BEIJING, May 21 (Xinhua) -- Beijing saw better air quality and more "good air" days in the first four months of this year, the Beijing Municipal Environmental Protection Bureau announced on Monday.

Average density of PM2.5 in Beijing stayed at 59 micrograms per cubic meter of air in the four months, down 22.4 percent year on year, the bureau said.

The capital saw 77 good air days from January to April, four days more than the same period last year. The number of heavy pollution days decreased by eight to seven, according to the bureau.

However, the air quality in April was not as good as the previous April, the bureau said. The average density of PM2.5 was 64 micrograms per cubic meter of air, up 20.8 percent compared with April 2017.

Experts said that high humidity, bad weather conditions, and an increase in construction sites after the winter were to blame for the rising PM2.5 density.

Average density of PM2.5 in Beijing dropped to 58 micrograms per cubic meter of air in 2017. It has met the goal set by the State Council in 2013.