How Should You Interpret an Air Purifier's CADR?

CADR is the clean-air output rate obtained when the Association of Home Appliance Manufacturers (AHAM) tests an air purifier according to a specified, AHAM-recognized standard. Its three main test categories are dust, smoke, and pollen. China's national air-purifier standard, issued in 2002, also introduced this metric. The latest national standard, GB/T 18801-2015, evaluates air purifiers mainly through five metrics: Clean Air Delivery Rate (CADR), Cumulative Clean Mass (CCM), Cleaning Energy Efficiency, Effective Room Size, and Cleaning Life Span. Other supporting metrics include noise and microorganism removal. Among the five main metrics, CADR and CCM are the two that most directly indicate purification performance: CADR determines applicable room area, while CCM determines purification life.

On the surface, CADR appears to provide an objective measure of an air purifier's capacity. On closer examination, however, it has significant limitations. As noted above, CADR was designed mainly around larger particles such as dust, smoke, and pollen, so the test method itself has constraints.

First, consider the simulated pollution source used in the test. GB/T 18801-2015 specifies cigarette smoke as the particulate source for CADR measurement. In typical cigarette smoke, PM2.5 accounts for about 50%, while particles larger than PM2.5 account for the other 50%. Cigarette smoke is an aerosol containing both solid and liquid particles. As the smoke disperses, particles coagulate: small particles gradually combine into larger ones, so fewer small particles remain over time and the larger particles become easier to count. CADR is calculated by measuring the number of particles per liter of air. This means that a filter-based air purifier can achieve a high CADR even with a medium-efficiency filter.

Second, consider the test environment. To keep the concentration uniform, the test chamber requires a circulation fan airflow of 500-700 m3/h. Such high airflow has little effect on filter-based purifiers, but it makes purifiers that rely on particle settling, such as electrostatic and negative-ion units, difficult to evaluate. Electrostatic purifiers often work better at lower air speeds. For negative-ion and other settling-based purifiers, this test method can be especially unfavorable because settled dust is stirred up again, preventing an objective measurement of performance.

In addition, CADR represents the amount of clean air produced over a period, but it does not define how clean that air must be. To achieve the same air cleanliness, one purifier may need several passes while another may need only one. Whether a device uses a filter or an electrostatic collector, its performance declines with repeated use. This introduces the question of purification life and, in turn, the CCM metric, which is not discussed in detail here. When choosing an air purifier, therefore, you should consider CCM as well as CADR.

Many entry-level and mid-range, single-function air purifiers achieve very high CADR values. High-end air purifiers must balance several factors, so their CADR may not show an obvious advantage.

Demand for air purifiers in China has risen rapidly in recent years, while national standards have struggled to keep pace. The mixed quality of products in the market can leave consumers unsure how to choose. Some sellers exploit this uncertainty and gaps in the standards by exaggerating one technology and disparaging others to gain market share. Some may even seek to influence the standard-setting process. As a result, consumers may buy an air purifier that fails to deliver the expected performance.

The 2015 national standard was, of course, a major improvement. Until better descriptive metrics are available, CADR remains an important reference for air purifiers, but it should be evaluated together with CCM and other indicators.