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Computing Methods of Air Purifier Applicable Area Varied Urging for Labelling of Clean Air Delivery Rate for Easy Comparison Pay Heed to Cost of Filter Replacements as 10-Year Expenses Plus Appliance Price Could Exceed $12,000

  • 2024.06.17

In the face of influenza outbreaks, many families use air purifiers to improve indoor air quality and protect against bacteria and viruses. The Consumer Council tested 10 models of air purifiers on the market and found that despite manufacturers often using “claimed applicable room size” to reflect a product’s ability to deliver clean air, the results revealed that the two were not necessarily commensurate. One model which claimed to be suitable for a 700 sq ft area had a slower smoke and dust removal speed than 3 models with a claimed applicable room size of less than 300 sq ft. Additionally, the purifying speeds for the removal of smoke and formaldehyde varied substantially among the same group of tested models, with differences as much as 2.7 times and 12.3 times respectively. As manufacturers use different test standards and computing methods for their products’ claimed applicable room size, it is difficult for consumers to make direct comparisons. The Council therefore urges manufacturers to adopt the Clean Air Delivery Rate[1] (CADR) value as used in test standards to indicate their products’ air purifying performance. Consumers are also reminded to consider not just the price of the product, but also the frequency and cost of replacing filters when making a purchase. One particular model, for instance, required replacing 2 filters each time, and due to their relatively short recommended replacement interval, if they were to be replaced every 6 months, the cumulative replacement cost across 10 years could amount to $7,524 in total. Factoring in both the price of the appliance plus the 10-year filter replacement expenses, the total operating cost could range from $2,600 to $12,412 among the models. Consumers should compare adequately and select a suitable model that aligns with their living environment and usage habits.

The Council purchased 10 models of air purifiers from different retailers and conducted the test in collaboration with the Electrical and Mechanical Services Department (EMSD). Priced between $1,798 and $7,880, all tested models came with High Efficiency Particulate Air (HEPA) filters which claimed to effectively filter cigarette smoke, dust, viruses, bacteria, etc. The Council’s test was conducted with reference to the US National Standard ANSI/AHAM AC-1, and the Mainland’s National Standard GB/T 18801, and measured the models’ purifying performance, quietness and convenience of use, while the EMSD was responsible for the safety tests. The 10 models were divided into 3 groups based on their claimed applicable room size: Group 1 had the largest applicable room size claims (from 646 sq ft to 1,000 sq ft); Group 2 had medium applicable room size claims (from 387 sq ft to 413 sq ft)[2]; whereas Group 3 had the smallest applicable room size claims (from 248 to 280 sq ft).

Particulate Matter Purifying Speed Could Vary by up to 2.7 Times Among Models in the Same Group

The foremost function of an air purifier is to purify air by removing particulate matter such as smoke, dust and pollen. The test measured the CADR values of particulate matter for each model, with a higher value indicating a faster and better air purifying performance. According to the test results, the 4 models in Group 1 varied considerably in their particulate matter removal speed, with the fastest model filtering smoke 2.7 times faster than the slowest model; whereas models in Groups 2 and 3 had similar performance within their respective group. Group 1 was measured with the highest average speed of 217.2 cubic ft/min in the removal of the 3 types of particulate matter, as compared to Group 2’s 155.5 cubic ft/min and Group 3’s 105.2 cubic ft/min. The results indicated that most air purifiers with larger applicable room size claims generally had higher CADR values and faster purifying speeds.

With reference to the US National Standard and based on the test results, the applicable maximum room size of the tested models was estimated to range from 123 sq ft to 461 sq ft, smaller than that claimed by manufacturers. The difference could be due to the varying test standards and methods adopted by manufacturers in computing the claimed applicable area, thus such claims might not be suitable for direct comparison with other models or for indicating a product’s purifying ability. A more objective approach would be to refer to the CADR values instead. For example, 1 model with a claimed applicable room size of 700 sq ft was remarkably slow in purifying particulates, and its speed in removing smoke and dust even fell below 100 cubic ft/min, the lowest among all tested models, meaning that it was even slower than 3 other models with the smallest applicable room size claims. The Council urges manufacturers to label the CADR value on their products and disclose the test standards adopted, so as to provide more objective and transparent references for consumers.

