Record pollution levels that saw Shanghai engulfed by acrid smog on Friday caused flight cancelations, shortages of face masks and made some expatriates reconsider their long-term plans to stay in the city.

"It's horrifying. I've never seen anything like this. I feel like I've had a constant hangover for four days," said Tom Duvalier of Chicago. "If you go down into the subway system, the same smell is in the air. It's everywhere."

The air quality index measured by the Shanghai Environmental Monitoring Center stood at 482 as of 6 pm, while the US Consulate gauged it at 503 at 2 pm - a reading "beyond the index". Levels above 300 are considered "hazardous".