The Beijing center recommended that children and the elderly stay indoors, and that others avoid outdoor activities.
The U.S. Embassy also publishes data for PM2.5 on Twitter, and interprets the data according to more stringent standards.
In the 24-hour period up to 10 a.m. Sunday, it said 18 of the hourly readings were "beyond index." The highest number was 755, which corresponded to a PM2.5 density of 886 micrograms per cubic meter. The U.S. Environmental Protection Agency's air quality index goes up to only 500, and the agency advises that anything greater than 300 would trigger a health warning of "emergency conditions," with the entire population likely affected.