Air Matters, a smartphone app that collects air quality information, has been told by Chinese authorities to stop displaying pollution levels that exceed an official cutoff point.

Wang Jun, one of the software engineers behind the app, told Sixth Tone on Tuesday that his team received a phone call on Jan. 5 from a provincial environmental protection bureau ordering them to cap their app’s maximum air quality index (AQI) reading at 500.

AQI is based on the concentration of harmful gases and small particles in the air. Different countries have their own formulas for calculating AQI, and China’s version does not go over 500: Even if air pollution is more severe, the reported AQI will not exceed that number.

Wang said that Air Matters had updated its app on Jan. 4 — a day before the phone call — to allow its AQI reading to go above this official cutoff point. “Our users wanted to know more about the exact number,” he said.

The update came after a weekslong spell of heavy air pollution affecting large parts of China. In late December, the AQI for some cities in the country’s north stood at 500 for days at a time. With local governments facing pressure from citizens to clear the skies, air pollution data is sensitive information.

“The next day [after the update], one of China’s provincial environment protection bureaus called and asked us to set the maximum numerical value back,” Wang said. Though he would not give the name of the province, he implied that it was one of the most polluted in China. They reverted the update after the phone call.

In 2014, the app — then called “China Air Quality Index” — was ordered by the Chinese government not to use air quality data from the U.S. embassy in Beijing. That order also came at a time when air pollution was a sensitive issue: on the same day foreign dignitaries arrived in the city for the Asia-Pacific Economic Cooperation (APEC) forum. In the lead-up to the summit, authorities had told factories in the area to halt production in order to make the skies as blue as possible — a phenomenon that earned its own sarcastic designation: “APEC blue.”

Development of the app started in 2011, around the time public awareness of air pollution began to rise in China. At the time, Wang worked for China’s largest internet search engine, Baidu. He and a colleague decided to experiment with noncommerical business models and came up with Air Matters. “We wanted to solve peoples’ needs rather than make commercial products,” said Wang.

The app has been downloaded more than 9 million times and delivers air pollution data on 52 countries from more than 11,000 monitoring sites. For China, most of the monitoring stations providing data are run by the Chinese government, although some comes from other sources, such as the U.S. consulate in Shanghai.

Netizen comments have suggested general resignation toward the latest news:  “We don’t get rid of the problem — we get rid of the the people who point out the problem,” wrote one user of microblog platform Weibo.

(Header image: A PM 2.5 monitoring app reads 475 at a park in Beijing, Jan. 4, 2017. VCG)