Hundreds of flights were grounded in Beijing Monday as heavy smog reduced visibility, highlighting the city’s chronic air pollution problems, the Financial Times reported. Nearly 500 flights from Beijing were canceled or delayed on Monday due to poor weather and pollution levels that were described as “hazardous” by the US embassy. Beijing residents have recently become more vocal about official pollution data that is far out of step with that published by the US embassy. “In Beijing and most other cities, the official data shows an increasing number of blue sky days. But to the ordinary people, the residents, they feel differently, because they cannot see the mountains or even the buildings during the blue sky days,” said environmentalist Ma Jun. Beijing’s data is based on a reading of larger PM10 particles in the air, while US measurements are based on smaller PM2.5 particles, which are considered more dangerous to health.