Despite Wang Zhangyi's excitement for a school-organized autumn outing on Friday, a trip he had been looking forward to for weeks, his mother was nothing but reluctant and anxious.

"I'm concerned for his health when he's exposed to serious air pollution and breathing smog," said Zhang Huihui, the mother of the 6-year-old who was recently enrolled in elementary school.

Serious pollution again blanketed Beijing on Friday, with high levels of PM2.5 pollution - particulate matter with a diameter smaller than 2.5 micrometers.