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An air purifier is only as effective as its filtration technology and its actual airflow rate through that technology relative to the room volume. The gap between manufacturer marketing claims and measurable performance can be substantial. The metrics and considerations below are drawn from AHAM (Association of Home Appliance Manufacturers) certification standards and ASHRAE guidance.
1. Understanding CADR
Clean Air Delivery Rate (CADR) is the standardised metric for air purifier airflow, expressed in cubic feet per minute (cfm) or cubic metres per hour (m³/h). It represents the volume of filtered air the unit delivers per unit time and is measured separately for three particle size classes: tobacco smoke (0.1–1 µm), dust (0.5–3 µm), and pollen (5–11 µm).
CADR certification is conducted by AHAM under ANSI/AHAM AC-1. Third-party CADR data from AHAM or Clean Air Stars is more reliable than manufacturer-stated figures. A unit with a tobacco smoke CADR of 200 cfm delivers 200 cubic feet of clean air per minute for particles in that size range — it does not mean the room air turns over 200 times per minute.
AHAM's "two-thirds rule" states that a unit's CADR should be at least two-thirds of the room's floor area in square feet. A 150 ft² room needs a minimum CADR of 100 cfm for tobacco smoke.
2. Sizing by Room Area
The standard industry formula assumes 8-foot ceilings and targets 4–5 air changes per hour (ACH) for general air quality maintenance:
Required CADR (cfm) = (Room area in ft² × ceiling height in ft × target ACH) ÷ 60
For a typical Canadian bedroom of 12 × 14 ft (168 ft²) with 8 ft ceilings targeting 5 ACH:
(168 × 8 × 5) ÷ 60 = 112 cfm minimum
For wildfire smoke events or households with asthma sufferers, 8–10 ACH is preferable, which roughly doubles the required CADR. Rooms with higher ceilings (9–10 ft, common in pre-1960s Canadian homes) require proportionally higher CADR.
Manufacturers frequently quote a "room coverage area" in square feet assuming only 2 ACH — half the AHAM-recommended target. Checking the CADR figure directly is more reliable than relying on stated coverage area.
3. True HEPA Filtration
True HEPA (High Efficiency Particulate Air) filters remove at least 99.97% of particles at 0.3 µm — the most penetrating particle size for fibre-based filtration, also known as the MPPS (most penetrating particle size). Above and below 0.3 µm, filter efficiency is actually higher.
True HEPA is distinct from "HEPA-type" or "HEPA-like" filters, which have no standardised minimum efficiency. Filters labelled H13 or H14 under EN 1822 exceed the standard HEPA threshold (H13: 99.95% at MPPS; H14: 99.995%).
HEPA filters capture particulate matter, pollen, most mould spores, and some bacteria but do not adsorb gases or odours. A purifier with HEPA filtration only will not reduce formaldehyde or cooking-related VOC concentrations.
4. Activated Carbon Filtration
Activated carbon (also called activated charcoal) adsorbs VOCs, odours, and some gases through physical and chemical adsorption. Effectiveness depends on carbon bed mass, contact time, and the specific compounds present. A thin carbon-impregnated pre-filter layer contains too little carbon to be effective for gaseous pollutant removal — a functional carbon stage requires at minimum a few hundred grams of granular activated carbon, with 1–2 kg being more meaningful for households with gas ranges or VOC sources.
Carbon beds saturate over time and must be replaced; they do not regenerate at room temperature. Some formaldehyde-specific variants use impregnated carbon (potassium permanganate or potassium iodide compounds) to convert formaldehyde to less harmful compounds.
5. UV-C and Photocatalytic Oxidation
UV-C lamps at 254 nm can inactivate bacteria and viruses by damaging DNA. In air purifiers, this is a secondary stage — the air must pass the lamp slowly enough for sufficient UV dose, and the UV-C irradiance must be appropriate for the target pathogens. Many consumer units with UV-C LEDs have insufficient irradiance or exposure time to provide meaningful germicidal effect.
Photocatalytic oxidation (PCO) uses UV light plus a titanium dioxide (TiO₂) catalyst to oxidise VOCs. Some PCO reactors can produce formaldehyde, acetaldehyde, and ozone as by-products if the reaction is incomplete — a concern well-documented in peer-reviewed literature. Unless the unit is specifically tested for by-product generation, PCO is not a recommended primary filtration choice for residential spaces.
6. Ionisers and Ozone
Ionisers charge airborne particles, causing them to precipitate onto surfaces or a collection plate. They do not remove particles from the room — they deposit them on nearby surfaces. Bipolar ionisation and plasma-based air "purifiers" marketed for HVAC installation remain poorly characterised in peer-reviewed literature for residential efficacy.
Ozone generators marketed as air purifiers produce ozone at concentrations that can exceed Health Canada's short-term guideline of 20 ppb (40 µg/m³) for occupied spaces. At concentrations sufficient to react with pollutants, ozone is hazardous to occupants. Health Canada advises against using ozone generators in occupied spaces.
7. Placement and Operating Conditions
Placement significantly affects performance. An air purifier placed in a corner with restricted airflow around its intake will deliver less clean air than its CADR rating suggests. Best practices:
- Place the unit at least 0.5 m from walls or furniture on the intake side
- Position near the primary pollution source (e.g., near the sleeping area for overnight use, near the cooking area for PM2.5 during meal preparation)
- Keep room doors closed to concentrate filtration in the target area
- Run continuously on medium setting rather than intermittently on high — consistent low-level filtration outperforms periodic bursts
8. Filter Replacement Intervals
HEPA filters in residential units typically require replacement every 6–12 months under continuous operation, though this varies significantly with air quality and occupancy. Manufacturer replacement schedules are often conservative (designed for worst-case use). Filter pressure drop indicators, if present, are more accurate guides.
Running a unit with a saturated or clogged filter does not harm the unit in most designs, but it progressively reduces CADR. A filter that has captured significant PM2.5 load will re-release some particles when airflow is stopped and started — running the unit continuously avoids this effect.
