SHIJIAZHUANG -- Hebei province, which neighbors Beijing and is known for severe winter smog, has recorded its best air quality since pollution control measures were implemented five years ago, local authorities said Friday.

Hebei's average PM2.5 level in 2017 was 65 micrograms per cubic meter, down 7.1 percent year-on-year, marking a decrease of nearly 40 percent from 2013.

PM2.5 is a measure of the density of hazardous particulate in the air. The World Health Organization recommends an annual PM2.5 level of 10 micrograms per cubic meter.

Hebei also recorded improvements in the density of other major pollutants such as sulfur dioxide, nitrogen dioxide and PM10, with sulfur dioxide down 63.5 percent compared with 2013.

While the number of heavy pollution days decreased from 80 in 2013 to 29, the province also enjoyed 202 "good air" days last year, 73 more than in 2013.

Hebei has experienced an unusual streak of clear air this winter. From October to November, PM2.5 levels dropped 37.4 percent year-on-year.