Many expatriates in Beijing used to find the capital a pleasant city to live in, especially right after the 2008 Olympics.
But some are now considering relocating because of its chronic air pollution.
The city's air quality improved yesterday morning, with readings of health-threatening PM2.5 respirable particles of about 100 micrograms per cubic metre of air, down from nearly 900 on Saturday. But the figure rose to above 200 in the afternoon as the sky turned milky, triggering concerns about the return of the smog that blanketed the city at the weekend.
The World Health Organisation recommends that PM2.5 levels be kept below 25 micrograms per cubic metre.
Joshua Dyer, a translator from the United States, recalled how different Beijing's air was when he arrived in 2008, when huge sums were being invested to improve the environment for the Olympics. "It was surprisingly good. Many blue skies," he said. "But what happened over the weekend was really shocking."
Dyer uses an air filter at home and puts on a mask when pollution readings are high. "The air pollution is one reason I know I can't stay much longer here. I feel the bad air affects me psychologically as well. I feel sluggish on heavily polluted days."
Youtube video: an extremely bad air day in Beijng
An American PR consultant, who arrived in Beijing two years ago, said he was planning to move, even though his employer wanted him to stay. "Many people are talking about whether to stay. The air pollution pushes them over the edge so that they really can't take it," he said.
The consultant said he thought Beijing's air pollution would be similar to that of Hong Kong when he first arrived in the capital, but he was always coughing after suffering from mild pneumonia last year. "Sometimes I end up taking aspirin to cure a headache after waking up," he said.
A Beijing-based American blogger working for a mainland newspaper said: "I have had more respiratory problems living in Beijing in 21/2 years than I had in my entire life in the US."
Some Hongkongers in Beijing also find the air pollution unbearable, even though they are not leaving Beijing. "I have to spend a fortune on buying an air purifier and am limited to staying at home," said Hongkonger Elaine Ho, who came to Beijing to join her husband three years ago.
A survey by the American Chamber of Commerce in China last year showed 36 per cent of 244 companies experienced difficulties recruiting senior executives because of air quality issues - up from 19 per cent in 2010.
Richard Saint Cyr, a doctor at Beijing United Family Hospital, said he was receiving more patients, not only because of pollution but also due to the flu season.
The doctor, who has lived in Beijing for six years, said air quality had deteriorated, but he still planned to raise his family in the capital. "When it comes to emotional health, my life here is quite interesting and exciting," he said.
Many mainland cities were still experiencing bad pollution yesterday, especially Shanghai, where the PM2.5 reading was 242. Liu Ronggen, 75, said he found it uncomfortable to breathe. "I had to wear a mask and I told my granddaughter not to do outdoor activities at her primary school," he said.
Additional reporting by Alice YanTopics: Environment Beijing air pollution