Beijing has issued a yellow alert for air pollution as smog has been blanketing the Chinese capital since Monday. The air quality index in the city has reached nearly 300 mark with poor visibility. Under China's four-tier warning system, red is the most severe, followed by orange, yellow and blue. Authorities have urged manufacturing companies to reduce pollution and ordered a halt to certain work at construction sites. Residents are advised to take protective measures. The smog is expected to disperse on Wednesday night.