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The roadside air pollution level in Causeway Bay had reached the "very high" level for five consecutive days last month.

According to the September monthly summary on air pollution index released by Environmental Protection Department, the roadside API in Causeway Bay exceeded 100 between September 19 and 23.

Air pollution level is considered to be "very high" when the API is between 101 and 200.

A roadside API of 167, a record high since the index was introduced in June 15, was measured in the month at the monitoring station in Causeway Bay. The station represented typical busy urban streets with very heavy traffic and surrounded by high-rise buildings.

The episode started on September 19 with the API at the Causeway Bay station reported at 106, reached a maximum of 167 on September 22, and dropped to 101 on September 23.

The winds prevailed during the period brought to Hong Kong some background ozone. Together with those formed locally by photochemical reaction of vehicle emissions, the ozone enhanced the oxidation of nitrogen oxide emitted from diesel vehicles into nitrogen dioxide.

Persistently high levels of nitrogen dioxide were measured at the Causeway Bay station as the pollutant did not disperse quickly until the wind picked up on September 24.

During the same period, the general APIs were also in the "high" API band with the highest reading of 96 measured at the Kwun Tong station on September 20. It was attributable to the elevated nitrogen dioxide level.

For the rest of the month, the APIs for the roadside stations ranged from 40 to 97 while that for the general stations from 20 to 87.

The API describes air pollution in five bands according to the potential impact on human health. The air quality is good when the API is from 0 to 25 (low), and acceptable when the index is from 26 to 50 (medium).

An API from 51 to 100 means that the air pollution is high. Regular exposure to such a level on a long-term basis could cause adverse health effects.

Any API higher than 100 means that the air pollution may pose an immediate health effect to some susceptible members of our community. At such levels, persons with heart or respiratory illnesses should reduce physical exertion or outdoor activities.

High air pollution and high APIs are frequently caused by respirable suspended particulates and nitrogen dioxide which are coming mainly from diesel motor vehicle exhausts.

Other common air pollutants include ozone and sulphur dioxide from both motor vehicles and industrial emissions.

End/Thursday, October 8, 1998

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