People stay indoors as air pollution in notoriously polluted Beijing goes off the index

People refused to venture outdoors and buildings disappeared into Beijing's murky skyline as the capital's air quality went off the index.

The Beijing Municipal Environmental Monitoring Center said on its website Sunday that the density of PM2.5 particulates had surpassed 700 micrograms per cubic meter in many parts of the city. Its index goes up to only 500.

The U.S. Embassy publishes data for the small particulate matter, which can penetrate deep into the lungs, but interprets the data according to more stringent standards.

In the 24-hour period up to 10 a.m. Sunday, it said 18 hourly readings were "beyond index." The highest was 886. The U.S.'s Environmental Protection Agency advises anything greater than 300 would trigger a health warning of "emergency conditions" with the entire population likely affected.