Fireworks and adverse weather conditions resulted in soaring PM 2.5 readings and reduced air quality in many Chinese cities during the Spring Festival holiday, the Ministry of Environmental Protection said.

PM 2.5 refers to fine particulate matter with a diameter of less than 2.5 microns. It is extremely hazardous to respiratory and cardiovascular systems, especially among children and the elderly.

Of the 74 cities that had applied a more comprehensive air quality monitoring standard and published daily reports on PM 2.5 by the end of 2012, 42.7 percent reported excessive readings during the week-long holiday.

The highest average single-day reading reached 426 micrograms per cubic meter, nearly 5.7 times the country's standard of 75 micrograms per cubic meter, the ministry said.

The days when excessive pollutants were recorded accounted for 45.3 percent of the holiday, which ran from Feb 9 to 15, while normal air conditions prevailed for 54.7 percent of this time.

The ministry said Tianjin saw the highest PM 2.5 concentration?- 577 micrograms per cubic meter?- among the 74 cities on Lunar New Year's Eve, from 10 am on Feb 9 to 2 am on Feb 10, followed by Shijiazhuang (527 micrograms per cubic meter) and Shanghai (511 micrograms per cubic meter).

Some cities also saw soaring readings for PM 10 and sulfur dioxide, the ministry said, adding that fireworks and unfavorable weather conditions were the main reasons for the poor air quality.

The latest research by the Chinese Academy of Sciences found a large amount of nitrogen-containing organic compounds in the recent smog that shrouded Beijing and nearby cities, key components of the photochemical smog 100 years ago in Los Angeles that killed more than 800 residents.

Research leader Wang Yuesi said it was the first time a large amount of nitrogen-containing organic compounds had been found in winter. He said scientists are conducting further research on the particulate matter collected.