Shanghai has added ozone density to its serious weather warnings. All warning signs have now been modified, the Shanghai Meteorological Bureau reported yesterday.

Shanghai is the country's first city to include ozone density in its weather warnings, according to the bureau.

The ozone density alert has two scales: yellow and orange. A yellow alert warns that in the next four hours, the ozone density in the city may rise over 100 parts per billion by volume.

It is suggested that people close windows and doors and avoid heavy physical exercise. Children, the elderly and people with respiratory diseases should stay indoors.

An orange alert warns that the ozone density is more than 120 ppb and that the condition will continue; or in the coming four hours, the ozone density might rise over 120 ppb.

When this happens all doors and windows should be closed and all outdoor activities should stop. In certain districts traffic control might be enforced.

The bureau said that over the past two years, the city had had 12 days when the ozone density reached a yellow alert level.

"Ozone is a serious pollutant which damages the respiratory system and accelerates degeneration of the lungs," said Geng Fuhai, vice director of the bureau's urban environmental and meteorological center.

"Over-dense ozone will also kill plant cells and reduce the yield from agriculture," he added.

Ozone is formed when volatile organic compounds are exposed to sunshine. VOCs, which can be widely found in the exhaust fumes of vehicles and factories, include a variety of chemicals such as carbon monoxide, Geng explained.

The new weather warnings are based on the latest system issued by the state-level bureau in the middle of October, said the bureau.

The new warnings have some changes and include haze, drought, and sandstorm warnings. Warnings for thunderstorms and hail will be divided into two separate parts.

And low temperature warnings have been changed to cold wave alerts.

Some practical hints are also going to be included in the new warnings - like advising people not to carry metal objects like badminton rackets or golf clubs outdoors when there is lightning about.

(Shanghai Daily November 2, 2007)