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  • Shanghai Air Part 4: The Dirty Story of Air Filters

Shanghai Air Part 4: The Dirty Story of Air Filters

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  • By admin
  • Date April 19th, 2012 09:15
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  • In China, selling air purifiers is good business - and its getting better everyday. One doesn't need to be a rocket scientist to relate the growth in sales to the growth in public awareness of air pollution.

    However, the irony is that they pollute more than they clean. Like many green technologies today, no one has bothered to step back and look at the big picture.

    Comparing air filters usually boils down to price vs. performance. However, the reality is that in the higher-end range most filters are about the same: most have a HEPA filter, strip out VOCs and strip out particulates. Most of them are about 99% effective at removing particulates 0.3 microns and larger. In most cases the decision comes down to the price and the sales pitch when in fact, the real deal breaker should be energy consumption.

    Air filters remove pollutants from the air by consuming energy. However, energy in China is mostly produced by burning coal, which in turn produces more pollutants. The question is, do air filters remove more than they produce?

    Air Filters

    A report published by China Environmental Sciences Press in 2006 [1] enables us to calculate that 1 watt of coal fired power produces about 16 grams of particulate matter.
    A further report published in 2010 [2] indicates that a fairly modern coal-fired power station produces about 357 grams of coal ash per watt, of which about 20% (70 grams) is light enough to become airborne. Of the 20%, only a fraction will actually become airborne, most likely dropping the figures to within range of the latter report.

    Now, consider that one of the most popular air purifiers on the market today consumes an incredible 215 watts for a flow rate of 440 m3/h. Using 16 grams per watt, that's 3440 grams (the weight of about 2 dozen apples) of particulates spewed into the air per hour.

    Using an average particulate (PM10) figure of 140 ug/m3, the air filter cleans 62 grams in an hour.

    In short, the air filter does a much better job polluting than it does cleaning up. What's worse, this doesn't even account for energy lost in power lines, the particulates created during mining and transportation, or the production of methane, NOx, SO2, CO2 and heavy metals.

    Meanwhile, another popular air purifier on the market uses 120 Watts for a flow rate of 830 m3/h. In theory, this means it could be operated at half speed to achieve the same effect as the one mentioned above. That's 3.5 times less pollution. Yet, even at 960 grams particulate matter, it is still polluting more than it is cleaning.

    Clearly, the challenge for manufacturers is to make air purifiers more energy efficient. Either that or we all revert to plants... lots of plants. Not only are they able to capture VOCs and particulates, they do so while capturing CO2. The only issue is that they compete for oxygen at night. This forces us to look for fresh air elsewhere, which will be the subject of the next article.

    Shanghai Air Series:
    Part 1: Indoor vs. Outdoors?
    Part 2: Windows... Open vs. Closed?
    Part 3: IKEA Unit Ruins Test Result?


    1. Xu Huaqing, et al. Report on China's Energy Environment Development (Zhongguo Nengyuan Huanjing Fazhan Baogao) (Beijing: China Environmental Sciences Press [Zhongguo huanjing kexue chuabanshe]: 2006)

    2. Yang Ailun, Zhou Hanhua, Su Miaohan, Tang Hongyuan, Zhao Xingmin, Huang Xu, Li Fei, Rashid Kang. The True Cost of Coal, An Investigation into Coal Ash in China, (Beijing: Greepeace, 2010)

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