Festival fireworks caused readings of PM2.5 to skyrocket in many cities during the Spring Festival holiday, indicating worsened air quality, the Ministry of Environmental Protection said.

Dense fog envelops Beijing on Feb. 17. [Sina.com]

Figures released Sunday by the ministry show that 42.7 percent of 74 Chinese cities reported higher-than-normal PM2.5 reading. The highest average reading in a single day was 426 micrograms per cubic meter, or 5.7 times the country's standard of 75 micrograms.

PM2.5 refers to fine particulate matter that is 2.5 microns or less in diameter.

PM2.5 readings were excessive in all 74 cities for about half of the holiday period, which lasted from Feb. 9 to 15, the ministry said.

North China's Tianjin muicipality saw PM2.5 level hit 577 micrograms per cubic meter from 10 a.m. Feb. 9, or Lunar New Year's Eve, to 2 a.m. Feb. 10. The readings were the highest among all 74 cities.

According to the ministry, a certain proportion of the cities also saw excessive readings for PM10 and sulfur dioxide (SO2).

The ministry blamed fireworks and unfavorable weather conditions for the worsened air quality.