BEIJING, Jan. 8 (Xinhua) -- A cold front on Sunday dispersed the smog which had been hovering over Beijing for the past week.

Beijingers saw blue sky again on a sunny weekend. People with children who had been cloistered indoors for the week took to parks and streets for the first time since the beginning of the year.

Beijing lifted its orange alert for air pollution on Saturday night and emergency measures such as vehicle restrictions and the suspension of factory production ended and construction work recommenced, according to the city pollution response headquarters.

Beijing has a four-tier alert system for pollution, with red the highest, followed by orange, yellow and blue. The orange alert means the air quality index is forecast to exceed 200 for three consecutive days. During high alerts, heavy polluting vehicles and trucks carrying construction waste are banned from roads and some manufacturing firms cut production.

Beijing issued an orange alert for air pollution on Dec. 29. A weak cold front dispersed the smog briefly on Jan. 2, then the city renewed the orange alert and extended it on Jan. 4.

Blue skies are forecast for the next few days.

According to the city's air quality surveillance center, except the southern Daxing District, air quality was "excellent" or "good" as of 10 a.m. Sunday. The density of PM 2.5 was less than 100 micrograms per cubic meter.8 Smog has become north China's biggest environmental issue in recent years, particularly in winter, when coal-fired heating boilers rev up and the wind drops. Despite drops in pollution indicators, smog still occurs frequently.