The operators of a popular pollution monitoring app installed on tens of thousands of phones in Beijing have been told to cap its highest AQI reading at 500, Sixth Tone reported today.

The article said authorities ordered the providers of the Air Matters pollution reading app to make sure its app never reports an AQI index number above 500 in China.

AQI is actually not a direct reading of pollution levels, but an indirect indicator that is derived from a variety of air quality data. The scale, which in both the US and China goes from 0 to 500, is meant to be a consumer-friendly indicator of relative air quality.

The scale, first derived by America's Environmental Protection Agency (EPA), stops at 500 because it was unimaginable at the time that the top end of the scale would be reached, in the absence of major disasters such as forest fires. However, the formula in principle has no limit, and numbers could go on for as high as you want – and in China, many hourly readings regularly exceed the 500 "beyond index" level.

One of the app's software engineers told the news outlet that an officer from the provincial environmental protection bureau called his team on January 5 and demanded they do not display AQI reading above 500, which coincides with the limit of both China and the US EPA's air quality index.

AirNow, a website that details much of America's air quality particulars and is spearheaded by the EPA, says "Think of the AQI as a yardstick that runs from 0 to 500. The higher the AQI value, the greater the level of air pollution and the greater the health concern." It also states clearly in its scale "Note: Values above 500 are considered beyond the AQI."

That sentiment is echoed by Yann Boquillod, co-founder of the AirVisual app, a competitor of Air Matters, who says: "The Americans [EPA] capped it at 500 because they probably didn't think it would ever go so high." He says the Chinese authorities did not make any demands about his team's readings and adds: "We extend the graph based on the formula used for calculating hazardous levels."

And while many experts might not have ever dreamt that PM2.5 levels could reach such alarmingly high levels in the past, that doesn't necessarily mean that they made a fatal oversight. In fact, many of us could easily dare to wonder if there really is that much of a difference between AQI 500 and AQI 799 – not in numerical terms, but in practicals terms.

That's because the behavior of air quality conscientious Beijingers on AQI 350 or even 400 days – nevermind the 500 cap – shouldn't be different than on days when the AQI goes north of 700. In fact, anything over 150 is considered "unhealthy" and any reading above that should mean that you tailor your behavior outside of the safety of an air-filtered environment accordingly.

To that end, Liam Bates – co-founder of Origins, which sells the Laser Egg AQI monitor and, like AirVisual, was not ordered by the Chinese authorities to cap their readings like Air Matters was – says that capping AQI readings at 500 is by no means limiting, but instead proper practice.

"The AQI scale is defined as going from 0-500. That is the maximum, end of story," he says, adding: "No one says the raw concentration (ug/m3) has to be limited. But the thing is, if you're using a scale someone made and is a government standard, you should follow it."

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Bates goes on to equate going above the 500 cap with a developer that created a weather app and opted to change basic temperature reading principals on a whim. "If it gave the option to view Fahrenheit, and then you decided that the difference with Celsius was too big beneath 0 degrees, so you just changed the formula – you can't do that."

And any foreigners that are riled up by the authorities asking an app developer to limit their readings to a prearranged cap might very well just be jumping to conclusions and falling back on hardened prejudices, at least in Bates' view. "It has nothing to do with hiding readings that are high," he says. "If that was the case maybe the app would be asked to stop showing PM2.5 concentrations. I feel it's a bit of a case of foreigners saying china is censoring everything again. But really those apps are just doing it incorrectly."

More stories by this author here.

Email: kylemullin@truerun.com

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