Air purifiers have become a standard appliance for defense against indoor pollutants. Selecting the correct unit begins with the industry-standard metric for performance: the Clean Air Delivery Rate (CADR). This rating provides a standardized, objective measure of an air purifier’s cleaning speed and efficiency. Selecting the right unit means matching the CADR rating to the specific volume and pollution concerns of your room. A high CADR unit may be overkill for a small office, while a low-rated unit will be ineffective in a large living area.
What is Clean Air Delivery Rate (CADR)?
Clean Air Delivery Rate (CADR) quantifies the volume of filtered air an air purifier produces per minute, expressed in cubic feet per minute (CFM). The rating combines two performance aspects: the volume of air moved by the unit and the filter’s efficiency at removing particles. This prevents manufacturers from using high airflow rates with ineffective filters or highly efficient filters that process little air.
The Association of Home Appliance Manufacturers (AHAM) developed and maintains the CADR standard through its Verifide program. AHAM tests purifiers for effectiveness against three particle types: smoke, pollen, and dust. Each pollutant receives a distinct CADR number, allowing consumers to prioritize based on their primary air quality concern. Smoke particles (0.09 to 1.0 microns) are the smallest and most difficult to trap, which is why the smoke CADR is typically the lowest reported number.
How CADR is Calculated and Tested
The CADR number is derived from a testing process performed in a controlled, sealed chamber measuring 1,008 cubic feet, following the ANSI/AHAM AC-1 standard. During the test, a specific concentration of a pollutant (smoke, dust, or pollen) is introduced into the chamber. The air purifier is activated at its highest fan speed for a set period, generally 20 minutes, while the contaminant reduction rate is continuously monitored.
The final CADR value is calculated by taking the rate at which particles are removed from the air and subtracting the rate at which those particles naturally settle. This calculation measures the purifier’s effectiveness by multiplying the air flow rate by the filter’s particle removal efficiency. The resulting number represents the volume of air completely cleaned of a specific pollutant each minute.
The Critical Relationship Between CADR and Room Size
The CADR rating must be matched to the room size where the air purifier will operate. An effective unit must be powerful enough to process the entire volume of air multiple times per hour. The relationship between CADR and room size is standardized by the “two-thirds rule,” an AHAM guideline for proper unit sizing.
This rule states that the air purifier’s smoke CADR rating should be at least two-thirds of the room’s square footage. For example, a 300 square foot room (15 feet by 20 feet) requires a smoke CADR of at least 200. A unit with a lower CADR will be underpowered and unable to clean the air fast enough to provide noticeable benefits. Conversely, a significantly higher CADR may be an unnecessary expense and likely operates at a higher, noisier fan speed.
Determining a Good CADR Rating for Your Space
An effective CADR rating meets or exceeds the two-thirds rule for your specific space. When matching a purifier to a room, use the smoke CADR as the benchmark for calculation. Since smoke particles are the smallest and hardest to capture, choosing a unit based on this rating ensures it can handle the full spectrum of airborne particulate matter, including larger dust and pollen.
For example, a 100 square foot bedroom requires a minimum smoke CADR of 67 CFM. A 200 square foot master bedroom or small living room requires at least 133 CFM. Large open-concept areas exceeding 400 square feet necessitate a unit with a smoke CADR of 267 CFM or higher. For severe air quality issues, such as wildfire smoke or strong allergies, AHAM recommends a smoke CADR equal to the room’s square footage for accelerated cleaning.
CADR Versus Air Changes Per Hour (ACH)
Air Changes Per Hour (ACH) offers a detailed perspective on an air purifier’s performance, especially for health-related concerns. While CADR measures the rate of clean air delivery, ACH indicates how many times the entire volume of air in a room is filtered every hour. This metric shows how quickly a pollutant is removed from the breathing space.
The ACH value is calculated using the air purifier’s CADR rating and the room’s total volume (square footage multiplied by ceiling height). The formula is ACH = (CADR in CFM $\times$ 60) $\div$ Room Volume in Cubic Feet. For standard air cleaning, an ACH of two to three is acceptable. However, for allergy sufferers, asthmatics, or high-risk environments, four or five ACH is advised. Achieving five air changes per hour means the air is completely filtered every 12 minutes, which quickly reduces airborne irritants.
Limitations of the CADR Rating
Despite its utility, the CADR rating does not provide a complete picture of an air purifier’s capabilities. The test is designed to measure the removal efficiency of particulate matter (smoke, dust, and pollen) but intentionally ignores gaseous pollutants. Consequently, a high CADR unit may perform poorly at removing Volatile Organic Compounds (VOCs), formaldehyde, or general household odors.
The CADR test also has limitations regarding particle sizes, as it does not fully account for ultra-fine particles smaller than 0.1 microns. Furthermore, the test is conducted over a short period with a brand-new filter at the highest fan speed. This means the reported CADR does not reflect performance degradation over time as the filter becomes loaded with pollutants.
Other Essential Factors When Buying an Air Purifier
Beyond the CADR rating, several factors contribute to the long-term effectiveness and practicality of an air purifier. Filtration technology is important, with True HEPA filters being the accepted standard because they capture 99.97% of airborne particles as small as 0.3 microns. To address the gaseous pollutants that CADR ignores, a substantial layer of activated carbon filtration is necessary for adsorbing odors and VOCs.
The noise level is a practical consideration, especially if the purifier is used in a bedroom where high fan speeds may be disruptive. Noise is measured in decibels (dB), and quiet operation typically falls in the 24 to 36 dB range. Energy consumption is also a factor, with Energy Star rated units being the most efficient. Finally, the long-term cost of ownership, specifically the frequency and expense of replacement filters, must be factored into the purchase decision.