（21）Disruption of ecological balance（the balance of ecosystem）
（22）Eruption of volcanoes                                 （23）Earthquake
（24）Avalanche                                                 （25）Landslide
（26）Hurricane                                                 （27）Shortage of water resources
（28）Shortage of fresh water                              （29）Harmful chemicals
（30）Poisonous/toxic gases                                 （31）Carbon monoxide
（32）Urban smog                                              （33）Industrial waste
（34）Hazardous nuclear waste                            （35）Radioactive pollutants
（36）Waste gas sent off from automobiles           （37）Car emission standard
（38）Non-biodegradable material                         （39）Throw-away lunchbox
（40）Recyclable product                                     （41）Renewable resources
（42）Conserve natural resources                         （43）Plastic bags
（44）Greenhouse effect                                      （45）Global warming
（46）Acid rain                                                   （47）Energy crisis