Last month's deadly riot in Lhasa, Tibet, not only left the city with casualties and economic loss, but also caused considerable environmental pollution when the air quality was monitored as the worst in its history.

"The air quality on March 14 was the worst on record since Lhasa began daily air quality reporting in 2000," said Zhang Yongze, the regional environmental protection bureau director on Thursday.

The density of sulfur dioxide was 10 micrograms per cubic meter on the day, double the average in the city. The nitrogen dioxide level was 60 micrograms per cubic meter, more than double compared to a normal day, according to air quality monitoring data from the bureau.

"Was the thick smoke, sewage and char produced by the arson in the riot the concrete actions to protect Tibet's environment as trumpeted by the Dalai clique?" asked Zhang indignantly.

Lhasa, known as the "City of Sunshine", is considered one of the purest places on the Earth. However, the plateau city was enveloped in dark clouds of smoke when the rioters set fires to schools, hospitals, homes and shops.

"The levels of sulfur dioxide and nitrogen dioxide in the air are two indicators marking industrial pollution and automobile emission. However, Lhasa is a city free of industrial pollution," Zhang said. "The major factor affecting the air quality here is sand dust."

The bureau's records showed that good air quality was recorded in 95 percent of the days in 2000, and further improved to 98 percent in 2007.

As life returned to normal in Lhasa, the good air was back again. The records on that day, however, have been noted.

The riot killed 18 civilians and one police officer and injured hundreds of others. The total property damage was more than 280 million yuan (40 million U.S. dollars).

(Xinhua News Agency April 18, 2008)