filtration is generally less well-characterized than particulate filtration, and consumers should be cautious about manufacturer claims regarding VOC removal [1]. Filter lifespan depends heavily on pollutant load — in high-VOC environments, carbon can saturate in weeks rather than months.