1.Something must be done to prevent factories sending out poisonous (poison) gases so as to keep the city from being polluted.
2.Despite the health problem caused by his excess weight, the wealthy businessman was still obsessed with making as much money as possible.
3.It was not until the early 18th century that the significance of the event was generally realized.
4.The fog was so heavy this morning that drivers could hardly make out the things just ten meters away.