Half of the Models Exceedingly Slow in Formaldehyde Removal

All Were Outstanding in Filtering H1N1 Virus

Renovation and new furniture could increase the concentration of formaldehyde indoor, prompting some consumers to consider air purifiers as a solution to lower indoor formaldehyde levels. Test results revealed significant disparity in the speed of formaldehyde removal among the models, ranging from 6.9 cubic ft/min to 139.2 cubic ft/min, a substantial 19.2-fold difference. Half of the tested models (5 models), were especially slow in removing formaldehyde, with a formaldehyde removal rate of lower than 15 cubic ft/min. Comparing among each group, models in Groups 1 and 3 showed greater variances in purifying speeds, with a difference of 2.6 times and 12.3 times respectively, while models in Group 2 exhibited similar performance with only a 0.59 times difference in speed.

All tested models performed exceptionally well in filtering the H1N1 influenza A virus, with a virus removal rate of over 99% after operating for 20 minutes, which even reached >99.99% after 1 hour of operation. Nonetheless, consumers should note that test environments differ from daily environments, and viruses can propagate via other means, such as attaching to garments or transmission through droplets. As such, it is inadvisable to solely rely on air purifiers for disease prevention, while other preventive measures should also be taken, such as putting on a face mask.

Filters Require Regular Replacement with 10-Year Cost Potentially Exceeding $7,500

The key to maximising an air purifier’s effectiveness lies in the condition of its filters. Timely replacement of filters is essential, otherwise their air purifying performance might be compromised. The recommended replacement frequency of the models’ filters ranged from as short as 6 to 12 months to as long as 10 years. Consumers should take into account the future costs of replacing filters in the long run. Considering the first decade of usage, only 2 models required no extra running costs as their filters could last for 10 years. 1 model’s filters had a particularly short recommended usage period, and if they were to be replaced every 6 months, the 10-year replacement cost could add up to $7,524. Factoring in both the price of the models which ranged from $1,798 to $7,880, plus the 10-year filter replacement expenses, the total operating cost could range from $2,600 to $12,412.

30% Models Had Inadequate Safety Warnings in User Manual

All models successfully passed the relevant safety tests by the EMSD. However, the user manuals of 3 models were somewhat inadequate, mainly in terms of warning statements or instructions, such as supervising children and the elderly when operating the appliance, or cautioning against young children playing with it.

Regarding the models’ convenience of use, 2 models came only with a basic instruction manual, requiring users to look up the detailed version on the official website. Furthermore, the filters of 2 models lacked installation direction markings to guide users, which might cause inconvenience during assembly or replacement.

Air purifiers are undoubtedly helpful in improving indoor air quality. However, poorly designed products or improper usage may not only fail to effectively filter allergens, but could contrarily release pollutants indoors. When using air purifiers, consumers could take heed of the following recommendations by the Hong Kong Medical Association and the Department of Health:

  • Recommendations by the Hong Kong Medical Association
    • Do not depend entirely on air purifiers for disease prevention, but also be mindful of maintaining good personal hygiene habits. Individuals with asthma or allergies should adhere to their doctor’s guidance on taking medication to manage their conditions, as air purifiers cannot fully relieve symptoms;
    • To enhance indoor air quality, besides using air purifiers, it is advisable to frequently keep the windows open to allow air circulation, keep the living environment clean, regulate indoor temperature and humidity, and minimise the use of chemicals and smoking. All of these could help reduce allergens, pollutants and bacteria in the air;
    • Consumers should replace filters regularly. Otherwise, bacteria and mould may breed easily, further contaminating indoor air. Ineffective filters could lead to accumulation of indoor air pollutants and aggravate allergic reactions and respiratory conditions;
  • As recommended by the Department of Health, some air purifiers come with built-in humidifying functions. Individuals with a weak immune system should use sterile or cooled boiled water with the product, and routinely clean and maintain the appliance as instructed to minimise the risk of Legionnaires’ disease.


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[1] The Clean Air Delivery Rate (CADR) is an indicator of an air purifier’s effectiveness in purifying the air. It measures the rate at which a device can deliver air free from specific pollutants. A higher CADR value denotes a faster removal speed of specific pollutants, and hence a better purifying ability.

[2] One sample in this group did not specify an applicable room size. The Council calculated the applicable room size based on its labelled maximum clean air delivery rate and the Mainland’s National Standard GB/T 18801:2015.