Mexico City has declared an environmental yesterday, ordering vehicles off the road and postponing the semi-finals of the first-division football league as a blanket of smog enveloped the sprawling capital.   <br />''<br />''A series of wildfires on the city's outskirts have combined with stagnant weather conditions to cloak the metropolitan area of more than 20 million people in a grey cloud. Pollution levels passed the critical 150 points on the authorities' air quality index. <br />'' <br />''<span style="color: #222222;">The Mexican football league said that under the recommendation of city officials, it had decided to postpone the first leg of the first-division semi-final between Leon and Mexico City club America from today night to tomorrow because of the pollution. </span><br />