A visitor swims in a thermal spring swimming pool on May 18, 2013, at Guanlan Lake Holiday Resort in Haikou city, Hainan province. [Photo/Xinhua]

Haikou city in Hainan province once again ranked the best among 74 Chinese cities tested monthly for air quality. The results came in the China National Environment Monitor Center's April report.

Haikou's air-quality index registered at 1.61, which is the lowest among the 74 cities in China. The air quality in a city in Hebei province ranked worst -- registering at 5.62.

Visitors play golf on May 18, 2013, at Guanlan Lake Holiday Resort in Haikou city, Hainan province. [Photo/Xinhua]

A visitor plays in a thermal spring swimming pool on May 18, 2013 at Guanlan Lake Holiday Resort in Haikou city, Hainan province. [Photo/Xinhua]