Beijing issued a red alert for air pollution Friday for the second time in a little more than a week. The Chinese capital as well as other parts of northern China expect smog from Saturday until Tuesday. The highest-level red alert prompts restrictions on vehicle use, factories, and construction. This weekend’s smog is expected to be worse than the last code red on Dec. 8. The PM2.5 pollution level is expected to exceed 500 micrograms per cubic meter. The World Health Organization recommends 25 micrograms per cubic meter as the maximum safe level.