Shanghai weather station issued high-temperature warning signal for the sixth consecutive days on July 30. At these sizzling hot days, ozone pollution occurs, reports China News Service on Thursday.

Data shows the city's primary pollutant is ozone since July 25. The air quality index (AQI) remained optimal level in the morning of July 25, but from 11 pm to 14 am, AQI surged from 58 to 114, almost doubled within two hours, because the ozone concentration continued to rise.

Different from smog, ozone pollution is easy to be ignored because it often appears at particular sunny days.

Volatile organic compounds (VOC), nitrogen oxides and strong light are the three elements which generates ozone, said Xiu Guangli, professor of East China University of Science and Technology.

The VOC, from industrial and catering emissions, and nitrogen oxides, from motor vehicles, power plants and ships, will chemically react in the bright light and lead to ozone pollution. So, ozone concentration in the air reaches the highest level around two to three o'clock in the afternoon.

"Prolonged exposure can cause eye irritation, coughing and other respiratory diseases," said Xiu Guangli, who also suggest that sensitive people should try to reduce travel when there are high ozone concentration in the air.

Since the 1990s, the ozone concentration has aroused concern of Shanghai authority and a series of policies have been issued including strengthening the monitor of oil quality and stimulating stricter vehicle emission standards.