HONG KONG — Air pollution has worsened markedly in Asian cities in recent years and presents a growing threat to human health, according to experts at a conference that began on Wednesday.

Clean Air Asia, a regional network on air-quality management, aggregated data from more than 300 cities in 16 Asian countries and found that levels of fine particulate matter — a key pollutant in terms of its impact on human health — were below targets recommended by the World Health Organization in just 16 cities, most of them in Japan.

Pollution levels in 70 percent of the cities, mostly in fast-growing, less developed countries like China, India, Bangladesh and Mongolia, exceed even the most lenient of several targets recommended by the W.H.O., the organization said.