{"id":5019,"date":"2024-12-18T20:38:00","date_gmt":"2024-12-18T12:38:00","guid":{"rendered":"https:\/\/smartairfilters.com\/?p=5019"},"modified":"2025-03-28T15:31:47","modified_gmt":"2025-03-28T07:31:47","slug":"how-accurate-air-quality-monitors","status":"publish","type":"post","link":"https:\/\/smartairfilters.com\/en\/blog\/how-accurate-air-quality-monitors\/","title":{"rendered":"Are Air Quality Monitors Accurate?"},"content":{"rendered":"

A few months ago, Smart Air tested several air quality monitors, Laser Egg, the Node, and the Dylos, against official PM2.5 readings. The results were way better than I expected<\/a>! All three machines correlated r<\/em> = .90 or higher with the gold-standard numbers, which is very high.<\/p>\n\n\n

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\"Air<\/figure><\/div>\n\n\n

<\/p>\n\n\n\n

But this test may have exaggerated its accuracy. See, the Laser Egg and the Node are probably calibrated to PM2.5 readings in Beijing, so their formulas work great here. But how well do they work in different environments?<\/p>\n\n\n\n

Why Your Air Quality Monitor Might Not Be Accurate in Shanghai<\/h2>\n\n\n\n

Lots of environmental differences could make particle counters less accurate outside of Beijing. Two examples:<\/p>\n\n\n\n

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  1. Beijing tends to be pretty dry, and we know humidity affects laser particle counts<\/a>.<\/li>\n\n\n\n
  2. Some cities may also have more of certain size particles. For example, Beijing might have more particles from the steel factories in Hebei, whereas Shanghai might have more vehicle exhaust particles. If these particles tend to be different sizes, the formula converting particle counts to micrograms would be less accurate.<\/li>\n<\/ol>\n\n\n\n

    Testing the Laser Egg in Guangzhou<\/h2>\n\n\n\n

    Smart Air DIYer Nikki in Guangzhou teamed up with me to test her Laser Egg against the US Consulate in Guangzhou.<\/p>\n\n\n

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    \"PM2.5<\/figure><\/div>\n\n\n

    <\/p>\n\n\n\n

    Nikki took 147 readings outside her apartment, which is 2.77 kilometers away from the consulate. This distance can make the readings less accurate, although my tests with the US Embassy in Beijing from 7 kilometers away<\/a> showed results nearly identical to tests done 200 meters away.<\/p>\n\n\n\n

    Results<\/h2>\n\n\n\n

    The Laser Egg correlated highly with the US Consulate (r\u00a0<\/em>= .88), but much lower\u00a0than in Beijing<\/a>\u00a0(r\u00a0<\/em>= .98). The Egg consistently overestimated PM2.5:<\/p>\n\n\n

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    \"Laser<\/figure><\/div>\n\n\n

    <\/p>\n\n\n\n

    The Laser Egg was off by an average of 23.53 micrograms. That is higher than in Beijing, where it was off by just 6.5 micrograms. (Open-source raw data<\/a> is free for all).<\/p>\n\n\n\n

    Is The Laser Egg Inaccurate Because of Humidity?<\/h2>\n\n\n\n

    I tested whether humidity was the culprit by analyzing humidity readings. Humidity ranged from 36% to 89%, so there was wide variation in humidity.<\/p>\n\n\n\n

    Adjusting for humidity increased the accuracy of the Laser Egg readings. The model went from explaining 76.8% of the variance in PM2.5 to 79.4%. The Egg clearly had less error when humidity was low (left) than when humidity was high (right):<\/p>\n\n\n

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    \"Kaiterra<\/figure><\/div>\n\n\n

    <\/p>\n\n\n\n

    The graphs below are a bit complicated, so let me break them down. First, if the Laser Egg were perfectly accurate, 1 microgram on the Laser Egg would equal 1 microgram from the US Consulate (blue line):<\/p>\n\n\n

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    \"Laser<\/figure><\/div>\n\n\n

    <\/p>\n\n\n\n

    But the Laser Egg isn’t 100% accurate. When humidity is low (< 50%), the actual relationship is off by a bit. In that case, 1 Egg microgram = .72 actual micrograms (middle line). And when humidity is high (> 50%), the actual relationship is even worse, 1 Egg microgram = .60 actual micrograms (right line).<\/p>\n\n\n

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    \"Laser<\/figure><\/div>\n\n\n

    <\/p>\n\n\n

    Bottom Line<\/div>
    Bottom Line: The Laser Egg's Accuracy<\/div>
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