* Lack of oxygen: Humans need oxygen to breathe. If there is not enough oxygen in the air, humans will become sick and eventually die.
* Lack of water: Humans need water to drink. If there is not enough water available, humans will become dehydrated and eventually die.
* Lack of food: Humans need food to eat. If there is not enough food available, humans will become malnourished and eventually die.
* Too much heat: Humans can only survive in a certain temperature range. If the temperature is too high, humans will become overheated and eventually die.
* Too much cold: Humans can only survive in a certain temperature range. If the temperature is too low, humans will become hypothermic and eventually die.
* Too much radiation: Humans can only survive in a certain amount of radiation. If there is too much radiation, humans will become sick and eventually die.
* Too much pollution: Humans can only survive in a certain amount of pollution. If there is too much pollution, humans will become sick and eventually die.
* Lack of gravity: Humans need gravity to stay on the ground. If there is no gravity, humans will float away and eventually die.
* Lack of atmosphere: Humans need an atmosphere to protect them from the sun's radiation. If there is no atmosphere, humans will be exposed to too much radiation and eventually die.
These are just some of the conditions that would make life difficult or impossible for humans to survive. If any of these conditions were to occur on Earth, it would have a devastating impact on human life.