The average density of PM2.5 particles polluting the air in Hebei province fell to 33 micrograms per cubic meter in April, down 26.7 percent year-on-year and hitting a record low for the month since monitoring of the particles began in 2013, the local environmental authority said.

The measurement is lower than the national second-level standard — an annual average level of 35 mcg/cu m. Reaching that would mean air quality is very good, the Hebei Bureau of Ecology and Environment said.

The national standard for the best air quality, in terms of PM2.5, is 15 mcg/cu m.

Of the 11 cities under the administration of Hebei, six saw their monthly average density of PM 2.5 in April fall to the national second-level standard, with Chengde boasting the lowest at 21 mg/cu m. Tangshan was highest at 40 mcg/cu m.

The monthly average concentration of other major pollutants in the province — sulfur dioxide, nitrogen dioxide and carbon monoxide — also dropped, the bureau said.

In past years, Hebei usually had six or seven cities that were listed among the 10 having the worst air conditions of the country's major monitored cities.

Since 2013, the province has strengthened its measures to improve air quality, including controlling dust at construction sites, cutting back on pollution from burning, reducing vehicle emissions and upgrading industrial production procedures to cut down pollutants.

Those efforts have been upgraded this year to yield better air quality.