Beijing authorities say a key measure of air pollutants showed a 19 percent drop in the first three months of the year.

The municipal government said Wednesday that the density of PM2.5 — harmful particles that are smaller than 2.5 microns in diameter — was 92.7 micrograms per cubic meter for the quarter, an improvement from 114 last year. The World Health Organization safety level is 25 micrograms.

Despite the improvement, Beijing remains one of the most polluted cities in China, according to the environmental group Greenpeace.

Beijing and national leaders have vowed to tackle the problem. Authorities say the closure of coal-fired factories and suspension of some construction sites have contributed to the decrease.