1. We must trace the source of these noxious gases before they asphyxiate us.
    在我们被这些有毒气体窒息之前, 必须查出其来源。

2. All air in Billion City would be passed through a cleaning system to remove dust particles, pollen, noxious gases and other pollutants.
    十亿人口城市中所有的空气都通过净化系统来消除灰尘、花粉、有害气体及其它污染物。