SHANGHAI, April 16 (Xinhua) -- Shanghai began on Monday to include unhealthy "haze" levels in its daily weather forecasts to warn people about air pollution, two months after the same move was taken by Beijing.

    No haze was reported in the city on Monday, a spokesman with the Municipal Meteorological Bureau said.

    Haze refers to weather with air humidity of 80 percent or below, and is different from fog, which occurs when humidity in the air is more than 90 percent. It forms when concentrations of dust and smog in the air are high.

    Beijing launched the same weather forecasting service on February 1. It classifies haze as classified light, moderate or heavy. Light haze means that outdoor visibility is between five and ten kilometers; with moderate haze, visibility is between two and five kilometers; heavy haze means visibility is less than two kilometers.

    A spokesman for the Beijing weather bureau said there were no "haze days" in the capital last month.

    Experts said that haze contains substances harmful to the respiratory tract and lungs so people should stay indoors during moderate and heavy haze days. Long exposure to haze can lead to coryza, bronchitis and even lung cancer.