As the city’s population rises along with an increasing number of cars, pollution is rising. Shenzhen had 131 smoggy days last year, the most in 50 years, said Zhu Xiaya, an observatory engineer. There were only eight hazy days in the 1970s, and 58 in the 1980s. The upward trend started in the 1990s, when there were 773 days of smog.

The poorer air quality has become a health threat. Experts warn residents to avoid outdoor exercise in the mornings when pollutants are more intense.

Most pollutants in the atmosphere can be breathed into the human body triggering respiratory illnesses and even lung cancer, said Mr. Wu, director of the environment and weather center of the Guangzhou tropical maritime climate research institute.

Low visibility on hazy days affects traffic to some extent. Motorists were advised to drive with caution on those days, said Zhang Xu, spokesman for a Shenzhen expressway company.